AIM (Aeronomy of Ice in the Mesosphere)
AIM is a minisatellite mission within NASA's SMEX (Small Explorer) program designed to provide frequent, low-cost access to space for a variety of missions (the AIM mission was selected in July 2002 with final approval in May 2004). The objective of AIM is to study the causes of Earth's highest-altitude clouds, which occur on the very edge of space. These clouds, referred to as PMCs (Polar Mesospheric Clouds), form in the coldest part of the atmosphere, about 50-90 km above the polar regions, every summer. 1)
PMCs are of special interest as they are sensitive to both global change and solar/terrestrial influences (study of the coupling between the heliosphere and the Earth's atmosphere). Recorded sightings of these silvery-blue, noctilucent or ”night-shining” clouds (NLCs) were first reported in 1885 at high latitudes. They have been increasing in frequency and extending to lower latitudes over the past four decades. They are called ”night shining” clouds by observers on the ground because their high altitude allows them to continue reflecting sunlight after the sun has set below the horizon.
The AIM mission will observe PMCs in their thermal, chemical and dynamic environment in which they form in order to determine the connection between PMCs and the meteorology. Specific parameters of the polar mesosphere to be measured are: PMC abundances, spatial distribution, particle size distributions, gravity wave activity, cosmic dust influx to the atmosphere and precise, vertical profile measurements of temperature, H2O, OH, CH4, O3, CO2, NO, and aerosols. The results from this mission will provide the basis for study of long-term variability in the mesospheric climate.
Figure 1: Artist's view of the AIM spacecraft in orbit to observe noctilucent clouds (image credit: Emily Hill Design)
AIM is a NASA PI (Principal Investigator) mission, lead by James M. Russell III of Hampton University (HU), Hampton, VA. The AIM team, led by HU, is made up of members from varies partner organizations (universities and institutions). In this setup, Hampton University is the prime contractor to NASA and manages all programmatic aspects of the project. LASP (Laboratory for Atmospheric and Space Physics) of the University of Colorado at Boulder is a major subcontractor to AIM, providing two instruments and the functions of mission operations and data acquisition. AIM data will be analyzed and prepared for public archiving by Hampton University with the assistance of GATS (Gordley &Associates Technical Software) Inc. of Newport News, VA. Further partners in AIM are: UAF (University of Alaska, Fairbanks), USU/SDL (Utah State University / Space Dynamics Laboratory), GMU (George Mason University), and BAS (British Arctic Survey). 2) 3) 4)
OSC (Orbital Sciences Corporation) of Dulles, VA, is the prime contractor to CAS/HU (Center for Atmospheric Sciences/Hampton University) for the spacecraft and payload integration. The AIM mission employs the LeoStar-2 bus of OSC, a 3-axis stabilized zero momentum platform. The spacecraft structure (hexagonal bus) has a diameter of 1.09 m and a length of 1.4 m. S/C power = 335 W (orbital average) using a fixed GaAs solar array; spacecraft mass of ~ 200 kg, design life of at least 2 years. 5) 6)
The spacecraft uses an aluminum honeycomb bus structure, with a deployed solar array wing canted at 50º. The solar array uses high efficiency solar cells on composite substrates, and the full array is assembled from 6 solar panel sections, and wrapped around the spacecraft (cells facing out) during launch. The panels are sequentially deployed into a flat panel and then canted away from the spacecraft body. The array provides 300 W of average power. The instruments SOFIE and CIPS are mounted to the nadir panel of the hexagonal structure, and CDE is mounted to the zenith panel. Instrument electronics for SOFIE are housed within the spacecraft body.
Figure 3: The cylindrical bus structure of AIM (image credit: NASA)
Attitude control is accomplished using 3 reaction wheels and 3 torque rods. The C&DH system employs a RAD 6000 computer (BAE). The ACS system uses a star tracker, gyro, magnetometer and sun sensors.
The spacecraft is inertially pointed during SOFIE observations, and nadir oriented with pitch and roll offsets to obtain the common volume CIPS observations. The spacecraft performs a subsolar yaw maneuver to keep the arrays sunlit over the daylight side of the orbit.
Figure 4: The AIM spacecraft with solar arrays in stowed configuration (image credit: NASA)
Legend to Figure 5: There are six separate solar array panels, which are integrated into a single deployed wing. In this orientation the nadir deck is pointed up towards the ceiling, and the SOFIE and CIPS instruments are visible in the photo. The photograph was taken in the clean room at the Orbital Sciences, Dulles, Va. facility.
Launch: An air launch of AIM on the Pegasus-XL launch vehicle of OSC took place on April 25, 2007 (air launch from an L1011 aircraft near the launch site: VAFB, CA, USA). 7)
Orbit: Sun-synchronous circular orbit, altitude = 600 km, inclination = 97.78º, LTAN = 12 hours.
RF communications: An S-band link is chosen via NASA's space/ground network and TDRS (Tracking and Data Relay Satellite) system. Communication is done through two helix antennas using an L-3 CXS-600B transceiver with S-band uplink and two downlink transmit capabilities for compatibility with the 2 Msample/s transmit rate to the ground network (GN), and the 2 ksample/s rate to TDRS.
Figure 6: Alternate view of the deployed AIM spacecraft (image credit: OSC)
• June 26, 2021: Clouds illuminated during the daytime by the Sun or at nighttime by the Moon are a common sight. It is rarer, however, to see clouds that form so high in the atmosphere that they continue to reflect sunlight hours after sunset, creating a spectacular display in the nighttime sky. 8)
- Noctilucent clouds, also called polar mesospheric clouds or “night-shining” clouds, occur most often near the poles and occasionally at lower latitudes. The phenomenon is seasonal and depends on just the right combination of atmospheric conditions.
- Scientists have called noctilucent clouds “the highest, driest, coldest, and rarest clouds on Earth.” Indeed, most of the planet’s clouds form in the tropopause, the layer of atmosphere closest to the ground, and occasionally in the stratosphere. In contrast, noctilucent clouds form in the mesosphere, at an altitude just over 50 miles (80 kilometers). They develop when water vapor aggregates and freezes around specks of meteor dust floating in the mesosphere. The amount of dust fluctuates, but there is generally enough to allow the clouds to grow when there is plenty of water vapor and when mesospheric temperatures plunge.
- Given that the clouds rely on extra-cold conditions, it might seem counterintuitive that they show up over the planet’s polar regions during late spring and summer. Cora Randall, the principal investigator for AIM-CIPS at the University of Colorado-Boulder, explained that air in the polar mesosphere ascends in the summer and descends in the winter. “So mesospheric air becomes colder in the summer and warmer in the winter,” she said. “This is just the opposite of what you might expect if the temperature were controlled just by sunlight.”
