SDO (Solar Dynamics Observatory)
SDO is a NASA satellite, considered to be a second-generation solar mission (also referred to as SOHO successor). SDO represents the first mission within NASA's LWS (Living With a Star) program, a space weather-focused and applications-driven research program. The goal of LWS is to understand the sun as a magnetic variable star and to measure its impact on life and society on Earth.
The overall SDO objective is to observe the dynamics of the solar interior, provide data on the sun's magnetic field structure, characterize the release of mass and energy from the sun into the heliosphere, and monitor variations in solar irradiance. The goal is to understand the dynamic state of the sun (its variability) on multiple temporal and spatial scales which influence life and technology on Earth - to enable the development of an operational capability for space weather prediction (the purpose of the LWS Program). 1) 2) 3) 4) 5) 6)
The SDO mission was assigned a number of mission objectives specifically designed to support the LWS goals of understanding the drivers of solar activity and variability that affect Earth and humanity. Specifically, SDO was designed to address seven science questions dealing with the sun's dynamic activity and its effect on the Earth: 7)
1) What mechanisms drive the quasi-periodic 11-year cycle of solar activity?
2) How is active region magnetic flux synthesized, concentrated, and dispersed across the solar surface?
3) How does magnetic reconnection on small scales reorganize the large-scale field topology and current systems and how significant is it in heating the corona and accelerating the solar wind?
4) Where do the observed variations in the Sun‘s EUV spectral irradiance arise, and how do they relate to the magnetic activity cycles?
5) What magnetic field configurations lead to the coronal mass ejections (CMEs), filament eruptions, and flares that produce energetic particles and radiation?
6) Can the structure and dynamics of the solar wind near Earth be determined from the magnetic field configuration and atmospheric structure near the solar surface?
7) When will activity occur, and is it possible to make accurate and reliable forecasts of space weather and climate?
• To provide nearly continuous coverage of solar activity
• To provide coverage of the regimes (interior, photosphere, corona) in which the activity occurs
• To provide sufficient data on the types of phenomena which impact Earth, near-Earth space and humanity
• To observe the solar variability over the relevant timescales (seconds to years).
Figure 1: Artist's rendition of the deployed SDO spacecraft (image credit: NASA)
The spacecraft is being designed and built at NASA/GSFC. The SDO design consists of a bus module and an instrument module (Figure 5); the instrument module employs a graphite composite structure to minimize thermal distortions The spacecraft bus module contains the S/C and instrument electronics. Redundant HGAs (High Gain Antennas) are mounted at the end of rigid booms (must be rigid due to required waveguides).
The spacecraft is 3-axis stabilized. The ACS (Attitude Control System) is a single-fault tolerant design. Its fully redundant attitude sensor complement includes 16 coarse sun sensors, a digital sun sensor (DSS), 3 two-axis inertial reference units (IRU), 2 star trackers (ST), and 4 guide telescopes. Attitude actuation is performed using 4 reaction wheel assemblies (RWA) and 8 thrusters, and a single main engine nominally provides velocity-change thrust. - The attitude control software has five nominal control modes: 3 wheel-based modes and 2 thruster-based modes. A wheel-based safehold running in the attitude control electronics box improves the robustness of the system as a whole. All six modes are designed on the same basic proportional-integral-derivative attitude error structure, with more robust modes setting their integral gains to zero. 8)
The ST and DSS combine to provide two-out-of-three single-fault tolerance fine attitude determination. Any one of the 4 AIA guide telescopes may be selected as the ACS CGT (Controlling Guide Telescope). Control is actuated using reaction wheel assemblies (RWA) and attitude control thrusters. Orbit change maneuvers can be accomplished using either the thrusters or a main engine, i.e. the RCS (Reaction Control Subsystem); the main engine will be used nominally for all long maneuvers performed in achieving geosynchronous orbit from the launch orbit.
The ACS supports five operational modes. These are: sun acquisition, inertial, science, ΔH and ΔV. One mode, namely safehold, operates solely in the ACE (Attitude Control Electronics) software. The SDO remains sun-pointing throughout most of its mission for the instruments to take measurements of the sun.
Figure 3: Overview of ACS components in the spacecraft (image credit: NASA)
Figure 4: Block diagram of the SDO attitude control electronics (image credit: NASA)
Onboard Ephemeris: The SDO onboard ephemeris predicts the locations of the Sun, Moon, spacecraft, and ground station in geocentric inertial coordinates, referred to the mean-equator-and-equinox of J2000 (GCI, mean-of-J2000). Each object's velocity is derived from differencing successive position vectors and dividing the result by the ephemeris task sample time (nominally 1 second). The solar ephemeris accuracy is better than 2 arcseconds during the 10 year SDO mission lifetime and has been validated by the JPL DE405 ephemeris.
The following key spacecraft technologies are being introduced:
• Ethernet chipset
• Ka-band transmitter
• APS (Active Pixel Sensor) star tracker.
SDO uses a bi-propellant propulsion system, an AKM (Apogee Kick Motor), to boost the spacecraft from a GTO (Geosynchronous Transfer Orbit) into a GSO (Geosynchronous Orbit). The Spacecraft design life is 5 years (10 years for expendables). The launch mass of SDO is about 3,200 kg.
Table 1: Overview of spacecraft parameters
Figure 6: Photo of the integrated SDO spacecraft (image credit: NASA)
Orbit: Inclined geosynchronous circular orbit (IGSO), altitude ~ 35,756 km, inclination = 28.5º, the spacecraft is positioned at a longitude of 102º W. The GSO permits nearly continuous observations of the sun and high data rates to the ground. Only two short eclipse periods per year are being encountered where the Earth's shadow grows to a maximum of about 72 minutes per day. Note: the inclined orbit will form a lemniscate, also referred to as analemma, (i.e., a figure 8 ground track) over the Earth during each day extending to ±28.5º in latitude (inclination) at the longitudinal position.
Figure 7: Illustration of SDO daily orbital trace of a figure 8 at the longitude of 102ºW with maximum latitude extensions of ±28.5º (image credit: NASA)
RF communications: Science data are downlinked in Ka-band (26.5 GHz) from its redundant onboard high-gain antennas at a data rate of 150 Mbit/s (includes data compression). There are no onboard recorders for the science data since the spacecraft is in continuous contact with the ground station. The TT&C data are in S-band (2215 MHz) using two onboard omni-directional antennas. - The continuous stream of science data from the SDO spacecraft will produce ~ 2 TByte of raw data every day.
• November 1, 2017: This sequence of images of Figure 8 shows the Sun from its surface to its upper atmosphere, all taken at about the same time on Oct. 27, 2017. The first shows the surface of the Sun in filtered white light; the other seven images were taken in different wavelengths of extreme ultraviolet light. Note that each wavelength reveals somewhat different features. They are shown in order of temperature from the first one at 6,000 degree C. surface, out to about 10 million degrees C. in the upper atmosphere. Yes, the Sun's outer atmosphere is much, much hotter than the surface. Scientists are getting closer to solving the processes that generate this phenomenon. 11)
Figure 8: On 27 Oct. 2017, the AIA (Atmospheric Imaging Assembly) instrument on SDO captured this image sequence of the Sun from its surface (left) to its upper atmosphere (right) in different wavelengths (image credit: NASA/GSFC/SDO)
• September 6, 2017: The sun emitted two significant solar flares on the morning of Sept. 6, 2017. The first peaked at 9:10 UTC and the second, larger flare, peaked at 12:02 UTC. NASA's SDO (Solar Dynamics Observatory), which watches the sun constantly, captured images of both events. Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth's atmosphere to physically affect humans on the ground, however — when intense enough — they can disturb the atmosphere in the layer where GPS and communications signals travel.
Figure 9: This animation shows both the X2.2 and the X9.3 flares that the Sun emitted on Sept. 6, 2017. The imagery was captured by the AIA instrument of NASA's SDO and shows light in the 131 angstrom wavelength (image credit: NASA/Goddard/SDO)
- The first flare is classified as an X2.2 flare and the second is an X9.3 flare. X-class denotes the most intense flares, while the number provides more information about its strength. An X2 is twice as intense as an X1, an X3 is three times as intense, etc.
- Both flares erupted from an active region labeled AR 2673, which also produced a mid-level solar flare on Sept. 4, 2017. The X9.3 flare was the largest flare so far in the current solar cycle, the approximately 11-year-cycle during which the sun's activity waxes and wanes. The current solar cycle began in December 2008, and is now decreasing in intensity and heading toward solar minimum. This is a phase when such eruptions on the sun are increasingly rare, but history has shown that they can nonetheless be intense. The radio black out from this particular flare is already passed.
• On May 25 2017, NASA's SDO mission saw a partial solar eclipse in space when it caught the moon passing in front of the sun. The lunar transit lasted almost an hour, between 6:24 and 7:17 UTC, with the moon covering about 89 percent of the sun at the peak of its journey across the sun's face. The moon's crisp horizon can be seen from this view because the moon has no atmosphere to distort the sunlight. 12)
- While the moon's edge appears smooth in these images, it's actually quite uneven. The surface of the moon is rugged, sprinkled with craters, valleys and mountains. Peer closely at the image, and you may notice the subtle, bumpy outline of these topographical features.
- Later this summer on Aug. 21, 2017, SDO will witness another lunar transit, but the moon will only barely hide part of the sun. However, on the same day, a total eclipse will be observable from the ground. A total solar eclipse — in which the moon completely obscures the sun — will cross the United States on a 110 km wide ribbon of land stretching from Oregon to South Carolina. Throughout the rest of North America — and even in parts of South America, Africa, Europe and Asia — a partial eclipse will be visible.
