1 The Aeronomy of Ice in the Mesosphere (AIM) Mission: Overview and early science results
2 Corresponding author: James M. Russell III*a
3 Co-authors: Scott M. Baileyb, Larry L. Gordleyc, David W. Ruschd, Mihály Horányid, Mark E.
4 Hervigc, Gary E. Thomasd, Cora E. Randalld, David E. Siskinde, Michael H. Stevense, Michael E.
5 Summersf, Michael I. Taylorg, Christoph R. Englerte, Patrick J. Espyh, William E. McClintockd
6 and Aimee W. Merkeld
7 *a Center for Atmospheric Sciences, Hampton University, Hampton, VA 23668, USA,
8 [email protected], phone (757)728-6893, fax (757)727-5090
9 b Bradley Department of Electrical and Computer Engineering, Virginia Polytechnical Institute
10 and State University, Blacksburg, VA 24061, USA
11 c GATS, Inc., 11864 Cannon Boulevard, Suite 101, Newport News, VA 23606, USA
12 d Laboratory for Atmospheric and Space Physics, 1234 Innovation Drive, Boulder, CO 80303,
13 USA
14 e Space Science Division, Naval Research Laboratory, Washington DC, 20375, USA
15
16 f Department of Physics, George Mason University, Fairfax, VA, 22030, USA
17
18 g Department of Physics, Utah State University, Logan, Utah 84322, USA
19
20 h Department of Physics, Norwegian University of Science and Technology, Høgskoleringen 5,
21 NO-7491, Trondheim, Norway
22
23
1 24 Potential Reviewers:
25 Matthew DeLand ([email protected])
26 Eric Shettle ([email protected])
27 Uwe Berger ([email protected])
28 Svetlana Petelina [email protected]
29 Abstract
30 The Aeronomy of Ice in the Mesosphere (AIM) mission was launched from Vandenberg Air
31 Force Base in California at 1:26:03 PDT on April 25, 2007 becoming the first satellite mission
32 dedicated to the study of polar mesospheric clouds. A Pegasus XL rocket launched the satellite
33 into a near perfectly circular 600 km sun synchronous orbit. AIM carries three instruments
34 specifically selected because of their ability to provide key measurements needed to address the
35 AIM goal which is to determine why these clouds form and vary. The instrument payload
36 includes a nadir imager, a solar occultation instrument and an in-situ cosmic dust detector.
37 Detailed descriptions of the science, instruments and observation scenario are presented. Early
38 science results from the first northern and southern hemisphere seasons show a highly variable
39 cloud morphology, clouds that are ten times brighter than measured by previous space-based
40 instruments, the presence of a previously suspected but never before measured layer of small
41 particles believed to be the cause of summertime radar echoes and a mesospheric ice layer that
42 extends from below the main northern hemisphere peak at 83kmup up to ~90km in one
43 continuous layer. Additionally, complex features were observed that are reminiscent of
44 tropospheric weather phenomena.
45 Key Words: Polar Mesospheric Clouds, Mesosphere, Remote Sensing, Gases, Particles, Global
46 Change
2 47 1. Introduction
48 Polar Mesospheric Clouds (PMCs) are Earth’s highest clouds, occupying the atmospheric region
49 near the high-latitude summer mesopause at ~ 83 km. They typically extend from 55° latitude to
50 the geographic pole, occurring in each summer in both hemispheres between about mid-May to
51 mid August in the North and mid-November to mid-March in the South. The polar upper
52 mesosphere becomes the coldest place on earth around summer solstice, when the temperature
53 plummets below 130K. Known as noctilucent, or “night-shining” clouds (NLCs) as seen by
54 ground based observers (Figure 1.), their name derives from the fact that they are seen at evening
55 or morning twilight, when the lower atmosphere is in darkness, but the upper atmosphere is still
56 sunlit. First identified over 120 years ago (Leslie, 1885), their nature as water-ice crystals was
57 not established until relatively recently [Hervig et al., 2001]. There is great interest in PMCs
58 because they are changing in ways that are not understood. They are appearing more frequently,
59 becoming brighter and they are being seen at lower latitudes than ever before [Deland et al.,
60 2006, 2007; Taylor et al., 2002]. This behavior suggests a possible connection with global
61 change occurring at lower altitudes.