- The CIPS instrument makes consistent observations of noctilucent clouds throughout their brief seasonal existence over the planet’s polar regions. Observers on the ground, however, can only see them when skies are free of low-level clouds. And because mesospheric clouds are so faint—dimmer than the daytime sky—they are only visible from the ground when the viewer is in darkness and the Sun is at an angle to illuminate the clouds from below.
- In recent years, skywatchers have observed the noctilucent clouds at especially low latitudes, spotting them from as far south as southern California in 2019 and 2020. According to Randall, data from NASA’s Microwave Limb Sounder indicates that conditions are favorable again this year for low-latitude viewing. High-altitude temperatures are lower than average and water vapor is higher than average, making it more likely that water vapor will condense into ice.
- “If viewing conditions are favorable on the ground, I would not be surprised to see more reports in the next week of noctilucent clouds down to 40 degrees north or even a bit lower,” Randall said. The photo above shows noctilucent clouds as viewed from Bainbridge Island, Washington (47.7 degrees north latitude) on June 16, 2021.
- Randall noted that one reason for the favorable cloud conditions could be an overall strengthening of atmospheric circulation. More ascending air can cause both more cooling and an increase in water vapor. Another reason could be the weak solar cycle, which means there is less ultraviolet radiation to break up water molecules at high altitudes. “I suspect the answer is complex,” Randall said, “and probably involves atmospheric wave activity.”
Figure 7: Noctilucent clouds form so high in the atmosphere that they continue to reflect sunlight hours after sunset, creating a spectacular nighttime display. The image shows a satellite-based view of noctilucent clouds on June 16, 2021. The image is centered on the North Pole and is stitched together from data acquired during several orbital passes by NASA’s Aeronomy of Ice in the Mesosphere (AIM) spacecraft. The satellite’s Cloud Imaging and Particle Size (CIPS) instrument measures albedo, or the amount of light reflected back to space by the clouds. Researchers use data from AIM to better understand the complexities of the mesosphere, and its relationship to other parts of the atmosphere, weather, and climate (image credit: NASA Earth Observatory image by Joshua Stevens, using data from the University of Colorado Laboratory for Atmospheric and Space Physics. Story and photograph by Kathryn Hansen)
• December 21, 2020: Summer in Antarctica is marked by days in which the Sun never sets, balmy temperatures that hover as high as freezing, and electric-blue clouds of ice. NASA's AIM (Aeronomy of Ice in the Mesosphere) mission spotted the summer’s first noctilucent, or night-shining, clouds on Dec. 8, 2020. In the days that followed, the fine wisps of cloud slowly grew into slight puffs high over Antarctica. Typically, they spin like cotton candy into a mass that blankets the poles, but this season is off to a slow start, and the clouds are sparser than usual. The season is also a late one: Scientists usually expect the Antarctic ice clouds to appear sometime in mid-November and run through mid-February. 9)
Figure 8: These AIM images span Dec. 8–Dec. 19, 2020, starting with AIM’s first observations of the Antarctic noctilucent cloud season. The colors — from dark blue to light blue and bright white — indicate the clouds’ albedo, which refers to the amount of light that a surface reflects compared to the total sunlight that falls upon it. Things that have a high albedo are bright and reflect a lot of light. Things that don’t reflect much light have a low albedo, and they are dark (image credits: NASA/HU/VT/CU-LASP/AIM/Joy Ng)
- The brilliant blue and white clouds drift about 50 miles overhead in a layer of the atmosphere called the mesosphere. During summer, this region has all three ingredients the clouds need to form: extremely cold temperatures (at -215 degrees Fahrenheit, it’s the coldest part of the atmosphere), water vapor, and meteor dust.
- In summer, the mesosphere is most humid, since relatively wet air circulating up from the lower atmosphere brings extra water vapor. Meteor dust comes from meteors, which are ground into dust when they plummet and burn through the atmosphere. Noctilucent clouds form when water molecules coalesce around the fine, otherworldly dust and freeze.
- Also known as polar mesospheric clouds (since they tend to huddle around the North and South Poles), the clouds help scientists better understand the mesosphere. The mesosphere is where the neutral atmosphere begins transitioning to the electrically charged gases of space. From the mesosphere up, the atmosphere is in constant motion, shaped by solar activity and near-Earth space from above and the lower atmosphere below.
- “Every year, we look at things that could predict when the season starts, and then we watch and try to gauge where our understanding is,” said James Russell, AIM principal investigator at Hampton University in Virginia. Some factors scientists consider are seasonal temperatures, the size of the ozone hole, atmospheric currents, and westerly winds.
- Unusual weather in Antarctica led scientists to expect late-blooming noctilucent clouds. The size of the ozone hole is at a record high for this time of year. Westerly winds are gusting unusually strong. The polar vortex, which locks in frigid air over the poles, is also very large. All this amounts to a long winter, late spring, and slow start to noctilucent clouds season.
- The fleeting clouds also help scientists study gravity waves, which are powerful waves of air that form when winds brush over disturbances at Earth’s surface, like mountaintops, or stir over severe weather systems like thunderstorms. Gravity waves rise through the sky, connecting the lower and upper atmosphere. Watching how they impact noctilucent clouds is one way to study how gravity waves affect the overall mesosphere. NASA’s AWE (Atmospheric Waves Experiment) mission, which launches in 2022, will also contribute to gravity wave research and complement AIM’s observations.
- It’s easy to think that gravity waves simply ripple straight up. But a study earlier this year found that the most influential gravity waves for the clouds — and that means, the upper atmosphere — might be the ones that rise like an escalator: up and across at the same time. The gravity waves that travel in this manner tend to form over tropical monsoons, then rise up from the tropics and across latitudes. The study analyzed eight seasons’ worth of noctilucent clouds, and combined observations from AIM and NASA’s TIMED mission.
- When AIM launched in 2007, scientists thought they understood the relationship between noctilucent clouds and the solar cycle, the Sun’s natural 11-year cycle of activity. But the connection seems to have disappeared in early 2005. Noctilucent clouds are sensitive to both water vapor and temperature in the upper atmosphere — and the solar cycle affects both at their altitude. Yet even as the Sun progressed through its regular ups and downs, the clouds have shone at more or less the same intensity. There appears to be a delicate balance that scientists don’t yet fully understand.
- “Noctilucent clouds are affected by influences from above, like the Sun, but also influences from below, like gravity waves,” said Scott Bailey, AIM deputy principal investigator at Virginia Tech. “Right now, it seems like the forces from below are in control.”