- The moon's rough, craggy terrain influences what we see on Earth during a total solar eclipse. Light rays stream through lunar valleys along the moon's horizon and form Baily's beads, bright points of light that signal the beginning and end of totality.
- The moon's surface also shapes the shadow, called the umbra, that races across the path of totality: Sunlight peeks through valleys and around mountains, adding edges to the umbra. These edges warp even more as they pass over Earth's own mountain ranges. Visualizers used data from NASA's LRO (Lunar Reconnaissance Orbiter), coupled with NASA topographical data of Earth, to precisely map the upcoming eclipse in unprecedented detail. This work shows the umbral shape varies with time, and is not simply an ellipse, but an irregular polygon with slightly curved edges.
Figure 10: Illustration of the lunar transit observed by NASA's SDO (image credit: NASA/GSFC, SDO/Joy Ng, producer)
• April 10, 2017: New research on solar storms finds that they not only can cause regions of excessive electrical charge in the upper atmosphere above Earth's poles, they also can do the exact opposite: cause regions that are nearly depleted of electrically charged particles. The finding adds to our knowledge of how solar storms affect Earth and could possibly lead to improved radio communication and navigation systems for the Arctic. 13)
- A team of researchers from Denmark, the United States and Canada made the discovery while studying a solar storm that reached Earth on Feb. 19, 2014. The storm was observed to affect the ionosphere in all of Earth's northern latitudes. Its effects on Greenland were documented by a network of global navigation satellite system, or GNSS, stations as well as geomagnetic observatories and other resources. Attila Komjathy of NASA/JPL (Jet Propulsion Laboratory), Pasadena, California, developed software to process the GNSS data and helped with the data processing. The results were published in the journal Radio Science. 14)
- Solar storms often include an eruption on the sun called a CME (Coronal Mass Ejection). This is a vast cloud of electrically charged particles hurled into space that disturbs the interplanetary magnetic field in our solar system. When these particles and the magnetic disturbances encounter Earth's magnetic field, they interact in a series of complex physical processes, and trigger perturbations in the Earth's magnetic field. Those perturbations are called geomagnetic storms. The interactions may cause unstable patches of excess electrons in the ionosphere, an atmospheric region starting about 80 km above Earth's surface that already contains ions and electrons.
- The 2014 geomagnetic storm was a result of two powerful Earth-directed CMEs. The storm initially produced patches of extra electrons in the ionosphere over northern Greenland, as usual. But just south of these patches, the scientists were surprised to find broad areas extending 500 to 1,000 km where the electrons were "almost vacuumed out," in the words of Per Høeg of the National Space Research Institute at the Technical University of Denmark, Lyngby. These areas remained depleted of electrons for several days.
- The electrons in the ionosphere normally reflect radio waves back to ground level, enabling long-distance radio communications. Both electron depletion and electron increases in this layer can possibly cause radio communications to fail, reduce the accuracy of GPS systems, damage satellites and harm electrical grids.
- "We don't know exactly what causes the depletion," Komjathy said. "One possible explanation is that electrons are recombining with positively charged ions until there are no excess electrons. There could also be redistribution — electrons being displaced and pushed away from the region, not only horizontally but vertically."
Figure 11: A solar eruption on Sept. 26, 2014, seen by NASA's Solar Dynamics Observatory. If erupted solar material reaches Earth, it can deplete the electrons in the upper atmosphere in some locations while adding electrons in others, disrupting communications either way (image credit: NASA)
• March 22, 2017: For 15 days starting on March 7, 2017, NASA's SDO (Solar Dynamics Observatory) returned visible light images of a yolk-like spotless sun. This is the longest stretch of spotlessness since the last solar minimum in April 2010, indicating the solar cycle is marching on toward the next minimum, which scientists predict will occur between 2019—2020. 15)
- The sun goes through a natural 11-year cycle marked by two extremes: solar maximum and solar minimum. Sunspots are dark regions of complex magnetic activity on the sun's surface, so the number of sunspots at any given time is used as an index for solar activity. Solar maximum is characterized by intense solar activity and the greatest sunspot number. Conversely, during solar minimum, the sun is least active and sunspot number is at its lowest. Solar activity, however, does not end as sunspot number decreases toward minimum. There are other sources of solar activity such as high-speed streams of solar material from coronal holes, which spark aurora and other space weather effects.
Figure 12: The SDO images here compare the sun on March 20, 2017, and February 27, 2014, during the last solar maximum when the sun sported numerous spots (image credit: NASA)
• November 3, 2016: On Oct. 19, 2016, operators instructed NASA's SDO spacecraft to look up and down and then side to side over the course of six hours, as if tracing a great plus sign in space. During this time, SDO produced some unusual data. Taken every 12 seconds, SDO images show the sun dodging in and out of the frame. SDO captured these images in extreme ultraviolet light, a type of light that is invisible to our eyes. Here, they are colorized in red. 16)
- SDO operators schedule this maneuver, one of a series of maneuvers that SDO completed on Oct. 27, 2016, twice a year to calibrate the spacecraft's instruments. Veering motions allow scientists to assess how light travels through SDO's instruments – whether light is reflected inside the instrument, for example – and how these instruments are changing over time.
- This particular maneuver is the EVE cruciform maneuver, designed to help SDO's Extreme ultraviolet Variability Experiment, or EVE, take accurate measurements of the sun's extreme UV emissions. EVE studies these emissions over time, so that we may better understand their role in influencing Earth's climate and local space environment.
Figure 13: Images from NASA's SDO during a routine EVE cruciform maneuver show the sun dodging in and out of the frame (image credit: NASA/GSFC, SDO, Joy Ng)
• October 20, 2016: While it often seems unvarying from our viewpoint on Earth, the sun is constantly changing. Material courses through not only the star itself, but throughout its expansive atmosphere. Understanding the dance of this charged gas is a key part of better understanding our sun – how it heats up its atmosphere, how it creates a steady flow of solar wind streaming outward in all directions, and how magnetic fields twist and turn to create regions that can explode in giant eruptions. Now, for the first time, researchers have tracked a particular kind of solar wave as it swept upward from the sun's surface through its atmosphere, adding to our understanding of how solar material travels throughout the sun. 17)
- Tracking solar waves like this provides a novel tool for scientists to study the atmosphere of the sun. The imagery of the journey also confirms existing ideas, helping to nail down the existence of a mechanism that moves energy – and therefore heat – into the sun's mysteriously-hot upper atmosphere, called the corona. A study on these results was published Oct. 11, 2016, in The Astrophysical Journal Letters. 18)
- "We see certain kinds of solar seismic waves channeling upwards into the lower atmosphere, called the chromosphere, and from there, into the corona," said Junwei Zhao, a solar scientist at Stanford University in Stanford, California, and lead author on the study. "This research gives us a new viewpoint to look at waves that can contribute to the energy of the atmosphere."
- The study makes use of the wealth of data captured by NASA's SDO (Solar Dynamics Observatory), NASA's IRIS (Interface Region Imaging Spectrograph), and the BBSO (Big Bear Solar Observatory) in Big Bear Lake, California. Together, these observatories watch the sun in 16 wavelengths of light that show the sun's surface and lower atmosphere. SDO alone captures 11 of these.
- "SDO takes images of the sun in many different wavelengths at a high time resolution," said Dean Pesnell, SDO project scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "That lets you see the frequencies of these waves – if you didn't have such rapid-fire images, you'd lose track of the waves from one image to the next."
- Though scientists have long suspected that the waves they spot in the sun's surface, called the photosphere, are linked to those seen in the lowest reaches of the sun's atmosphere, called the chromosphere, this new analysis is the first time that scientists have managed to actually watch the wave travel up through the various layers into the sun's atmosphere.
- When material is heated to high temperatures, it releases energy in the form of light. The type, or wavelength, of that light is determined by what the material is, as well as its temperature. That means different wavelengths from the sun can be mapped to different temperatures of solar material. Since we know how the sun's temperature changes throughout the layers of its atmosphere, we can then order these wavelengths according to their height above the surface – and essentially watch solar waves as they travel upwards.
- The implications of this study are twofold – first, this technique for watching the waves itself gives scientists a new tool to understand the sun's lower atmosphere. "Watching the waves move upwards tells us a lot about the properties of the atmosphere above sunspots – like temperature, pressure, and density," said Ruizhu Chen, a graduate student scientist at Stanford who is an author on the study. "More importantly, we can figure out the magnetic field strength and direction."
- The effect of the magnetic field on these waves is pronounced. Instead of traveling straight upwards through the sun, the waves veer off, taking a curved path through the atmosphere. "The magnetic field is acting like railroad tracks, guiding the waves as they move up through the atmosphere," said Pesnell, who was not involved in this study.
- The second implication of this new research is for a long-standing question in solar physics – the coronal heating problem. -The sun produces energy by fusing hydrogen at its core, so the simplest models suggest that each layer of the sun should be cooler as you move outward. However, the sun's atmosphere, called the corona, is about a hundred times hotter than the region below – counter to what you would expect.
- No one has as-yet been able to definitively pinpoint the source of all the extra heat in the corona, but these waves may play a small role. "When a wave travels upwards, a number of different things can happen," said Zhao. "Some may reflect back downwards, or contribute to heating – but by how much, we don't yet know."