62 Due to their sensitivity to changes in the environment, PMCs are expected to respond to long-
63 term global change, and may be a modern phenomenon associated with the rise of greenhouse
64 gases in the industrial era [Thomas et al., 1989]. Indeed, an increase of ~5% per decade in PMC
65 brightness has occurred over the past 27 years [DeLand et al., 2007]. Additional interest in
66 mesospheric clouds stems from the unique conditions of cloud formation in the weakly-ionized
67 D-region (a ‘dusty plasma’), and their close relationship with a mesospheric layer of strong radar
68 reflectance (Polar Mesospheric Summer Echoes). At least three factors are believed to control
69 PMC formation: (1) temperature, (2) water vapor, and (3) cosmic dust or some other nucleation
70 source. The detailed role of water vapor has not yet been established, although it is of critical
3 71 importance in the cloud saturation and growth processes. There is now evidence that the clouds
72 redistribute water vapor [Summers et al., 2001], which in turn could affect the ozone
73 concentration and heat balance [Siskind et al., 2005]. There are many open questions regarding
74 cloud nucleation, microphysics (Rapp and Thomas, 2006), chemistry, gravity waves, transport,
75 and cosmic dust influx. Despite the sophistication of present-day modeling [e.g. Berger and
76 Lübken, 2006], these uncertainties hamper our ability to understand the origin of the observed
77 changes, nor can we project with confidence future changes. In short, we do not understand what
78 causes a mesospheric cloud to form, or how it evolves. To establish the basis for new predictive
79 PMC models, high precision measurements of PMCs and key chemical constituents are required
80 for various conditions of temperature, cosmic dust influx, and H2O concentrations.
81 The Aeronomy of Ice in the Mesosphere (AIM) satellite mission was launched from Vandenberg
82 Air Force Base on April 25, 2007 with the overall goal to resolve why PMCs form and why they
83 vary. A Pegasus XL rocket placed the AIM satellite into a near circular (601 km apogee, 595 km
84 perigee), 12:00 AM/PM sun-synchronous orbit. By measuring PMCs and the thermal, chemical
85 and dynamical environment in which they form, AIM will quantify the connection between these
86 clouds and the meteorology of the polar mesosphere. In the end, this will provide the basis for
87 study of long-term variability in mesospheric climate and its relationship to global change. The
88 results of AIM will be a rigorous test and validation of predictive models that then can reliably
89 use past PMC changes and current data to assess trends as indicators of global change. This goal
90 is being achieved by measuring PMC abundances, spatial distribution, particle size distributions,
91 gravity wave activity, cosmic dust influx to the atmosphere and precise vertical profiles of
92 temperature, H2O, CH4, O3, CO2, NO, and aerosols.
93 The AIM science team has established a set of six objectives designed to address the mission
94 goal. The first objective is to understand the cloud microphysics, i.e. the relationship between
4 95 temperature, water vapor and size distribution under a variety of conditions and times in the
96 PMC season, and for different cloud types. The second objective is focused on assessing the role
97 of atmospheric gravity waves in PMC formation. It is clear that waves are important because
98 their presence is obvious in the many ground-based photographs of the clouds. The third
99 objective addresses the question of the influence of temperature in controlling the length of the
100 PMC season. Clearly, extremely cold temperatures (on the order of 140K or less) are essential
101 because where the clouds, form fifty miles above the surface, the pressure is one hundred
102 thousand times less than at the surface and the air is one hundred thousand to a million times
103 drier than surface desert air. The fourth objective addresses upper mesosphere water vapor
104 chemistry and how it changes during the PMC season. As noted earlier, the clouds are known to
105 be made up of water ice crystals and interactions between H2O and O3 and the role of solar
106 Lyman-α radiation which controls water vapor at the cloud altitude needs to be understood. The
107 fifth objective is to asses the sources of seed particles needed for PMC formation by providing a
108 nucleation site to begin the particle growth process. The last objective combines results from the
109 first five to validate coupled chemistry, dynamics and PMC microphysical models and to assess
110 the ability to quantify long-term changes, and predict future changes in PMCs based on these
111 models and historical data sets.