- The start of the 2020 Antarctic noctilucent cloud season marks roughly one year into the current solar cycle, which began in December 2019. As AIM continues imaging the clouds, scientists hope the growing long-term record will yield clues to these puzzles.
- “Since AIM launched, we’ve found out the processes controlling the clouds are very complex,” Russell said. “We need as much data as we can get.”
• May 28, 2020: Ice-blue clouds are drifting high above the Arctic, which means the Northern Hemisphere’s noctilucent cloud season is here. NASA's AIM spacecraft first spotted wisps of noctilucent, or night-shining, clouds over the Arctic on May 17. In the week that followed, the ghost-like wisps grew into a blur, quickly filling more of the Arctic sky. This is the second-earliest start of the northern season yet observed, and the season is expected to run through mid-August. 10)
Figure 9: These animated images show AIM’s observations from the first week of the Arctic noctilucent cloud season, which began on May 17, 2020. The colors — from dark blue to light blue and bright white — indicate the clouds’ albedo, which refers to the amount of light that a surface reflects compared to the total sunlight that falls upon it. Things that have a high albedo are bright and reflect a lot of light. Things that don’t reflect much light have a low albedo; they are dark (image credit: NASA/HU/VT/CU-LASP/AIM/Joy Ng)
- The seasonal clouds hover high above the ground, about 50 miles overhead in a layer of the atmosphere called the mesosphere. Most meteors burn up when they reach the mesosphere; there are enough gases there to slough plummeting meteors into nothing more than dust and smoke. Noctilucent clouds form when water molecules congregate around the fine dust and freeze, forming ice crystals. The icy clouds, reflecting sunlight, shine bright blue and white. They first appear in summer — around mid-May in the Northern Hemisphere and mid-November in the Southern — when the mesosphere is most humid, with the season’s heat lofting moisture up to the sky.
- “Every year, twice a year, the start of the season is a big event for us,” said Jim Russell, AIM principal investigator at Hampton University in Virginia. “The reason we’re excited is we’re trying to find out what the causes of the season’s starting are and what does it really mean with regard to the larger picture in the atmosphere.”
- Also known as polar mesospheric clouds (because they tend to huddle around Earth’s poles), these clouds help scientists better understand the mesosphere and how it’s connected to the rest of the atmosphere, weather and climate.
- Scientists are eager to see what this Arctic season brings. For the most part, the brilliant clouds usually cling to the polar regions. But sometimes, they stray south. Last year, they were spotted as far south as southern California and Oklahoma — lower latitudes than have ever been seen before, Russell said. The new season is another chance to better understand the fleeting clouds and their possible migration south. Some evidence indicates this could be the result of changing atmospheric conditions.
- “With every year, we get new data to help us put together a picture of the atmosphere,” Russell said.
- Launched in 2007, AIM is a NASA-funded mission managed by NASA’s Goddard Space Flight Center in Greenbelt, Maryland. The mission is led by the AIM principal investigator from the Center for Atmospheric Sciences at Hampton University.
• February 11, 2020: The AIM mission is operating and about to complete its 13th year in orbit on 25 April. The two AIM instruments are operating nominally and collecting excellent data (Ref. 12).
• January 2020: A paper has been published. The authors have updated long‐term trends in mesopause temperature, airglow emission intensities and noctilucent clouds (NLC) based on ground‐based observations conducted in the Moscow region (Russia). Trends in mesopause temperature and airglow emissions have been derived for the period 2000‐2018 (19 years), and long‐term trends in NLC characteristics have been obtained for 1968‐2018 (51 years). Trends in airglow emissions have been estimated separately for winter and summer seasons. 11)
• January 9, 2019: AIM is fully operational and collecting scientific data using nadir measurements of Rayleigh backscattered sunlight made by the CIPS (Cloud Imaging and Particle Size) high spatial resolution camera and limb extinction using the SOFIE(Solar Occultation For Ice Experiment) solar occultation approach. It is routinely making measurements of Polar Mesospheric Clouds during the cloud season by both CIPS and SOFIE, and SOFIE profiles at all times of temperature, O3, H2O, CH4, NO and limb extinction at eight wavelengths ranging from 0.87 to 4.6 µm. Outside the PMC region, CIPS provides high spatial resolution measurements of gravity waves near 50 km altitude over a broad latitude range extending ranging from high northern to high southern latitudes (Ref. 12).
• January 30, 2018: After more then 10 years on orbit, the AIM mission is fully operational with planned operations going through September 2023. 12)
• January 7, 2018: The sky over Antarctica is now glowing electric blue with noctilucent, or night-shining, clouds. That’s according to recent images from NASA’s AIM spacecraft (Aeronomy of Ice in the Mesosphere), which monitors these clouds for the whole Earth. The season for night-shining clouds in the Southern Hemisphere is November to April, so they are right on schedule. These are ice clouds, and Earth’s highest clouds, located some 50 miles (80 km) above the ground in a layer of the atmosphere called the mesosphere. The clouds – made of ice crystals – are seeded by fine debris from disintegrating meteors. 13) 14)
- The season for noctilucent clouds in the north is May to September. In both hemispheres, they happen when it’s summertime, when water vapor wafts up into the high atmosphere, providing the moisture needed to form these spectacular ice clouds at the edge of space. - As for the electric-blue glow, it comes from sunlight shining through the high clouds.
- Cora Randall, a member of the AIM science team at the University of Colorado’s Laboratory for Atmospheric and Space Physics, said: ”The current season began on November 19. Compared to previous years of AIM data, this season seems to be fairly average, but of course one never knows what surprises lie ahead, particularly since the southern hemisphere seasons are so variable.”
- If you were in Antarctica now, would you see these clouds shining overhead? Not likely, since there’s 24-hour daylight shining on that part of the globe now. But we’re past the December solstice, meaning that summer is waning in the Southern Hemisphere. People outside the Antarctic, at relatively high Southern Hemisphere latitudes, might be able to glimpse the clouds, especially as their sunsets come earlier and night lengthens on that part of the globe.
- We frequently see images of noctilucent clouds, taken from the ground, during the northern summer. Our friends at high northern latitudes – typically from northern Europe and Scandinavia – capture them.