Figure 14: Scientists analyzed sunspot images from a trio of observatories — including the Big Bear Solar Observatory, which captured this footage — to make the first-ever observations of a solar wave traveling up into the sun's atmosphere from a sunspot (image credit: BBSO/Zhao et al)
Figure 15: Scientists used data from NASA's SDO, NASA's IRIS, and the BBSO to track a solar wave as it channeled upwards from the sun's surface into the atmosphere (image credit: Zhao et al/NASA/SDO/IRIS/BBSO)
• April 19, 2016: Solar flares are intense bursts of light from the sun. They are created when complicated magnetic fields suddenly and explosively rearrange themselves, converting magnetic energy into light through a process called magnetic reconnection – at least, that's the theory, because the signatures of this process are hard to detect. But during a December 2013 solar flare, three solar observatories captured the most comprehensive observations of an electromagnetic phenomenon called a current sheet, strengthening the evidence that this understanding of solar flares is correct. 19)
- These eruptions on the sun eject radiation in all directions. The strongest solar flares can impact the ionized part of Earth's atmosphere – the ionosphere – and interfere with our communications systems, like radio and GPS, and also disrupt onboard satellite electronics. Additionally, high-energy particles – including electrons, protons and heavier ions – are accelerated by solar flares.
- Unlike other space weather events, solar flares travel at the speed of light, meaning we get no warning that they're coming. So scientists want to pin down the processes that create solar flares – and even some day predict them before our communications can be interrupted.
- "The existence of a current sheet is crucial in all our models of solar flares," said James McAteer, an astrophysicist at New Mexico State University in Las Cruces and an author of a study on the December 2013 event, published on April 19, 2016, in the Astrophysical Journal Letters. "So these observations make us much more comfortable that our models are good."
- And better models lead to better forecasting, said Michael Kirk, a space scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, who was not involved in the study. "These complementary observations allowed unprecedented measurements of magnetic reconnection in three dimensions," Kirk said. "This will help refine how we model and predict the evolution of solar flares."
Looking at Current Sheets:
- A current sheet is a very fast, very flat flow of electrically-charged material, defined in part by its extreme thinness compared to its length and width. Current sheets form when two oppositely-aligned magnetic fields come in close contact, creating very high magnetic pressure. Electric current flowing through this high-pressure area is squeezed, compressing it down to a very fast and thin sheet. It's a bit like putting your thumb over the opening of a water hose – the water, or, in this case, the electrical current, is forced out of a tiny opening much, much faster. This configuration of magnetic fields is unstable, meaning that the same conditions that create current sheets are also ripe for magnetic reconnection.
- "Magnetic reconnection happens at the interface of oppositely-aligned magnetic fields," said Chunming Zhu, a space scientist at New Mexico State University and lead author on the study. "The magnetic fields break and reconnect, leading to a transformation of the magnetic energy into heat and light, producing a solar flare."
- Because current sheets are so closely associated with magnetic reconnection, observing a current sheet in such detail backs up the idea that magnetic reconnection is the force behind solar flares. "You have to be watching at the right time, at the right angle, with the right instruments to see a current sheet," said McAteer. "It's hard to get all those ducks in a row."
- This isn't the first time scientists have observed a current sheet during a solar flare, but this study is unique in that several measurements of the current sheet – such as speed, temperature, density and size – were observed from more than one angle or derived from more than method.
- The multi-faceted view of the December 2013 flare was made possible by three solar-watching missions: NASA's SDO (Solar Dynamics Observatory) and STEREO (Solar and Terrestrial Relations Observatory) and Hinode (Solar-B), a collaboration between JAXA, NASA, UKSA and ESA.
- Even when scientists think they've spotted something that might be a current sheet in solar data, they can't be certain without ticking off a long list of attributes. Since this current sheet was so well-observed, the team was able to confirm that its temperature, density, and size over the course of the event were consistent with a current sheet.
Figure 16: During a December 2013 solar flare, three NASA missions observed a current sheet form – a strong clue for explaining what initiates the flares. This animation shows four views of the flare from NASA's Solar Dynamics Observatory, NASA's Solar and Terrestrial Relations Observatory, and JAXA/NASA's Hinode, allowing scientists to make unprecedented measurements of its characteristics. The current sheet is a long, thin structure, especially visible in the views on the left. Those two animations depict light emitted by material with higher temperatures, so they better show the extremely hot current sheet (image credit: NASA/JAXA/SDO/STEREO/Hinode, courtesy Zhu, et al.)
• Nov. 18, 2015: An international team of researchers, led by Queen's University Belfast (Northern Ireland), has devised a high-precision method of examining magnetic fields in the Sun's atmosphere, representing a significant leap forward in the investigation of solar flares and potentially catastrophic 'space weather'. 20)
- The technique pioneered by the Queen's-led team, published in the journal Nature Physics, will feed into the DKIST (Daniel K Inouye Solar Telescope) project (which will be the largest solar telescope in the world when construction is finished in 2019 on the Pacific island of Maui), as well as allowing greater advance warning of potentially devastating space storms. The new technique allows changes in the Sun's magnetic fields, which drive the initiation of solar flares, to be monitored up to ten times faster than previous methods. 21)
- The Queen's-led team, who span universities in Europe, the Asia-Pacific region and the USA, harnessed data from both NASA's premier space-based telescope, SDO (Solar Dynamics Observatory), and the ROSA (Rapid Oscillations in the Solar Atmosphere) multi-camera system (ground-based), which was designed at Queen's University Belfast, using detectors made by Northern Ireland company Andor Technology.
- Lead researcher David Jess from Queen's Astrophysics Research Center said: "Continual outbursts from our Sun, in the form of solar flares and associated space weather, represent the potentially destructive nature of our nearest star. Our new techniques demonstrate a novel way of probing the Sun's outermost magnetic fields, providing scientists worldwide with a new approach to examine, and ultimately understand, the precursors responsible for destructive space weather.
Specific research results include:
1) The datasets used provided unprecedented images of all layers of the Sun's tenuous atmosphere, allowing the team to piece the jigsaw puzzle together of how magnetic fields permeate the dynamic atmosphere. Images captured by NASA's SDO and STEREO spacecrafts provided million-degree vantage points of how these magnetic fields stretch far out into the Sun's corona (the region of the Sun's atmosphere visible during total solar eclipses).
2) Waves propagated along magnetic fields, similar to how sound waves travel through the air on Earth. The speed at which these waves can travel is governed by the characteristics of the Sun's atmosphere, including its temperature and the strength of its magnetic field. The waves were found to propagate with speeds approaching half a million (500,000) mph, and when coupled with temperatures of around 1,000,000 degrees in the Sun's outer atmosphere, the researchers were able to determine the magnetic field strengths to a high degree of precision.
3) The strength of the magnetic fields decreases by a factor of 100 as they travel from the surface of the Sun out into the tenuous, hot corona. While the magnetic fields have decreased in strength, they still possess immense energy that can twist and shear, ultimately releasing huge blasts towards Earth in the form of solar flares. The team's methods provide a much faster way of examining magnetic field changes in the lead up to solar flares, which can ultimately be used to provide advanced warning against such violent space weather.
Figure 17: Left: An image of our sun taken by NASA's SDO, showing million degree plasma being channeled into loop-like shapes by the immense magnetic fields. Right: A zoom-in of the highly magnetic region of the sun's corona studied by David Jess and colleagues from Queen's University Belfast, Northern Ireland (image credit: Queen's University Belfast)
• On Sept. 13, 2015, as NASA's SDO kept up its constant watch on the sun, its view was photobombed not once, but twice (Figure 18). Just as the moon came into SDO's field of view on a path to cross the sun, Earth entered the picture, blocking SDO's view completely. When SDO's view of the sun emerged from Earth's shadow, the moon was just completing its journey across the sun's face. 22)
- Though SDO sees dozens of Earth eclipses and several lunar transits each year, this is the first time ever that the two have coincided. This alignment of the sun, moon and Earth also resulted in a partial solar eclipse on Sept. 13, visible only from parts of Africa and Antarctica.
- SDO's orbit usually gives us unobstructed views of the sun, but Earth's revolution around the sun means that SDO's orbit passes behind Earth twice each year, for two to three weeks at a time. During these phases, Earth blocks SDO's view of the sun for anywhere from a few minutes to over an hour once each day.
Figure 18: SDO captured this image of Earth and the moon transiting the sun together on Sept. 13, 2015. The edge of Earth, visible near the top of the frame, appears fuzzy because Earth's atmosphere blocks different amounts of light at different altitudes. On the left, the moon's edge is perfectly crisp, because it has no atmosphere. This image was taken in extreme ultraviolet wavelengths of 171Å. Though this light is invisible to our eyes, it is typically colorized in gold (image credit: NASA, SDO)
• August 31, 2015: As navigators know, Earth's magnetic axis is tilted with respect to its rotation axis. Such misalignment had not been expected in the Sun — but now it's been seen. NASA's SDO (Solar Dynamics Observatory) has been trained on the Sun for half of the current 11-year cycle of solar activity. Using SDO's HMI (Helioseismic and Magnetic Imager), Adur Pastor Yabar of the Institute of Astrophysics of the Canary Islands and his colleagues created daily maps of the line-of-sight strength and polarity of the Sun's magnetic field for each of the mission's first 1700 days. The Sun's rotation period varies with latitude. It's 25.5 days at the equator and 34.4 days at the poles. 23) 24)
- To look for variations not associated with differential rotation, Pastor Yabar and his colleagues averaged each map over all longitudes in 1º-wide latitudinal belts. When the researchers Fourier-transformed the entire sequence of binned maps, they discovered that a more-or-less monthly oscillation showed up at every latitude on every day. Random sprouting of active regions that rotate in and out of view could conceivably account for the oscillation, but when the researchers excluded active regions, the oscillation persisted. Their latitudinal belts did not sample the same parts of the Sun, as they had assumed, but wobbled up and down over the solar disk—hence the oscillation. Based on that and other lines of evidence, Pastor Yabar and his colleagues concluded that the Sun's magnetic and rotational axes must be misaligned. Dynamo models that presume alignment will require modification.