112 The AIM experiment includes three instruments: SOFIE (Solar Occultation For Ice Experiment),
113 an infrared (IR) solar occultation differential absorption radiometer; CIPS (Cloud Imaging and
114 Particle Size experiment), a panoramic UV imager; and CDE (Cosmic Dust Experiment), an in-
115 situ cosmic dust detector. In this paper we describe the AIM mission including the instruments
116 and their data products and the overall mission implementation.
5 117 2. The AIM Science Instruments
118 The Solar Occultation for Ice Experiment (SOFIE)
119 SOFIE (Fig 2.) is performing satellite solar occultation measurements to determine vertical
120 profiles of temperature, pressure, the abundance of five gaseous species (H2O, O3, CO2, CH4, and
121 NO) and PMC particle extinction at eleven wavelengths. Occultation measurements are
122 accomplished by monitoring solar intensity as the satellite enters or exits the Earth’s sunlit side.
123 The ratio of solar intensity measured through the atmosphere (V) to the exoatmospheric solar
124 intensity (Vexo) yields atmospheric transmissions, which are the basis for retrieving the desired
125 geophysical parameters. SOFIE uses differential absorption radiometry, with eight channel pairs
126 covering wavelengths (λ) from 0.29 to 5.32 µm (see Table 1). Gordley et al., [2008] and Hervig
127 et al., 2008, provide details of the SOFIE instrument design, characteristics, calibration, retrieval
128 methods and on-orbit performance in companion papers in this special issue. Specific gases are
129 targeted by measuring solar intensity in two wavelength regions; one where the gas is strongly
130 absorbing and an adjacent region where the gas is weakly absorbing. SOFIE measures the strong
131 and weak band radiometer signals (VS and VW) and the difference of these which is amplified by
132 an electronic gain G∆V to give the relation:
133 ∆V = (Vw – Vs) G∆V (1)
134 While a single band measurement alone can be sufficient to retrieve gas mixing ratios in the
135 lower mesosphere and stratosphere, SOFIE will characterize the tenuous regions where PMCs
136 form extending into the lower thermosphere. At these altitudes, atmospheric and gaseous
137 abundances are low and the corresponding signals can approach typical noise levels.
138 While a single band measurement alone can be sufficient to retrieve gas mixing ratios in the
139 lower mesosphere and stratosphere, SOFIE will characterize the tenuous regions where PMCs
140 form extending into the lower thermosphere. At these altitudes, atmospheric and gaseous
6 141 abundances are low and the corresponding signals can approach the noise levels. However,
142 common mode optics errors and electronics errors can be greatly reduced by measuring the
143 difference in intensity between adjacent strong and weak absorption bands. Benefits are realized
144 because a variety of atmospheric and instrumental effects are nearly identical in the strong and
145 weak bands, and are therefore are greatly reduced by differencing band pairs. For example, Fig.
146 3 shows the SOFIE water vapor channel band pair locations indicated by the orange horizontal
147 bars, along with calculated transmission spectra for relevant gaseous absorbers and PMCs. Note
148 that PMC absorption is nearly identical in the H2O weak and strong bands, and thus will be
149 almost completely removed in the difference signal measurement. Another benefit is that
150 electronic gain can be applied to the difference signals, allowing the measurement precision to be
151 enhanced to a level consistent with the detector noise. Six SOFIE channels are designed to
152 measure gaseous signals, and two are dedicated to particle measurements. Particle measurements
153 are also obtained from the gas channel weak bands. Measurements in two CO2 bands will be
154 used to simultaneously retrieve temperature and CO2 mixing ratio [Gordley et al., 2008]. The
155 SOFIE sun sensor provides measurements of atmospheric refraction angle with sub-arc second
156 knowledge precision that is being used to retrieve temperature profiles at altitudes below about
157 50 km.
158 SOFIE Measurement Geometry
159 SOFIE provides spacecraft sunset measurements at latitudes between about 65° and 85°S and
160 spacecraft sunrise measurements at latitudes between about 65° and 85°N [See Gordley et al.,
161 2008]. SOFIE observes 15 sunrise and 15 sunset occultations per day, and consecutive sunrises
162 or sunsets are separated by ∼96 minutes in time or ∼24° in longitude. The SOFIE field of view
163 (FOV) is 1.8 arc minutes vertical by 6.0 arc minutes horizontal which corresponds to about 1.5 x
164 5 km, respectively, at the tangent point. The SOFIE measurements, consisting of 16 radiometer
7 165 and 8 difference signal measurements, are sampled at 20 Hz, which corresponds to a vertical
166 distance of 150 m in the atmosphere. The vertical resolution of 1.5 km combined with the 3
167 km/sec solar sink or rise rate sets the natural frequency of the data set at ∼2 Hz, which is the rate
168 at which the FOV vertical dimension is swept through the atmosphere.