Figure 10: Noctilucent (“night-shining”) cloud over Antarctica in early January, 2018, from NASA’s AIM satellite (image credit: NASA, HU, VT, CU LASP)
• November 9, 2016: NASA honored the AIM Flight Operations Team (FOT) with a Group Achievement Award for its exceptional engineering and innovative achievement enabling the AIM mission to continue operations without command uplink. 15)
- The AIM spacecraft has now gone 1193 days without command uplink capability, lost early in this single-string, low-cost mission due to a defect in the command receiver exacerbated by spacecraft charging and radiation effects. The extraordinary innovation and dedication of the AIM FOT clearly saved this mission, enabling significant scientific advancements in understanding the coldest region on Earth. Their efforts have pushed the limits in spacecraft automation beyond anything accomplished before. They enabled science operations to continue despite an anomaly that could have ended the AIM mission prematurely, and significantly reduced the cost of operating the mission during the extended mission phase.
- Ten days into the mission, AIM began having difficulty attaining lock on the uplink subcarrier. Since then, the AIM spacecraft experienced varying periods, from hours to weeks, between contacts with successful command uplink. The first extended outage of approximately four days occurred a couple of weeks after the problem surfaced. This drove the FOT to examine new ways of operating the spacecraft. The FOT made massive software changes while they could, to allow the spacecraft to operate autonomously for long periods without command uplink. The initial efforts focused on providing a system that was robust against extended command outages without making significant changes to the risk posture of the program.
- Then, the priority shifted to developing and testing groundbreaking techniques for the command and control of a deaf satellite. The first efforts focused on producing an expedited instrument commissioning sequence using stored commands and developing a stored command sequence to execute in the event of another extended outage. Next, the FOT increased the onboard command storage capability, improved the on-board orbit knowledge and provided for an autonomous downlink of recorded data when AIM passed over a scheduled ground station. Then the FOT proceeded to fully automate the spacecraft to handle science observation sequences and to perform autonomous orbit maneuvers using the onboard telemetry monitors that are nominally used for onboard fault detection and correction sequences. This provided the AIM mission with a means for continuing science observations in the event of an extended command outage.
- Having achieved a level of autonomy that would meet mission requirements, the FOT worked on improving the science data quality and the robustness of the system. This involved incorporating the ground based mission-planning software into flight software. In parallel, the FOT developed a process to modulate the RF signal through the TDRSS link in ways that are observable to the flight software (a.k.a. Morse code commanding), without requiring the receiver to lock onto the subcarrier. This allows the operations team to trigger pre-loaded stored command sequences to perform emergency recovery operations. The capability to build custom command sequences is enabling new science as the AIM orbit evolves during extended mission operations.
- The AIM spacecraft will be reconfigured to perform science during upcoming years of "full-sun" condition when the autonomous state vector routine and the on-board mission planning software will no longer function due to lack of sunrises and sunsets. These innovative modifications continue to enable robust ongoing spacecraft operations with no loss of science data quality or quantity. The FOT implemented extensive, opportunistic and highly creative operational changes to both the spacecraft and science instruments, resulting in the ongoing return of over 98% of the science data on a continuous basis after accomplishing 100% of the science return for the mission. Their accomplishment has enormous potential for the future conscious implementation of spacecraft autonomy, potentially requiring fewer operators and lower cost to NASA. This was all accomplished with no change of funding from NASA and resurrected a mission that could have been a total loss extending it into years of spectacular science results.
• June 6, 2017: The AIM spacecraft continues to perform well. 16)
- As of February 2017, the AIM orbit plane is nearly perpendicular to the Earth - sun vector. As a result the sun does not rise or set as viewed from the satellite, and SOFIE measurements are not possible. This will change by early October, 2017, when SOFIE will resume measurements. SOFIE science and housekeeping parameters all indicate a stable and healthy instrument. SOFIE V1.3 data are available online through February 2017.
- Siskind et al. 17) used SOFIE and MLS satellite data to categorized the inter-annual variability of winter and springtime upper stratospheric methane (CH4). They showed the effects of this variability on the chemistry of the upper stratosphere throughout the following summer. Years with strong wintertime mesospheric descent followed by dynamically quiet springs, such as 2009, lead to the lowest summertime CH4.
- Years with relatively weak wintertime descent, but strong springtime planetary wave activity, such as 2011, have the highest summertime CH4. By sampling Aura MLS to the SOFIE measurement locations, it was demonstrated that summertime upper stratospheric ClO almost perfectly anti-correlates with the CH4 (Figure 11). This is consistent with the reaction of atomic chlorine with CH4 to form the reservoir species, hydrochloric acid (HCl). The summertime ClO for years with strong uninterrupted mesospheric descent is about 50% greater than in years with strong horizontal transport and mixing of high CH4 air from lower latitudes. Small, but persistent effects on ozone are also seen such that between 1 and 2 hPa, ozone is about 4–5% higher in summer for the years with the highest CH4 relative to the lowest. This is consistent with the role of the chlorine catalytic cycle on ozone.
- These dependencies may offer a means to monitor dynamical effects on the high-latitude upper stratosphere using summertime ClO measurements as a proxy. Additionally, these chlorine-controlled ozone decreases, which are seen to maximize after years with strong uninterrupted wintertime descent, represent a new mechanism by which mesospheric descent can affect polar ozone. Finally, given that the effects on ozone appear to persist much of the rest of the year, the consideration of winter/spring dynamical variability may also be relevant in studies of ozone trends.
Figure 11: The color contours on the left are zonal mean WACCM/NOGAPS difference fields for August 2009 minus August 2008 for ClO (top) and CH4 (bottom). The vertical dashed white line is the mean latitude of the SOFIE occultations for August. On the right, a vertical profile of the model difference at the SOFIE occultation latitude (solid line with plus symbols) is compared with MLS ClO and SOFIE CH4 (data are dotted/dashed curves with stars). Note that x axes for the right panels are reversed from one another since the ClO change is positive, while the CH4 change is negative (image credit: NRL, Hampton University, Virginia Tech, University of Colorado)
• December 02, 2016: Strictly speaking, noctilucent or “night shining” clouds don’t glow in the dark the way the bioluminescent algae or fireflies do. Also called polar mesospheric clouds, they float high enough in the atmosphere to capture a little bit of stray sunlight, even after the Sun has fallen below the horizon. In this way, they have more in common with an actor being illuminated by a spotlight in front of a dark curtain. 18) 19)
- Noctilucent clouds commonly occur at high latitudes in the late spring and early summer, before 24-hour daylight sets in. They form around both the North and South Pole, roughly 50 to 85 km above the Earth’s surface. NASA's AIM captured images of the clouds over Antarctica on November 29, 2016, a few weeks after they first appeared on November 17. The mosaic of Figure 12 was created using satellite data from several passes over the Antarctic. The AIM instrument measures cloud albedo—the amount of light reflected back to space. Lighter areas correspond to more brightly lit clouds, while areas with no data appear in black.