• July 2015: The SDO (Solar Dynamics Observatory) mission is approved by NASA for its first extended mission. NASA will work with the mission to address the findings that were raised by the Senior Review panel, to allow the mission to continue its successful operations. The SDO mission will be invited to the 2017 Heliophysics Senior Review. 25)
• June 2015: The SDO instruments provide a combination of imaging and spectral data that sample the Sun from its interior to the outer corona. These instruments provide a cadence and uniform quality of data that well characterize the photospheric magnetic fields (both longitudinal and vector), represent the thermal structure in the chromosphere and corona, allow the probing of different depths in the solar convection zone and summarize the broad UV spectral irradiance changes due to both long-term and very short-term solar activity variation. High data quality and uniformity combined with physically-appropriate time cadence and full disk coverage make the SDO data indispensable for studies of large-scale connectivity in the solar atmosphere. The general availability of many SDO data products with minimal latency allows for complementary usage in the context of space weather situational awareness and forecasting efforts. Some SDO data will also be used to develop important synergy between solar and stellar astrophysics. The science impact from this complex mission has been major. Recent science results cited from the SDO prime mission phase include breakthroughs in coronal seismology, developing a better understanding of evolution of magnetic fields in solar upper atmosphere decoding the importance of helicity and global connectivity to the eruption of magnetic systems, and establishing the hierarchy of organized flows in the convection zone. 26)
- SDO spacecraft / instrument health and status: With one exception, all instruments are in a good health and expected continue taking a good quality of data. EVE/MEGS-A has failed, and there are no expectations for this instrument to provide further solar irradiance data in the 5–38 nm band. HMI (Helioseismic Magnetic Imager) reboots of the flight software and anomalous upsets have not affected the science data significantly due to their low occurrence rate. All SDO spacecraft subsystems are operating nominally with two exceptions, neither of which interferes with SDO's ability to continue normal operations for at least five more years.
- SDO is a valuable asset to the HSO (Heliophysics System Observatory). It provides a breadth of observational information that will enable cutting-edge investigations; the data are presently used by many NASA Heliophysics missions, as well as for near-real-time space-weather forecasting by US and international agencies.
• May 6, 2015: The sun emitted a significant solar flare, an X2.7 class flare, peaking at 22:11 UTC on May 5, 2015. NASA's SDO (Solar Dynamics Observatory), which watches the sun constantly, captured an image of the event. This flare was also accompanied by a coronal mass ejection that blasted a mass of plasma into space. Both are triggered by disruptions of areas of intense magnetic fields. Solar flares are powerful bursts of radiation. Harmful radiation from a flare cannot pass through Earth's atmosphere to physically affect humans on the ground, however — when intense enough — they can disturb the atmosphere in the layer where GPS and communications signals travel. 27)
Legend to Figure 19: Each image shows a different wavelength of extreme ultraviolet light that highlights a different temperature of material on the sun. By comparing different images, scientists can better understand the movement of solar matter and energy during a flare. From left to right, the wavelengths are: visible light, 171 Å, 304 Å, 193 Å and 131 Å. Each wavelength has been colorized.
• In early May 2015, the SDO mission is providing 5 years of science data to the scientists and the public. The project has been watching Solar Cycle 24 rise to solar maximum, storing about 7 PByte of data, releasing almost 200 million images, and having about 1900 scientific papers published. 28)
• On March 11, 2015, an X2-class fare erupted. The bright flash of the flare was followed by streams of dark ejecta that move across the sun to the left. The flare did cause some radio blackouts on Earth when it disturbed our ionosphere. The flare was also associated with a coronal mass ejection. The images were taken in extreme ultraviolet light. 29)
Figure 20: A strong flare X2-class flare erupted on March 11, 2015 into space from an active region that was roughly facing towards Earth (image credit: NASA)
• February 11, 2015 marked the 5th anniversary of SDO on orbit. The mission is providing very detailed images of the Earth-facing side of the sun 24 hours a day. NASA released two videos on this occasion (links in Ref. 30) showcasing highlights from the last five years of sun watching. The first is a time lapse of the past five years. Different colors represent different wavelengths of extreme ultraviolet light, ultraviolet light, and visible light, which in turn correspond to solar material at different temperatures. Additionally SDO returns solar magnetic field data that helps scientists study solar activity. 30)
- The second video showcases highlights from the last five years. Watch the movie to see giant clouds of solar material hurled out into space, the dance of giant loops hovering in the corona, and huge sunspots growing and shrinking on the sun's surface.
- Five years into its mission, SDO continues to send back tantalizing imagery to incite scientists' curiosity. For example, in late 2014, SDO captured imagery of the largest sun spots seen since 1995 as well as a torrent of intense solar flares. Solar flares are bursts of light, energy and X-rays. They can occur by themselves or can be accompanied by what's called a coronal mass ejection, or CME, in which a giant cloud of solar material erupts off the sun, achieves escape velocity and heads off into space. In this case, the sun produced only flares and no CMEs, which, while not unheard of, is somewhat unusual for flares of that size. Scientists are looking at that data now to see if they can determine what circumstances might have led to flares eruptions alone.
• On January 19, 2015, the AIA (Atmospheric Imaging Assembly) instrument on SDO captured its 100 millionth image of the sun. AIA uses four telescopes working in parallel to gather eight images of the sun – cycling through 10 different wavelengths — every 12 seconds. The SDO downloads ~1.5 TB of data per day. AIA is responsible for about half of that. Every day it provides 57,600 detailed images of the sun that show the dance of how solar material sways and sometimes erupts in the solar atmosphere, the corona. 31)
Legend to Figure 21: The dark areas at the bottom and the top of the image are coronal holes — areas of less dense gas, where solar material has flowed away from the sun.
• June 10, 2014: The sun emitted a significant solar flare, peaking at 11:42 UTC on June 10, 2014 (X 2.2 flare). NASA's SDO (Solar Dynamics Observatory) – which typically observes the entire sun 24 hours a day - captured images of the flare. The sun released a second X-class flare (X 1.5), peaking at 12:52 UTC. 32) On June 11, 2014, the active region (AR2087) blazed again with a third flare at 9:05 UTC (X 1.0 class flare).
Figure 22: A solar flare bursts off the left limb of the sun in this image captured by SDO on June 10, 2014, at 11.42 UTC. This is classified as an X 2.2 flare, shown in a blend of two wavelengths of light: 171 and 131 Ä, colorized in gold and red, respectively (image credit: NASA, SDO)
• April 2014: New research that uses data from SDO, to track bright points in the solar atmosphere and magnetic signatures on the sun's surface, offers a way to probe the star's depths faster than ever before. The technique opens the door for near real-time mapping of the sun's roiling interior – movement that affects a wide range of events on the sun from its 22-year sunspot cycle to its frequent bursts of X-ray radiation called solar flares. 33) 34)
Figure 23: Brightpoints in the sun's atmosphere, left, correspond to magnetic parcels on the sun's surface, seen in the processed data on the right. Green spots show smaller parcels, red and yellow much bigger ones. Images based on data from NASA's SDO captured at 15:00 GMT on May 15, 2010 (image credit: NASA)
- One of the most common ways to probe the sun's interior is through a technique called helioseismology in which scientists track the time it takes for waves – not unlike seismic waves on Earth — to travel from one side of the sun to the other. From helioseismology solar scientists have some sense of what's happening inside the sun, which they believe to be made up of granules and super-granules of moving solar material. The material is constantly overturning like boiling water in a pot, but on a much grander scale: A granule is approximately the distance from Los Angeles to New York City; a super-granule is about twice the diameter of Earth.
- Instead of tracking seismic waves, the new research probes the solar interior using the HMI (Helioseismic Magnetic Imager) on SDO, which can map the dynamic magnetic fields that thread through and around the sun. Since 2010, Scott McIntosh has tracked the size of different magnetically-balanced areas on the sun, that is, areas where there are an even number of magnetic fields pointing down in toward the sun as pointing out. Think of it like looking down at a city from above with a technology that observed people, but not walls, and recording areas that have an even number of men and women. Even without seeing the buildings, you'd naturally get a sense for the size of rooms, houses, buildings, and whole city blocks – the structures in which people naturally group.
- The team found that the magnetic parcels they mapped corresponded to the size of granules and supergranules, but they also spotted areas much larger than those previously noted — about the diameter of Jupiter. It's as if when searching for those pairs of men and women, one suddenly realized that the city itself and the sprawling suburbs was another scale worth paying attention to. The scientists believe these areas correlate to even larger cells of flowing material inside the sun.
- The researchers also looked at these regions in SDO imagery of the sun's atmosphere, the corona, using the AIA (Atmospheric Imaging Assembly) instrument. They noticed that ubiquitous spots of extreme ultraviolet and X-ray light, known as brightpoints, prefer to hover around the vertices of these large areas, dubbed g-nodes.
