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)
Figure 2: Illustration of the AIM spacecraft (image credit: OSC, CAS/HU)
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)
Figure 5: Photo of the AIM spacecraft with the solar array fully deployed (image credit OSC)
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)
• 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. 8)
- 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 7 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 7: This image shows the noctilucent clouds over Antarctica, captured by NASA's AIM spacecraft on November 29, 2016 (image credit: NASA Earth Observatory, map by Jesse Allen, using preliminary Polar Mesospheric Cloud data from the University of Colorado/LASP)
• 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. 9)
• March 8, 2016: 10) 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). 11) 12)
Figure 8: 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. 13)
- 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. 14)
- 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. 15)
Figure 9: This composite image shows noctilucent clouds over the Arctic captured by NASA's AIM spacecraft on June 10, 2015 (image credit: LASP, University of Colorado, Joshua Stevens)
Legend to Figure 9: 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. 20).
• 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. 16)
- 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 10: 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. 17)
- 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.
Figure 11: The 2013/2014 winter air temperature in Indianapolis is correlated with the frequency of noctilucent clouds over Antarctica (image credit: AIM Science Team)
Legend to Figure 11: 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. 20).
• 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. 18)
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. 19).
• January 2013: The AIM spacecraft continues to perform nominally. The subsystems remain healthy and functional. 19)
- 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. 20)
• 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. 21)
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 12: 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. 23). During the last period of bitlock on May 23, the project loaded several products to improve three areas of the spacecraft's performance.
Figure 13: 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 14: Noctilucent clouds over Edmonton, Canada observed on July 20, 2011 (image credit: NASA) 22)
• 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. 25)
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. 26)
• 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: 27)
- 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. 26).
• 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. 28)
• 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. 29)
• 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. 30)
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 15: One of the first ground sightings of noctilucent clouds in the 2007 season over Budapest, Hungary on June 15, 2007 (image credit: NASA)
Figure 16: 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.
Table 1: Overview of instrument mass, power, data rate and dimensions
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. 31) 32)
Figure 17: 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. 33) 34)
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 18.
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. 25).
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. 26).
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 19: Schematic view of a single CIPS camera and its elements (image credit: LASP)
Figure 20: Photo of the CIPS camera assembly (image credit: CU/LASP, Ref. 26)
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 21: 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.
Figure 22: CDE sensors mounted on the top of the spacecraft (image credit: LASP)
Legend to Figure 22: 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 22) 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. 35) 36) 37) 38)
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 23). 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 23: Block diagram of the optical system of SOFIE (image credit: USU/SDL, Hampton University)
Figure 24: 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 function.
• 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.
Table 3: Retrieval characteristics of SOFIE
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 25: Photo of the SOFIE instrument (image credit: USU/SDL and GATS Inc.)
Table 4: SOFIE retrievals and data products
Figure 26: 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).