- The AIM mission first observed noctilucent clouds over the Arctic in 2007, and the clouds seem to be appearing earlier and at lower latitudes. They have been observed as far south as Utah and Colorado. Research has also shown that they have been getting brighter.
Figure 12: Data from the AIM spacecraft shows the sky over Antarctica is glowing electric blue due to the start of noctilucent, or night-shining, cloud season in the Southern Hemisphere. This data was collected from Nov. 17-28, 2016 (image credit: NASA/HU/VT/CU-LASP/AIM/Joy Ng, producer)
• September 26, 2016: The AIM spacecraft and its instruments (SOFIE and CIPS) are working very well, in their 9th year on orbit. The project is dealing with a temporary issue brought on by orbit precession which has caused the loss of data for about a month. However, the problem is understood and steps are underway to fix the problem. The full collection mode should be reached in about two weeks. 20)
• March 8, 2016: 21)
The SOFIE instrument continues to operate normally. Software
modifications to accommodate the changing orbit and updated instrument
operations are ongoing. The changes are confined to level 1 and are
addressing times when the SOFIE attenuator balance procedure approached
exoatmospheric heights, and the new operation sequence (post September
2015) which has longer events and no balance procedure. SOFIE
observations of temperature versus altitude have been used to
characterize GWs (Gravity Waves). 22) 23)
Figure 13: Vertical profiles of GW amplitude and PE (Potential Energy) from SOFIE results for May 2007 through December 2014 (image credit: AIM Science Team)
• July 10, 2015: The AIM mission is approved by NASA to continue planning against the current budget guidelines. Any changes to the guidelines will be handled through the budget formulation process. The AIM mission will be invited to the 2017 Heliophysics Senior Review. 24)
- June 2015: The 2015 Heliophysics Senior Review panel undertook a review of 15 missions currently in operation in April 2015. The panel found that all the missions continue to produce science that is highly valuable to the scientific community and that they are an excellent investment by the public that funds them. 25)
- AIM demonstrated that water vapor injections into the upper mesosphere and lower thermosphere (MLT) from space traffic potentially affect polar mesospheric cloud properties and compensate for stronger water vapor photolysis during increased solar activity. Observational evidence from AIM and TIMED signal decreases in MLT composition and temperature during the last decade, indicating possible anthropogenic cooling effects due to rising concentrations of the greenhouse gas CO2. Insights like these into the sources of natural and human induced changes in the Earth system address a primary science theme of the Earth Science Division.
• June 19, 2015: In the late spring and summer, unusual clouds form high in the atmosphere above the polar regions of the world. As the lower atmosphere warms, the upper atmosphere gets cooler, and ice crystals form on meteor dust and other particles high in the sky. The result is noctilucent or “night-shining” clouds (NLCs)—electric blue wisps that grow on the edge of space. 26)
Legend to Figure 14: This image is a composite of several satellite passes over the Arctic, and the clouds appear in various shades of light blue to white, depending on the density of the ice particles. The instrument measures albedo—how much light is reflected back to space by the high-altitude clouds.
• Feb. 27, 2015: The AIM mission is in its 8th year on orbit and is operating very well. The CIPS (Cloud Imager and Particle Size) and SOFIE (Solar Occultation For Ice Experiment) instruments are working without flaw. The dust instrument, CDE (Cosmic Dust Experiment), was turned off about 4 years ago. So everything is working very well (Ref. 31).
• October 31, 2014: All systems on AIM are functioning nominally. Also, in order to mitigate the effects of the solar eclipse which occurred on October 23rd, AIM was transitioned to its backup attitude control mode (TMON/RTS Control) prior to the eclipse, and transitioned back to OOMP (the normal control mode) on the evening of October 24, 2014. 27)
- The CIPS instrument continues to perform well, with no health issues. The project is gearing up for the start of the Southern Hemisphere (SH) season, which should begin later in November or early December. It was previously suggested that interhemispheric teleconnections triggered by planetary wave activity in the SH winter stratosphere led to a rapid decrease in PMCs (Polar Mesospheric Clouds) in the Northern Hemisphere (NH) that began about 40 DFS (Days From Solstice). The PMCs recovered significantly after reaching a minimum near DFS 45, peaking near DFS 53; the season ended shortly thereafter. See the first figure below, which shows the daily PMC frequency at 80°N latitude for all NH PMC seasons observed by AIM; the red curve shows 2014.
Figure 15: CIPS PMC frequencies at 75ºN latitude for all NH seasons observed by AIM (image credit: AIM Science Team)
- The SOFIE instrument continues to operate nominally, and is collecting high quality data on the state of the middle atmosphere. Observations of the Northern Hemisphere (NH) 2014 season are complete and relevant data files and figures are available on the SOFIE and AIM web pages.
• April 2014: New data from NASA's AIM spacecraft have revealed "teleconnections" in Earth's atmosphere that stretch all the way from the North Pole to the South Pole and back again, linking weather and climate more closely than simple geography would suggest. 28)
- The AIM science team at HU (Hampton University) and at LASP (Laboratory for Atmospheric and Space Physics), University of Colorado, Boulder, CO, found that the winter air temperature in Indianapolis, Indiana (or any other city in the USA), is well correlated with the frequency of NLCs (Noctilucent Clouds) over Antarctica. NLCs are Earth's highest clouds. They form at the edge of space 83 km above our planet's polar regions in a layer of the atmosphere called the mesosphere. Seeded by "meteor smoke," NLCs are made of tiny ice crystals that glow electric blue when sunlight lances through their cloud-tops.
- While in the first part of the AIM mission, the project's attention was focused on a narrow layer of the atmosphere where NLCs form. Now the project team is finding out this layer manifests evidence of long-distance connections in the atmosphere far from the NLCs themselves.
- One of these teleconnections links the Arctic stratosphere with the Antarctic mesosphere. Stratospheric winds over the Arctic control circulation in the mesosphere. When northern stratospheric winds slow down, a ripple effect around the globe causes the southern mesosphere to become warmer and drier, leading to fewer NLCs. When northern winds pick up again, the southern mesosphere becomes colder and wetter, and the NLCs return.
- In January 2014, a time of year when southern NLCs are usually abundant, the AIM spacecraft observed a sudden and unexpected decline in the clouds. Interestingly, about two weeks earlier, winds in the Arctic stratosphere were strongly perturbed, leading to a distorted polar vortex.
- The AIM science team believes that this triggered a ripple effect that led to a decline in NLCs half-way around the world. This is the same polar vortex that made headlines this winter (2013/2014) when parts of the USA experienced crippling cold and ice.