- By opening up a way to peer inside the sun quickly, these techniques could provide a straightforward way to map the sun's interior and perhaps even improve our ability to forecast changes in magnetic fields that can lead to solar eruptions.
• On March 29, 2014, an X-class flare (CME) erupted from the right side of the sun... and vaulted into history as the best-observed flare of all time. The flare was witnessed by four different NASA spacecraft and one ground-based observatory — three of which had been fortuitously focused in on the correct spot as programmed into their viewing schedule a full day in advance. 35) 36)
Figure 24: This combined image shows the March 29, 2014, X-class flare as seen through the eyes of different observatories. SDO is on the bottom/left, which helps show the position of the flare on the sun. The darker orange square is IRIS data. The red rectangular inset is from Sacramento Peak. The violet spots show the flare's footpoints from RHESSI (image credit: NASA/GSFC)
The telescopes involved were:
- NASA's IRIS (Interface Region Imaging Spectrograph)
- NASA's SDO (Solar Dynamics Observatory)
- NASA's RHESSI (Reuven Ramaty High Energy Solar Spectroscopic Imager)
- JAXA's Solar-B/Hinode Observatory
- National Solar Observatory's Dunn Solar Telescope located at Sacramento Peak in New Mexico.
Numerous other spacecraft provided additional data about what was happening on the sun during the event and what the effects were at Earth.
- NASA's STEREO mission and the SOHO mission of ESA and NASA both watched the great cloud of solar material that erupted off the sun with the flare.
- NOAA's GOES satellite tracked X-rays from the flare, and other spacecraft measured the effects of the flare as it came toward Earth.
This event was particularly exciting for the IRIS team, as this was the first X-class flare ever observed by IRIS (launched on June 28, 2013). IRIS provided scientists with the first detailed view of what happens in this region during a flare.
Figure 25: A closeup of the sunspot at the root of the March 29, 2014, X-class flare taken by the HMI instrument of SDO (image credit: NASA)
Figure 26: EUV radiation streams out of an X-class solar flare as seen in this image captured on March 29, 2014 by the AIA instrument of SDO (image credit: NASA, Ref. 36)
• On January 30, 2014, the SDO got its own private solar eclipse showing from its geosynchronous orbital perch. Twice a year during new phase, the moon glides in front of the sun from the observatory's perspective. The events are called lunar transits rather than eclipses since they're seen from outer space. Transits typically last about a half hour, but at 2.5 hours, today's was one of the longest ever recorded. The next one occurs on July 26, 2014. 37)
Legend to Figure 27: The times of each photo are given in CST (Central Standard Time). In the last frame, the moon is silhouetted against the solar corona. At maximum about 90% of the sun was covered.
• January 07, 2014: An enormous sunspot, labeled AR1944 (Active Region 1944), slipped into view over the sun's left horizon late on Jan. 1, 2014. The sunspot steadily moved toward the right, along with the rotation of the sun, and now sits almost dead center, as seen in the image (Figure 28) from NASA's Solar Dynamics Observatory.
Sunspots are dark areas on the sun's surface that contain complex arrangements of strong magnetic fields that are constantly shifting. The largest dark spot in this configuration is approximately two Earths wide, and the entire sunspot group is some seven Earths across. 38)
Sunspots are part of what's known as active regions, which also include regions of the sun's atmosphere, the corona, hovering above the sunspots. Active regions can be the source of some of the sun's great explosions: solar flares that send out giant bursts of light and radiation due to the release of magnetic energy, or coronal mass ejections that send huge clouds of solar material out into space. As the sunspot group continues its journey across the face of the sun, scientists will watch how it changes and evolves to learn more about how these convoluted magnetic fields can cause space weather events that can affect spaceborne systems and technological infrastructure on Earth.
- On January 7, 2014 (18:18:34 UTC), as the giant AR1944 sunspot turned toward Earth, it erupted with a powerful X1.2-class flare.
Figure 28: One of the largest sunspots in the last nine years, labeled AR1944, was seen in early January 2014, as captured by NASA's SDO. An image of Earth has been added for scale (image credit: NASA)
• Nov. 9, 2013: The Sun is currently acting like it's in solar maximum. Our Sun has emitted dozens of solar flares in since Oct. 23, 2013, with at least six big X-class flares.
Figure 29: The AIA instrument of SDO captured this image of the sun showing an X1.1 class flare on Nov. 8, 2013 (image credit: NASA)
• On October 21, 2013, NASA successfully launched a Black Brant IX sounding rocket at 10:00 hours UTC from the White Sands Missile Range, N.M., carrying instrumentation to support the calibration of the EVE (EUV Variability Experiment) aboard SDO. EVE measures the total extreme ultraviolet output of the sun, called its irradiance. 39)
As part of the planned SDO/EVE program, the rocket calibration flight occurs about once a year to accurately determine the long-term variations of the solar extreme ultraviolet irradiance. This kind of calibration is known as an under-flight. It uses a near-replica of the SDO/EVE instrument to gather a calibrated sounding rocket observation in coordination with the orbital satellite's observations.
Comparison of the two data sets then validates the accuracy of the SDO/EVE data, providing crucial calibration of any long-term changes in the orbital instrumentation. This was the fourth under-flight calibration for the EVE instrument. The previous flight was successfully conducted on June 23, 2012.
• May 15, 2013: According to "SpaceWeather.com," AR1748 (sunspot active region 1748) has produced "the strongest flares of the year so far, and they signal a significant increase in solar activity." In only two days, sunspot AR1748 has produced four X-flares. The latest X-flare from this active sunspot occured on May 15th at 0152 UT. NASA's SDO (Solar Dynamics Observatory) captured the extreme ultraviolet flash. 40)
The AR1748 has produced an X1.7-class flare (0217 UT on May 13), an X2.8-class flare (1609 UT on May 13), an X3.2-class flare (0117 UT on May 14), and an X1-class flare (0152 on May 15). These are the strongest flares of the year, and they signal a significant increase in solar activity. 41)
Figure 30: An X3.2-class flare observed by SDO's AIA instrument at 0114 UT, 14 May 2013 (NASA/SDO/AIA)
Figure 31: SID (Sudden Ionospheric Disturbance) events on May 13, 2013 (station 21.75 kHz), image credit: Roberto Battaiola, Pantigliate, Milan, Italy (Ref. 41)
Legend to Figure 31: SID events make themselves known by the effect they have on low-frequency radio signals. When a SID passes by, the atmosphere overhead becomes a good reflector for radio waves, allowing signals to be received from distant transmitters. Battaiola monitored a faraway 21.75 kHz radio station to receive the SIDs over his location.
• On April 3, 2013 the SDO project performed an HMI roll maneuver. The entire SDO spacecraft is spun around so that HMI can verify its operation and measure some calibration data. 42)
- Twice a year, SDO performs a 360º roll maneuver about the axis on which it points toward the Sun. This produces some unique views; the rolls are necessary to help calibrate the instruments, particularly the HMI instrument, which is making precise measurements of the solar limb to study the shape of the Sun. The rolls also help the science teams to know how accurately the images are aligned with solar north. 43)
• The SDO mission and its payload are operating nominally in 2013. On Feb. 11, 2013, SDO was 3 years on orbit providing an enornous amount of information. 44)
Figure 32: SDO track of the rising level of solar activity as the sun ascends toward the peak of the latest 11-year sunspot cycle (image credit: NASA) 45)
Legend to Figure 32: These six images from SDO, chosen to show a representative image about every six months, track the rising level of solar activity since the mission first began to produce consistent images in May, 2010. The period of solar maximum is expected in 2013. The images were taken in the 171 Angstrom wavelength of extreme ultraviolet light.
• On April 4, 2012, the SDO spacecraft performed a 360º spin. It rolled completely about its axis– something it does twice a year . This maneuver helps the HMI (Helioseismic and Magnetic Imager ) instrument, one of three instruments onboard SDO, take measurements of the solar limb to study the shape of the sun. The roll helps scientists remove optical distortions from the images and to precisely determine the boundaries of the sun's horizon, or "limb". Accumulated over time, such data shows whether the sun's sphere changes in concert with the 11-year solar cycle, during which the sun moves through periods of greater and lesser activity as evidenced by the changing frequency of giant solar eruptions. 46)
• In 2012, the spacecraft and its payload are operating nominally. On Feb. 11, 2012, the SDO spacecraft was 2 years on orbit.
The SDO mission serves as a clear example of the importance of the systems engineering role across all phases of the mission development lifecycle and its contribution to mission success. The best metric to evaluate the success of this approach is in the successful launch and the on-orbit performance of the SDO mission itself, which, despite challenging technical drivers and development obstacles along way, is currently enabling ground-breaking science after only two years into its five-year mission life (Ref. 7).
Figure 33: AIA instrument image observed at 171 Ä showing the current conditions of the quiet corona and upper transition region of the Sun (image credit: NASA) 47)
• Sept. 2011 (the late phase of solar flares): Over the course of a year, the science team used the EVE (Extreme ultraviolet Variability Experiment) instrument on SDO to record data from many flares. EVE doesn't snap conventional images. T. Woods is the principal investigator for the EVE instrument and he explains that it collects all the light from the sun at once and then precisely separates each wavelength of light and measures its intensity. This doesn't produce pretty pictures the way other instruments on SDO do, but it provides graphs that map out how each wavelength of light gets stronger, peaks, and diminishes over time. EVE collects this data every 10 seconds, a rate guaranteed to provide brand new information about how the sun changes, given that previous instruments only measured such information every hour and a half or didn't look at all the wavelengths simultaneously – not nearly enough information to get a complete picture of the heating and cooling of the flare.
Recording extreme ultraviolet light, the EVE spectra showed four phases in an average flare's lifetime (Figure 34). The first three have been observed and are well established (though EVE was able to measure and quantify them over a wide range of light wavelengths better than has ever been done).
- The first phase is the hard X-ray impulsive phase, in which highly energetic particles in the sun's atmosphere rain down toward the sun's surface after an explosive event in the atmosphere known as magnetic reconnection. They fall freely for some seconds to minutes until they hit the denser lower atmosphere, and then the second phase, the gradual phase, begins.
- Second phase: Over the course of minutes to hours, the solar material, called plasma, is heated and explodes back up, tracing its way along giant magnetic loops, filling the loops with plasma. This process sends off so much light and radiation that it can be compared to millions of hydrogen bombs.
- The third phase is characterized by the sun's atmosphere — the corona — losing brightness, and so is known as the coronal dimming phase. This is often associated with what's known as a CME (Coronal Mass Ejection), in which a great cloud of plasma erupts off the surface of the sun.
But the fourth phase, the late phase flare, spotted by EVE was new. Anywhere from one to five hours later for several of the flares, they saw a second peak of warm coronal material that didn't correspond to another X-ray burst. The late phase turns out to be different, the emissions happen substantially later, and it happens after the main flare exhibits that initial peak. 48) 49)
To try to understand what was happening, the team looked at the images collected from SDO's AIA (Advanced Imaging Assembly) as well. They could see the main phase flare eruption in the images and also noticed a second set of coronal loops far above the original flare site. These extra loops were longer and become brighter later than the original set (or the post-flare loops that appeared just minutes after that). These loops were also physically set apart from those earlier ones.
Legend of Figure 34: This image shows a single moment from May 5, 2010. Instead of a conventional picture, the EVE produces graphs like this, called spectra, that show the total intensity of any given EUV wavelength of light coming off of the sun.
The intensity, the project recorded in those late phase flares, is usually dimmer than the X-ray intensity. But the late phase goes on much longer, sometimes for multiple hours, so it's putting out just as much total energy as the main flare that typically only lasts for a few minutes. Because this previously unrealized extra source of energy from the flare is equally important to impacting Earth's atmosphere, Woods and his colleagues are now studying how the late phase flares can influence space weather (Ref. 48).
• In 2011, the spacecraft and its payload are operating nominally.
Legend to Figure 35: The image captures a dark coronal hole just about at sun center. Coronal holes are areas of the sun's surface that are the source of open magnetic field lines that head way out into space. They are also the source regions of the fast solar wind, which is characterized by a relatively steady speed of approximately 800 km/s. As the sun continues to rotate, the high speed solar wind particles blowing from this hole will likely reach Earth in a few days and may spark some auroral activity. 50)
• On Aug.1, 2010, a most unusual solar event occurred. Nearly the entire Earth-facing side of the Sun erupted in a tumult of activity, comprising a large solar flare, a solar tsunami, multiple filaments of magnetism lifting off the solar surface, radio bursts and half a dozen coronal mass ejections (CMEs). At the same time, NASA's three solar spacecraft, SDO and the two STEREO spacecraft, were ideally positioned to capture both the action on the Earth-facing side of the Sun, and most activity around the backside, leaving a wedge of only 30 degrees of the solar surface unobserved. 51) 52)
Explosions on the sun are not localized or isolated events, according to Karel Schrijver and Alan Title of LMSAL (Lockheed Martin, Solar & Astrophysics Laboratory). Instead, solar activity is interconnected by magnetism over breathtaking distances. Solar flares, tsunamis, coronal mass ejections--they can go off all at once, hundreds of thousands of miles apart, in a dizzyingly-complex concert of mayhem.
For several decades, scientists studying the sun have observed solar flares that appear to occur almost simultaneously but originated in completely different areas on the Sun. Solar physicists called them "sympathetic" flares, but it was thought these near-synchronous explosions in the solar atmosphere were too far apart – sometimes millions of kilometers distant – to be related. But now, with the continuous high-resolution and multi-wavelength observations with the SDO, combined with views from the twin STEREO spacecraft, the scientists are seeing how these sympathetic eruptions — sometimes on opposite sides of the sun — can connect through looping lines of the Sun's magnetic field. 53)
Figure 36: Locations of key events are labeled in this extreme ultraviolet image of the sun, obtained by the Solar Dynamics Observatory during the Great Eruption of Aug. 1, 2010. White lines trace the sun's magnetic field (image credit: Karel Schrijver and Alan Title of LMSAL)
• On May 14, 2010, SDO passed a major milestone when it completed its post-launch check out (end of commissioning phase) and officially began its five-year science mission to study the sun (phase E). The project at NASA/GSFC declared SDO an operational mission. All of the instruments and the spacecraft are performing extremely well. SDO is now sending 1.5 TB of data/day to Earth, and will continue to do so at least until the end of the prime phase of the mission in 2015. 54) 55)
The SDO has allowed scientists for the first time to comprehensively view the dynamic nature of storms on the sun. Solar storms have been recognized as a cause of technological problems on Earth since the invention of the telegraph in the 19th century. 56)
• On April 19, 2010, SDO observed a massive eruption on the sun — one of the biggest in years. Astronomers have seen eruptions like this before, but rarely so large and never in such fluid detail. Coronal rain has long been a mystery. It's not surprising that plasma should fall back to the sun. After all, the sun's gravity is powerful. The puzzle of coronal rain is how slowly it seems to fall. The rain appears to be buoyed by a 'cushion' of hot gas.
Using the AIA (Atmospheric Imaging Assembly) instrument with an array of ultraviolet telescopes, SDO can remotely measure the temperature of gas in the sun's atmosphere. Coronal rain turns out to be relatively cool—"only" 60,000 K. When the rains falls, it is supported, in part, by an underlying cushion of much hotter material, between 1,000,000 and 2,200,000 K. 57)
Figure 37: Coronal rain. Encircled are two plasma streamers, one hitting the sun's surface and another incoming behind it (image credit: NASA)
Legend to Figure 38: False colors trace different gas temperatures. Reds are relatively cool (~60,000 K); blues and greens are hotter (> 1,000,000 K). SDO is able to monitor not just one small patch of sun, but rather the whole thing--full disk, atmosphere, surface, and even interior. 58)
Figure 39: An erupting prominence observed by the AIA instrument on March 30, 2010 (image credit: NASA)
• Following several precise propulsion burns to circularize its orbit, SDO arrived "on station" on March 16, 2010. All systems are operating nominally.
Sensor complement: (HMI, AIA, EVE)
The SDO sensor complement consists of three instruments which are pointed toward the sun to provide continuous, high cadence (cyclic) observations of the full solar disk and coronal imaging in multiple wavelengths to improve the understanding and forecasting of the sun's impact on our terrestrial environment. 59) 60)
• HMI (Helioseismic and Magnetic Imager) measures the surface magnetic fields and the flows that distribute it on global and local solar scales. A study of the origins of solar variability using solar oscillations and the longitudinal photospheric magnetic field to characterize and understand the sun's interior and the various components of magnetic activity.
• AIA (Atmospheric Imaging Assembly) images the solar outer atmosphere. A study of coronal energy storage and release evidenced in rapidly evolving coronal structures over a broad temperature range that are intrinsically tied to the Sun's magnetic field and irradiance variations.
• EVE (EUV Variability Experiment), a spectrometer/spectrograph providing the solar full-disk distribution of the spectral irradiance in the EUV and UV ranges that cause variations in composition, density, and temperature of the Earth's ionosphere and thermosphere. A study of the sun's transient and steady state coronal plasma emissions that are driven by variations in the solar magnetic field.
HMI (Helioseismic and Magnetic Imager)
The HMI instrument is being developed at LMSAL (Lockheed Martin, Solar & Astrophysics Laboratory) in Palo Alto, CA (PI: P. Scherrer of Stanford University). HMI is a joint project of the Stanford University, Hansen Experimental Physics Laboratory, and LMSAL, with key contributions from the High Altitude Observatory of NCAR, and the HMI Science Team. The overall objective of HMI is to extend the capabilities of the SOHO/MDI (Michelson Doppler Imager) instrument with continuous full-disk coverage at considerably higher spatial and temporal resolution line-of-sight magnetograms with the optional channel for full Stokes polarization measurements [I = (I; Q; U; V)] and hence vector magnetogram determination (3-D imagery of the sun's interior employing a technique known as helioseismology, which maps the inside of the sun by measuring the velocity of low-frequency sound waves that ricochet below its surface). 61) 62) 63) 64) 65)
Note: Since the two instruments, HMI and AIA, are both being developed at LMSAL, there is a lot of organizational synergism and cooperation between the two instruments on all levels.
HMI makes interference measurements of the motion of the solar photosphere to study solar oscillations and measurements of the polarization in a spectral line to study all three components of the photospheric magnetic field. HMI produces data to determine the interior sources and mechanisms of solar variability and how the physical processes inside the sun are related to surface magnetic field and activity. It also produces data to enable estimates of the coronal magnetic field for studies of variability in the extended solar atmosphere. HMI observations will enable establishing the relationships between the internal dynamics and magnetic activity in order to understand solar variability and its effects, leading to reliable predictive capability, one of the key elements of the LWS (Living With a Star) program.