Legend to Figure 16: Changes in surface temperatures near Indianapolis, IN (blue, left and bottom scales) are well correlated with changes in the Arctic stratosphere (orange, right and bottom scales) and with changes in noctilucent clouds (PMCs) at 77º S latutude two weeks later (red, left and top scales).
• January 2014: The AIM spacecraft continues to perform nominally in its 7th year on orbit. The project received funding to operate through September 2018 (Ref. 31).
• June 2013: Every summer, something strange and wonderful happens high above the north pole. Ice crystals begin to cling to the smoky remains of meteors, forming electric-blue clouds with tendrils that ripple hypnotically against the sunset sky. This year, NLCs (Noctilucent Clouds) are getting an early start. NASA's AIM spacecraft started seeing them on May 13. 29)
The early start is extra-puzzling because of the solar cycle. Researchers have long known that NLCs tend to peak during solar minimum and bottom-out during solar maximum—a fairly strong anti-correlation.
• On April 25, 2013, the AIM spacecraft was 6 years on-orbit. AIM continues to operate nominally. A lunar eclipse occurred on May 10 , 2013 but the coarse sun sensors remained locked on the sun and therefore had no impact on the spacecraft operations (Ref. 30).
• January 2013: The AIM spacecraft continues to perform nominally. The subsystems remain healthy and functional. 30)
- AIM is currently funded to operate through September 2013. The project submitted a proposal to the NASA Senior Review process for continued operations through 2018. That proposal is in review with a decision expected in June, 2013. 31)
• August 2012: A key ingredient of Earth's strangest clouds does not come from Earth. New data from NASA's AIM spacecraft shows that "meteor smoke" is essential to the formation of NLCs (Noctilucent Clouds). Using data from the SOFIE (Solar Occultation for Ice Experiment), the project found that about 3% of each ice crystal in a noctilucent cloud is of meteoritic origin. 32)
The inner solar system is littered with meteoroids of all shapes and sizes — from asteroid-sized chunks of rock to microscopic specks of dust. Every day Earth scoops up tons of the material, mostly the small stuff. When meteoroids hit our atmosphere and burn up, they leave behind a haze of tiny particles suspended 70 km to 100 km above Earth's surface.
In the 19th century, NLCs were confined to high latitudes—places like Canada and Scandinavia. In recent times, however, they have been spotted as far south as Colorado, Utah and Nebraska. The reason, James Russell (PI of AIM mission) believes, is climate change. One of the greenhouse gases that has become more abundant in Earth's atmosphere since the 19th century is methane (CH4). It comes from landfills, natural gas and petroleum systems, agricultural activities, and coal mining.
When methane makes its way into the upper atmosphere, it is oxidized by a complex series of reactions to form water vapor. This extra water vapor is then available to grow ice crystals for NLCs.
Figure 17: The graphic shows how methane, a greenhouse gas, boosts the abundance of water at the top of Earth's atmosphere. This water freezes around "meteor smoke" to form icy noctilucent clouds (image credit: Hampton University, NASA)
• Status of July 20, 2012: All of the AIM spacecraft subsystems continue to perform well (Ref. 34). During the last period of bitlock on May 23, the project loaded several products to improve three areas of the spacecraft's performance.
Figure 18: Astronauts on board the ISS took this picture of noctilucent clouds near the top of Earth's atmosphere on July 13, 2012 Image credit: HU, NASA)
• The AIM spacecraft and its instruments are operating nominally (except for a command workaround) in 2012 - in its 5th year on orbit. The AIM mission has been extended by NASA through the end of FY12.
• The AIM spacecraft and its instruments are operating nominally in 2011 (Ref. 2).
Figure 19: Noctilucent clouds over Edmonton, Canada observed on July 20, 2011 (image credit: NASA) 33)
• For the first time scientists have a comprehensive data set showing the formation and seasonal variation of the clouds over both poles. The mission is providing high quality data on cloud nucleus particle size, size variation with altitude, particle shape and its altitude dependence, and other characteristics that describe the onset and end of the PMC season. In addition scientists are observing the interplay between particles, water vapor and temperature variations, brightness variability over the entire polar cap region, and space and time variability. 36)
All AIM spacecraft systems have been functioning nominally since launch - except for the command receiver. The receiver has had periods of intermittent command signal rejection, but the AIM Flight Operations team has been able to successfully work around these difficulties. 37)
• The autonomous operations concept for AIM has evolved over its first year on orbit. On May 20, 2008, AIM has been selected for extended mission funding following the 2-year Explorer baseline mission. The extension from June 2009 through September 2012 will allow tracking the evolution of mesospheric clouds for an additional seven seasons and provide data to address key outstanding questions including: 38)
- Are there variations in PMCs that can be explained by changes in solar irradiance and particle input?
- What changes in mesospheric properties are responsible for north/south differences in PMC features?
- What controls interannual variability in PMC season duration and latitudinal extent?
- What is the mechanism of teleconnection between winter temperatures and summer hemisphere PMCs?
- What is the global occurrence rate of gravity waves outside the PMC domain?
• Routine science data processing started in February 2008. All data products are available to the public via the internet at the main AIM web page (Ref. 37).
• As of December 2007, AIM has provided the first global-scale view of the clouds over the entire 2007 Northern Hemisphere season with an unprecedented horizontal resolution of 5 km x 5 km. 39)
• Full science operations began on May 22, 2007. In June 2007, the AIM instruments captured the first images of noctilucent clouds over the Arctic region. 40)
• The commissioning of the spacecraft proceeded nominally through attaining normal pointing mode. Nine days after launch, the satellite started to have problems locking on the command uplink subcarrier modulation. This was the beginning of the intermittent operation of the transceiver that has continued ever since. Over the next couple of weeks many different uplink configurations were tested to characterize the performance of the receiver. 41)
The initial efforts focused on providing a system that was robust against extended command outages without making significant changes to the risk posture of the program. Once that had been accomplished, the priority shifted to developing and testing groundbreaking techniques for the command and control of a deaf satellite. These enhancements are being used to ensure AIM continues to collect great science data on the mysterious clouds that appear on the edge of space.
Figure 20: One of the first ground sightings of noctilucent clouds in the 2007 season over Budapest, Hungary on June 15, 2007 (image credit: NASA)
Figure 21: Noctilucent clouds over the Arctic region as seen by the AIM instruments (image credit: NASA)
Sensor complement: (CIPS, CDE, SOFIE)
The sensor complement consists of three instruments. Initially, the mission was planned with 4 instruments, but SHIMMER (Spatial Heterodyne Imager for Mesospheric Radicals) of NRL was deleted due to budgetary problems.