The HMI observation goals are being addressed in a coordinated investigation in a number of parallel studies:
• Convection-zone dynamics and the solar dynamo
• Origin and evolution of sunspots, active regions and complexes of activity
• Sources and drivers of solar activity and disturbances
• Links between the internal processes and dynamics of the corona and heliosphere
• Precursors of solar disturbances for space-weather forecasts.
HMI will observe the full solar disk in the Fe I absorption line at 6173 Å (goal of 1 arcsecond resolution). The HMI instrument will produce measurements in the form of filtergrams in a set of polarizations and spectral line positions at a regular cadence for the duration of the mission that meet these basic requirements:
4) Full-disk Doppler velocity and line-of-sight magnetic flux images with 1.5 arcsec resolution at least every 50 seconds
5) Full-disk vector magnetic images of the solar magnetic field with 1.5 arc-sec resolution at least every 10 minutes.
The primary observables (Dopplergrams, longitudinal and vector magnetograms, and continuum intensity images) will be constructed from the raw filtergrams and will be made available at full resolution and cadence. Other derived products such as subsurface flow maps, far-side activity maps, and coronal and solar wind models that require longer sequences of observations shall be produced and made available.
In effect the solar turbulence is analogous to earthquakes. In manner similar to how seismologists can learn about the interior of the Earth by studying the waves generated in an earthquake. HMI's helioseismologists learn about the structure, temperature and flows in the solar interior.
The HMI instrument consists of a refracting telescope, a polarization selector, an image stabilization system (ISS), a narrow-band tunable filter. In addition, there are two 4096 x 4096 pixel CCD cameras with mechanical shutters and control electronics. The twin cameras of HMI operate independently. One is referred to as the "Doppler camera"; the objective is to measure the line-of-sight component of the magnetic field and velocity vectors. The second camera is referred to as "Magnetic camera"; the objective is to measure the vector magnetic field and line of sight velocities. 66)
The optics package consists of the following elements:
- Telescope section
- Polarization selectors - 3 rotating waveplates for redundancy
- Focus blocks
- ISS (Image Stabilization System)
- 5 element Lyot filter. One element tuned by rotating waveplate
- 2 tunable Michelson interferometers. 2 waveplates and 1 polarizer for redundancy
- Reimaging optics and beam distribution system
- 2 functionally identical CCD cameras - "Doppler" and "Magnetic"
The combined Lyot-Michelson filter system in HMI produces a transmission profile with a FWHM of 76 mÅ. The tuning positions are 69 mÅ apart from each other.
Figure 40: Principal optics package components of the HMI instrument (image credit: Stanford University)
Figure 41: Photo of the HMI instrument (image credit: NASA)
Figure 42: Optical layout of the HMI instrument (image credit: Stanford University)
Table 2: Overview of HMI observation requirements
PCU (Polarization Calibration Unit):
HMI polarization calibration requires the input of fixed polarization states into the instrument and the measurement of the observed parameters with the HMI. The PCU creates the polarization states by using a linear polarizer and retarder (wave plate) that can be inserted into the optical path and rotated independently. The PCU consists of a TCP/IP control interface (Newport XPS-C4) and two mechanical units (size: 787 mm x 508 mm x 203 mm), with 175 mm clear apertures that house the polarization optics. Each mechanical unit contains a linear and a rotational stage. The linear stages (Newport IMS300CC) move the polarization optics into and out of the optical path with a linear position resolution of 1.25 microns. The rotational stages (Newport RV240CC) move the calibration optics to any given angle with a resolution of 0.001º. 67)
Figure 43: HMI accommodation on SDO (image credit: Stanford University)
Figure 44: Functional block diagram of the HMI (image credit: Stanford University)
AIA (Atmospheric Imaging Assembly):
The AIA instrument is being designed and developed at LMSAL (Lockheed Martin Solar and Astrophysics Laboratory), Palo Alto, CA; (PI: Alan Title, LMSAL). The AIA science team includes scientists and engineers from many national and international institutions. The SAO (Smithsonian Astrophysical Observatory) has a major role in the AIA program.
The objective is to provide an unprecedented view of the solar corona, taking images that span at least 1.3 solar diameters in multiple wavelengths nearly simultaneously, at a resolution of about 1 arcsec and at a cadence of 10 seconds or better. The primary goal of the AIA science investigation is to use these data, together with data from other SDO instruments, as well as from other observatories, to significantly improve our understanding of the physics behind the activity displayed by the sun's atmosphere, which drives space weather in the heliosphere and in planetary environments. 68) 69)
Themes of the AIA Investigation
1) Energy input, storage, and release: the 3-D dynamic coronal structure.
2) Coronal heating and irradiance:thermal structure and emission.
3) Transients: sources of radiation and energetic particles
4) Connections to geospace: material and magnetic field output of the sun
5) Coronal seismology: a new diagnostic to access coronal physics
Table 3: Overview of AIA observation requirements for various science themes
AIA instrument design overview:
• Four ST (Science Telescopes), each with 8 science channels
- 7 EUV channels in a sequence of Fe line and He 304 Å
- 1 UV channel with CTN, 1600 Å, 1700 Å filters
• Active secondaries for image stabilization. Each ST is equipped with an ISS (Image Stabilization System)
• Four GT (Guide Telescopes)
• Four 4096 x 4096 pixel thinned back-illuminated CCDs (the sampling of 0.6 arcsec requires a 4096 x 4096 pixel detector). Note, the AIA and HMI CCDs: a 4096 x 4096 pixel science-grade CCD with 12 µm pixel pitch developed by ev2 and RAL, are currently the largest CCD to have ever flown in a space mission (Ref. 66).
• Full CCD readout in 2.5 seconds
• Reconfiguration of all mechanisms in 1 second (filter wheels, sector shutter, focal plane shutters)
• Onboard data compression via several lookup tables
The AIA design provides the following instrument capabilities:
• Seven EUV (Extreme Ultraviolet) and three UV/visible channels. Four of the EUV wavelength bands open new perspectives on the solar corona, having never been imaged or imaged only during brief rocket flights. The set of six EUV channels that observe ionized iron allow the construction of relatively narrow-band temperature maps of the solar corona from below 1 MK to above 20 MK.
• A field of view (FOV) exceeding 41 arcmin (or 1.28 solar radii in the EW and NS directions), with 0.6 arcsec pixels
• A detector full well > 150,000 electrons and ~ 15 e/photon, with a camera readout noise of ≤ 25 electrons
• A sustained 10 second cadence during most of the mission
• A capability to adjust the observing program to changing solar conditions in order to implement observing programs that are optimized to meet the requirements of specific scientific objectives. This allows, for example, a 2 second cadence in a reduced field of view for flare studies.
• Provision of images in multiple EUV and UV pass bands. The basic observables are full-sun intensities at a range of wavelengths. Together, these will comprise the data archive, which is freely accessible to the research community and, with limitations dictated by resources, to other interested parties.
Derived data products, such as coronal thermal charts, maps of variability, and comparisons to HMI magnetograms and to (non-)potential field extrapolations will be made available regularly through the data-processing pipeline for a subset of the data for use in evaluation of the data and to aid the discovery of phenomena and cataloging of events. Software will be made available to researchers to create these data products for other datasets; a core library of easy-to-use, publicly-available software will be developed as part of the SolarSoft IDL environment to enable and support the investigations that are required to meet the primary AIA science goals
Table 4: Definition of AIA instrument spectral bands
Table 5: AIA instrument design characteristics
Figure 46: Illustration of a single AIA science telescope with quad selector (image credit: LMSAL)
Figure 47: AIA science telescope assembly (image: credit: LMSAL)
Figure 48: Optical layout of the AIA science telescope (image credit: LMSAL)
Figure 49: AIA telescope array mounted on IM (Instrument Module), image credit: LMSAL
AIA camera systems:
• The camera systems with CCD detectors are key elements of HMI & AIA. The HMI and AIA instrument use identical cameras and CCDs except that the AIA CCDs are back-side thinned.
• Each CCD detector array has a size of 4096 x 4096 pixels with 12 µm pixels (they were provided by e2v technologies ltd., Chelmsford, Essex, UK)
• CEB (Camera Electronics Box): 8 Mpixel/s via 2 Mpixel/s from 4 ports simultaneously
The AIA instrument has a data rate allocation of 67 Mbit/s (max, using data compression). The data is communicated over the IEEE 1355 high-rate science data bus (SpaceWire).
Camera readout electronics: Each AIA and HMI CCD (Figure 45) is driven and read out through its own dedicated CEB (Camera Electronics Box). It has dimensions of 152 mm x 131 mm x 95 mm and a mass of 2.9 kg. The enclosure walls are 5 mm thick aluminum to ensure sufficient attenuation of space radiation over mission life. During exposures the CCD and CEB consumes 12 W rising to 17 W during readout. The CEB contains four electronics cards mounted above a separately screened input filter and DC-DC power converter. A photo of the assembled unit, minus front panel and lid, is reproduced in Figure 50 (Ref. 66).