The instrument mass values (Table 1) include the electronic boxes. The bus size includes the solar arrays in deployed configuration.
CIPS (Cloud Imaging and Particle Size):
CIPS is an instrument designed and developed at CU/LASP (University of Colorado / Laboratory for Atmospheric and Space Physics), Boulder, CO. The objective of CIPS is to take imagery of the clouds to determine when and where they form, and to document what they look like. 42) 43)
Figure 22: Illustration of CIPS (image credit: CAS/HU)
CIPS images the PMC cloud deck with a resolution of 2 km, and measures the scattering phase function of PMCs along with other microphysical properties such as particle size and water content. The instrument consists of four wide angle cameras with a combined FOV (Field of View) of 80º x 120º. The camera FOV is centered about nadir providing an image size of 1440 km x 960 km at an altitude of 83 km. The clouds are imaged at the UV bandpass of 265 nm (±5 nm), taking advantage of the strong absorption characteristic of ozone at this wavelength to enhance the contrast of the cloud scattering with respect to the background Rayleigh scattering. 44) 45)
Each camera has an overlapping FOV and a pixel size at the nadir of ~2 km. The FOV of the camera system is 80º-120º, centered at the sub-satellite point, with the 120º axis along the orbit track as shown in Figure 23.
Figure 23: CIPS FOV projected to 83 km; the satellite velocity vector direction is to the right (image credit: LASP, Ref. 37)
CIPS is a panoramic UV (narrow bandwidth with a center at 265 nm) nadir-pointing imager. Each of the cameras has a custom-designed 9 element lens system (Lakin Optical Systems) and a narrow bandpass optical filter from Barr Associates. The UV cloud image is focused onto the CsTe photocathode on a Hamamatsu image intensifier (converting UV to visible) that is fiber coupled to an Atmel CCD. An instrument microprocessor stores and processes all camera images for transmission to the spacecraft (Ref. 36).
The combination of images from the four cameras is referred to as a scene. CIPS records scenes of atmospheric and cloud radiance in the summer hemisphere from the terminator to ~40º latitude along the sunlit portion of the orbit. The near-polar orbit and cross-track FOV will cause the observation swaths to overlap at latitudes higher than about 70º, so that nearly the entire polar cap will be mapped daily by the 15-orbit per day coverage. In the nominal pointing mode, the CIPS images extend poleward to about 85º latitude in each hemisphere (Ref. 37).
Each camera has a focal ratio of 1.12, a focal length of 28 mm, a 25 mm lens diameter and includes an interference filter and a CCD (Charge Coupled Device) detector system. The throughput of the optical elements and their sizes are designed for a 71% measurement precision of the background sunlit Earth. The custom UV filters were manufactured by Barr associates and centered at 265 nm. The CCD detectors are coupled with Hamamatsu V5181U-03 image intensifiers (40 mm diameter active area) and have 2048 x 2048 useful pixels that are electronically binned in 4 x 8 combinations for an effective 340 (cross track) x 170 (along track) pixel images. The signal in each pixel is digitized to12 bit resolution. On average, 26 images are produced per orbit in the summer polar region with special ‘first light’ images just beyond the terminator.
Imaging is achieved with this body-fixed camera assembly using an exposure time of 1 s, which, when combined with the FOV, yields the nadir spatial resolution of ~2 km. Between four and seven exposures of the same cloud volume are made during a satellite overpass, at a rate of one scene every 46 s. Each CCD is equipped with a DSP (Digital Signal Processing) interface that incorporates a lossless Huffman compression algorithm, reducing data volume by about a factor of two. Therefore ,each scene produces 523kB of data yielding approximately 18 MB per orbit.
Figure 24: Schematic view of a single CIPS camera and its elements (image credit: LASP)
Figure 25: Photo of the CIPS camera assembly (image credit: CU/LASP, Ref. 37)
CDE (Cosmic Dust Experiment):
CDE is an in-situ dust detector designed and developed at LASP. CDE is mounted on the zenith side of the spacecraft, providing a very wide field of view and looking away from the Earth. The objective is to measure the influx of dust particles into the upper atmosphere, the PMC (Polar Mesospheric Cloud) region.
CDE is a copy of the SDC (Student Dust Counter) developed for the New Horizons spacecraft of NASA (launch Jan. 19, 2006), that is now traveling to Pluto and the Kuiper Belt (Pluto flyby in 2015).
Both CDE and SDC comprise an array of impact detectors made from polyvinylidene fluoride (PVDF). PVDF is an electrically polarizable material. When physically impacted by a high speed particle, a small change in the polarization takes place, and that depolarization signal can be sensed as a change in electrical charge by fast analog electronics. Both particle mass and velocity contribute to the signal.
To minimize redesign in the CDE effort, the CIPS instrument electronics provide the interface to the CDE instrument, and the CIPS electronics were designed to mimic the New Horizons spacecraft interface. The AIM payload originally included an instrument platform assembly (IPA) as an integrating structure that would have permitted the instruments to be assembled and tested as a suite, and then installed on the spacecraft as a complete unit.
Figure 26: Illustration of the CDE device (image credit: CAS/HU)
CDE observations integrated over several days are expected to show the temporal variability of the cosmic dust influx that could influence the formation of PMCs. The cosmic dust delivered to the mesosphere is most likely ablated to particle radii of ~0.2 nm, which coagulate to PMC nucleation sites of ~1 nm. Recent results from global-scale models reveal that variations in the influx of meteoric material can dramatically affect the availability of nucleation sites in the polar summer mesosphere. Although the availability of nucleation sites depends strongly on equator-ward transport from the polar summer mesosphere, large uncertainties exist in the models regarding total influx of material, the initial radius of the dust and the coagulation efficiency. The temporal variability of meteoric material measured by CDE will be used with observed variations in PMCs and model studies to assess the role that extraterrestrial forcing plays in PMC formation and variability. Such studies will involve sorting out other sources of variability of ice properties, and thus will probably require several PMC seasons to build up an adequate database.
Legend to Figure 27: Each patch of the 12 active PVDF sensors has a surface area of about 85 cm2. The panel is mounted to point towards the local zenith direction at all times, minimizing the impact rates from orbital debris.
The CDE goal to measure an expected impact rate of ≥ 100 hits/week requires a mass threshold of ≤ 4 x 10-12 g and a total sensitive surface area of ≥ 0.1 m2. To meet this requirement CDE (Figure 27) consists of 12 active PVDF patches with surface areas of 85 cm2 each. In addition,there are two other sensors (identical to the front side patches), on the back side of CDE that cannot be hit by dust. These reference detectors are being used to measure the noise background. The CDE dynamic range provides mass resolutions within a factor of ≤ 3 in the mass range of 4 x10-12 g ≤ m ≤ 4 x 10-9 g covering an approximate size range in particle radius of 0.8 µm ≤ a ≤ 8 µm.
Each of the 14 sensors has an adjustable threshold to optimize CDE operations. To follow the possible degradation of its performance due to ageing, CDE has onboard calibration capabilities for its electronics. Internal signals can be injected in each of the 14 channels with amplitudes covering its entire dynamical range. Each dust hit generates a science event, where the time, channel number and the impact charge are recorded, in addition to all relevant housekeeping data. Using appropriate averages this can be turned in to a time-dependent global map to show the possible spatial and temporal variability of the amount of cosmic dust entering the atmosphere.
SOFIE (Solar Occultation For Ice Experiment):
The SOFIE instrument is designed and developed at USU/SDL (Utah State University / Space Dynamics Laboratory) at Logan, UT. SOFIE is of SABER (Sounding of the Atmosphere using Broadband Emission Radiometry) heritage flown on TIMED (launch Dec. 7, 2001). The objective of SOFIE is to observe the following atmospheric constituents by the use of the solar occultation technique: temperature, PMCs, carbon dioxide (CO2), methane (CH4), nitric oxide (NO), ozone (O3) and aerosols.
SOFIE is an 8-channel differential absorption radiometer covering the spectral range from 290 nm (UV) to 5.26 µm (MWIR). Six channels are designed to measure gaseous signals, and two are dedicated to particle measurements. Measurements in two carbon dioxide bands are being used to simultaneously retrieve profiles of temperature and the carbon dioxide mixing ratio. 46) 47) 48) 49)
Each SOFIE channel uses two detectors, one that samples a spectral region where the target gas is strongly absorbing, and one that samples a weakly absorbing region. Measuring the difference of these signals allows precise isolation of the target gas signal. Once the gaseous contribution is isolated, the remaining signals can be used to infer particle extinction, so that particle measurements will be obtained from every channel.
Radiation entering the SOFIE telescope passes through a field stop which defines the instantaneous field of view (FOV). The field stop provides an angular field of view of 1.8 arcmin vertical by 6.0 arcmin horizontal. The FOV dimensions at the tangent point are about 1.2 km vertical by 4.1 km horizontal.
Optics: SOFIE uses a cassegrain telescope with a 10.16 cm entrance pupil. An elliptical steering mirror (16.76 cm x 11.55 cm) directs the incoming beam onto a focusing mirror and then to a secondary mirror (Figure 28). The backside of the secondary mirror contains a pickoff mirror that directs a portion of the beam into the sun sensor module. The main beam passes through a field stop that determines the instantaneous field of view (IFOV). The field stop is 1.95 arcmin vertical x 4.74 arcmin horizontal, which is 1.50 km x 3.63 km when projected to the 83 km limb path tangent point. The beam is chopped at 1000 Hz using a tuning fork device, and directed into the CSM (Channel Separation Module) where the science measurements are accomplished.
Figure 29: Illustration of the SOFIE instrument (image credit: USU/SDL and GATS Inc.)
Table 2: Band specification of SOFIE
The SOFIE instrument includes a solar tracking system with the ability to acquire the sun, track it through an occultation, and perform scans as required for various on-orbit calibration sequences. The pointing system consists of two principal components: the sun sensor and steering mirror.
• The sun sensor uses a
radiation hardened focal plane array (FPA) image sensor with 1024 x
1024 pixels. The FPA field of view is 2.04º in azimuth and
2.025º in elevation. The FPA diodes are 15 µm in size and
subtend roughly 7.14 arcsec at tangent. The sun sensor center
wavelength is 705 nm with a bandwidth of ± 5 nm. Incoming light
is dispersed by the sun sensor optics according to the Airy disc
• Pointing is accomplished using a steering mirror at the aperture entrance. The steering mirror provides ± 1.6º of rotation in both elevation and azimuth. Optical gain magnifies these angles by a factor of 2 in elevation and sqrt(2) in azimuth. As a result, the range of optical rotation provided by the mirror is 4.5º in azimuth by 6.4º in elevation. The steering mirror has a maximum slew rate of > 0.8º/s. The pointing resolution is better than 0.8 arcsec.
Signal conditioning electronics: Three measurements are accomplished for each channel, the weak and strong band radiometer signals (Vw and Vs) and the difference of these signals (ΔV). Output signals from the detector preamp undergo signal conditioning including synchronous rectification at 1000 Hz. SOFIE signals are digitized using a 14 bit converter operating in the range of ±3 V.
SOFIE measurement geometry: SOFIE provides spacecraft sunset measurements at latitudes between about65º and 85ºS and sunrise measurements at latitudes between about 65ºN and 85ºN. SOFIE observes 15 sunrise and 15sunset occultations per day,and consecutive sunrises or sunsets are separated by ~96 min in time or ~24º in longitude. The SOFIE FOV (Field of View)at the tangent point is ~1.5 km vertical by ~4.4 km horizontal. The SOFIE measurement suite,consisting of 16 radiometer and 8 difference signal measurements, is sampled at 20 Hz, which corresponds to a vertical distance of ~145 m in the atmosphere. The vertical resolution of 1.5 km combined with the ~3 km s-1 solar sink or rise rate sets the natural frequency of the data set at ~2 Hz,which is the rate at which the FOV vertical dimension is swept through the atmosphere.
SOFIE instrument performance: SOFIE performance was characterized in laboratory calibration studies before launch and detailed characterizations have been completed in orbit. Laboratory calibration sequences addressed important instrument characteristics including measurement background and noise, FOV, response linearity, relative spectral response,time response,absolute gain,and difference signal gain. Because the basic measurements are ratios of signals used to determine atmospheric transmissions, absolute radiometric calibration is not important,except to ensure that the exoatmospheric solar view generates signals near the upper limit of the data acquisition system. Performance of the SOFIE measurement and retrieval system in-orbit is excellent in all cases with noise levels at or below laboratory values. The retrieval precision and altitude range based on data analysis thus far are summarized in Table 3.
In all cases SOFIE performance meets or exceeds AIM science requirements. Note that SOFIE CO2 and NO retrievals are currently not operational, but will be available in future data versions.
Figure 30: Photo of the SOFIE instrument (image credit: USU/SDL and GATS Inc.)
Table 4: SOFIE retrievals and data products
Figure 31: Temperature profile of the standard atmosphere (image credit: Emily Hill Design)
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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).