The upper-most card carries four video processing and digitization ASICs operating in parallel at 2 Mpixel/s and each connected to one of the CCD's quadrant readout amplifiers. The second card in the stack provides all of the CCD's low-noise DC bias voltages. Supplies to each of the CCD's output amplifiers are buffered separately to minimize crosstalk between channels. An 8-channel 10-bit DAC ASIC enables software programming and fine adjustment of the bias supplies. Telemetry circuitry internal to the CEB allows monitoring of the CEB's secondary power rails, CCD bias voltages and the CCD and CEB operating temperatures. The third card carries a waveform generator and sequencer ASIC and sufficient clock driver buffers to enable CCD readout through any or all of its quadrant readout amplifiers. The final card provides a SpaceWire communications interface with the main AIA or HMI control electronics. A single link is used for programming the CEB's ASICs and registers, commanding a CCD readout and the return of the CCD's digitized video data at 200 Mbit/s.
A key component of the camera electronics is a custom-designed and space-qualified CCD video signal processing and digitization ASIC. It provides 2 Mpixel/s video amplification, CDS processing and 16 bit digitization of a 1 V input signal. The design is fully-differential to aid rejection of common-mode noise. A 10-bit DAC enables ± 500 mV of programmable DC offset to be introduced into the video signal and a 7-bit programmable x1-x3 gain amplifier enables the ADC to be matched to the required CCD signal swing. The ADC is a 16 bit fully-differential pipelined converter using feedback capacitor switching in the amplifier stages, and over-ranging at intervals in order to minimize differential non-linearity due to capacitor mismatching and amplifier gain errors. Triple-voting logic is used to enhance the single-event upset tolerance of the logic and registers. The ASIC was manufactured on a 0.35 µm 3.3 V CMOS process known for its excellent tolerance to ionizing radiation. With its inputs grounded, the ASIC's noise is 3.5 ADU rms in 16 bits or 53 µV rms. The CCD provides ~ 4.5 µV/ e- and so the equivalent noise is ~ 12 e- rms. The combined noise floor of the CCD and electronics is ~ 4 ADU rms or ~ 16 e- rms. The power consumption from a 3.3 V supply is 400 mW (Ref. 66).
Figure 51: Photo of the AIA telescope array (image credit: NASA)
EVE (EUV Variability Experiment)
The Extreme ultraviolet Variability Experiment (EVE) has been designed and developed at LASP (Laboratory for Atmospheric and Space Physics) of the University of Colorado (CU) at Boulder, CO (PI T. Woods). The science team consists of members from: CU/LASP, USC (university of Southern California), NRL (Naval Research Laboratory), MIT/LL (Massachusetts Institute of Technology/ Lincoln Laboratory), NOAA, and the University of Alaska, Utah State University. The objective is to measure the solar extreme ultraviolet (EUV) irradiance with unprecedented spectral resolution, temporal cadence, accuracy, and precision. Use of physics-based models of the solar EUV irradiance to advance the understanding of the solar EUV irradiance variations based on the activity of the solar magnetic features. 70) 71) 72) 73)
Specific EVE science objectives are:
1) Specify the solar EUV spectral irradiance and its variability on multiple time scales.
- EUV: 0-105 nm (0.1 nm resolution at >10 nm) and H I Lyman-á(121.6 nm)
- Time Scales: < 20 s cadence, continuous sequence
2) Advance current understanding of how and why the solar EUV spectral irradiance varies.
- Use AIA & HMI solar images to understand the interactions of the solar magnetic fields and the evolution of the solar features (e.g., plage, active network) and how these affect the solar EUV variations
3) Improve the capability to predict the EUV spectral irradiance variability
- Develop new forecast and nowcast models of the solar EUV irradiance for use in the NOAA space weather operations
4) Understand the response of the geospace environment to variations in the solar EUV spectral irradiance and the impact on human endeavors
- Use solar EUV irradiances with thermosphere and ionosphere models to better define the solar influences on Earth's atmosphere
- Input EVE solar data near real-time into NOAA operational atmospheric models to improve accuracy of solar storm warnings and satellite drag calculations and to predict better communication disruptions
The EVE measurement approach is to observe simultaneously the solar EUV irradiance with different instrument types (multiple subsystems and technology) to meet the wavelength, resolution, and accuracy requirements.
Table 6: Overview of EVE instrument modules and measurements
Table 7: EVE instrument parameters
Figure 52: Overview of the EVE instrument (image credit: CU/LASP)
The EVE instrument consists of the following elements/modules: MEGS, ESP, and EEB.
• MEGS (Multiple EUV Grating Spectrograph). A set of 2 Rowland-circle grating spectrographs that measure the 5-105 nm spectral irradiance with 0.1 nm spectral resolution and with 10 second cadence. The MEGS have laminar groove profile (50% duty cycle of grooves) to suppress even orders.
- MEGS-A uses single, holographic, spherical grating at 80º grazing incidence
- MEGS-B uses dual, holographic, spherical grating, used near normal incidence
- CCD array type of size: 1024 x 2048 pixels (CCID-28 devices of MIT/LL, heritage: flown on Chandra and XMM/Newton)
- Back-thinned, back-illuminated
- Passively cooled to -100º C
Figure 53: Cross-section of the MEGS optics system (image credit: CU/LASP)
- MEGS-A has two slits and two filters: Slit 1: Mo/C, 5-20 nm; Slit 2: Si, 17.0 -37.0 nm
- MEGS-B has one slit and no primary filter. Additional removable filters for higher order checks.
Table 8: Overview of MEGS-A parameters
Figure 54: Schematic view of the MEGS-A device (image credit: CU/LASP)
Table 9: Overview of MEGS-B parameters
Figure 55: Schematic view of the MEGS-B device (image credit: CU/LASP)
• MEGS-SAM (Multiple EUV Grating Spectrograph-Solar Aspect Monitor). The objective is to provide pulse height analysis of X-ray photons. The device provides also MEGS pointing information with precision of 9 arcseconds. MEGS-SAM has a wavelength coverage of 0.1 -7 nm with a spectral resolution of 0.01-1 nm, and a spatial resolution with 10 arcsec/pixel. Detector: pinhole illuminates the MEGS-A CCD.
Table 10: Overview of MEGS-SAM parameters
Figure 56: Schematic of the MEGS-SAM device (image credit: CU/LASP)
• MEGS-P: Photometer for Lyman-α H I 121.6 nm and He I 58.4 nm emissions.
- Technique: grating + filter photometer
- MEGS-P channels are located in MEGS-B entrance baffles, providing a resolution of 5 nm
- Detector: IRD Si photodiode
- Filter: Acton Lyman-α filter and Al/Sn foil filter
Table 11: Parameters of the MEGS-P device
Figure 57: Schematic of the MEGS-P device (image credit: CU/LASP)
• ESP (EUV Spectrophotometer): A transmission grating spectrograph with stable Si photodiodes to provide solar X-ray measurement short of 5 nm, calibrations for MEGS sensitivity changes and higher time cadence (0.25 s). The ESP is very similar to the SOHO SEM instrument. ESP is of SEM instrument heritage flown on SOHO and also of TIMED heritage.
Table 12: Parameters of the ESP device
Figure 58: Optical layout of the ESP instrument (image credit: CU/LASP)
• EEB (EVE Electronics Box): Electronics that control the MEGS and ESP instruments and provides an interface to/from the SDO spacecraft.
EVE data products:
• Near real-time space weather data product of the solar EUV irradiance for NOAA SEC operations
• High quality solar EUV irradiances on 10 s cadence and averaged over 1 day provided daily to EVE's archive and FTP distribution center.
SDO ground system:
Data reception and spacecraft commanding will be conducted via a dedicated and newly implemented ground station at White Sands, NM. The SDO ground system consists of five major elements: 74)
1) SDOGS (SDO Ground Station), located at White Sands, NM and co-located with the WSGT (White Sands Ground Terminal) for TDRS service support. Two dual-feed antennas of 18 m diameter (S-band and Ka-band) are being allocated for SDO science data acquisition and TT&C operations support. A major function of the DDS is to continuously receive the high-rate science telemetry from the SDOGS Ka-band system and to deliver the science data to the SOCs in near real-time.
2) DDS (Data Distribution System), located at White Sands, NM.
• Receives the science telemetry data, processes it into files and distributes them to the instrument teams in near-real-time
• Provides a short-term (30 day) storage capability and supports data retransmissions as needed
• Provides the remote monitor and control capabilities of the DDS and SDOGS, from the MOC
3) MOC (Missions Operations Center), located at GSFC
• Supports the conventional real-time TT&C functions, which allows the Flight Operations Team (FOT) to monitor the health and status of the observatory and to control its operations
• Provides mission planning, trending and analysis, remote control and monitoring of DDS and ground station functions, and flight dynamics functions, including attitude determination and control and orbit maneuver computations and execution.
4) SOC (Science Operations Center). The 3 SOCs are located at the PI home institutions:
• They provide real-time health and safety monitoring as well as the command function for the science instrument
• Provision of science mission planning
• Science data processing, analysis, archiving, and distribution to the user community
5) GRN (Ground Communications Network)
• Provides connectivity between each of the ground system elements supporting all levels of data exchange and voice communications for SDO mission operations.
- One Optical Carrier Level 3 (OC3) network to AIA (67 Mbit/s) from DDS
- One Optical Carrier Level 3 (OC3) network to HMI (55 Mbit/s) from DDS
- One T3 circuit to EVE (7 Mbit/s) from DDS
- TT&C data: Four T1 circuits from MOC to/from SDOGS for S-band housekeeping telemetry and commands, two per SDOGS antenna site (restore time is < 1minute).
Figure 59: Ka-band end-to-end data flow configuration (image credit: NASA)
Figure 60: S-band end-to-end data flow configuration (image credit: NASA)
Figure 61: Overview of the SDO ground system (image credit: NASA)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).