169 SOFIE Instrument Performance
170 SOFIE performance was characterized in laboratory calibration before launch and now detailed
171 characterizations have been done in orbit. In-orbit performance is excellent with noise levels at
172 or below laboratory values. Ground radiative calibration sources included a solar emulator
173 blackbody (SEB) that achieved an emitting temperature of ∼3000K. Radiance from the SEB was
174 equivalent to 28% - 46% of the exoatmospheric solar signal in bands 5 – 16, and less than 8% of
175 the exoatmospheric signal in bands 1 - 4. Bands 1 - 4 were stimulated with a xenon lamp. The
176 Sun was viewed in the laboratory by directing the solar image into the SOFIE aperture using
177 external pointing mirrors. Calibration sequences addressed important instrument characteristics
178 including measurement background and noise, FOV, response linearity, relative spectral
179 response, time response, absolute gain, and difference signal gain. Because the measurements
180 are used to determine transmissions (τ = V / Vexo), absolute radiometric calibration is not
181 important, except to ensure that the exoatmospheric solar view generates signals near the upper
182 limit of the data acquisition system. In all cases the measured instrument characteristics were
183 within design specifications based on AIM science requirements. Expected on-orbit
184 performance of the SOFIE measurement/retrieval system was characterized by simulating the
185 SOFIE measurements and then retrieving the desired geophysical parameters from these signals.
186 These exercises included key SOFIE performance characteristics measured during calibration
187 and testing, and were used to determine salient characteristics of the retrieved parameters. The
188 retrieved quantities, measurements used, and expected altitude range of reliable retrievals are
8 189 summarized in Table 2. Gordley et al. [2008] give more detail on the bands used and their
190 properties.
191 SOFIE Physical PMC Properties
192 SOFIE multi-wavelength PMC extinction measurements are used to infer a variety of physical
193 cloud properties including ice mass density, particle shape, and particle size distribution [Hervig
194 et al., 2008a]. PMCs are identified using 3.19 µm extinction profiles based on a threshold
195 approach considering the combined measurement and retrieval noise floor (10-7 km-1). The
196 infrared (IR) measurements are due entirely to absorption and are thus proportional to the
197 particle radius cubed. As a result, the IR measurements are directly proportional to particle
198 volume density (or ice mass density). The conversion from extinction to ice mass density is
199 based on theoretical relationships that have total uncertainties (due to uncertainties in the
200 refractive index and particle shape and size) of between 2 and 10% for λ > 2.7 µm. Particle
201 shape is determined by comparing measured extinction ratios to modeled signals from clouds
202 comprised of oblate and prolate spheroids with varying axial ratios (AR). Certain SOFIE spectral
203 band extinction ratios within the IR are sensitive to AR and nearly independent of particle size.
204 The SOFIE measurements yield both oblate (AR > 1) and prolate (AR < 1) solutions for AR < ∼4,
205 and a unique solution when AR > ∼4. The unique combination of UV thru IR PMC
206 measurements provides excellent information concerning the PMC size distribution. The size
207 distribution retrievals assume the unimodal Gaussian form, which describes the concentration
208 versus radius in terms of the total particle concentration (N), mode radius (rm), and distribution
209 width (∆ r). Theoretical uncertainties (assuming perfect measurements) in N, rm, and ∆ r are less
210 than a few percent for 20 < rm < 180 nm and 5 < ∆ r < 25 nm. SOFIE results are discussed later.
9 211 The Cloud Imaging and Particle Size Experiment (CIPS)
212 The CIPS instrument is a panoramic UV imager. CIPS provide images of the atmosphere at a
213 variety of scattering angles in order to remove the Rayleigh scattering component of the signal,
214 determine cloud presence, infer particle sizes, provide the spatial morphology of the clouds, and
215 constrain the parameters of cloud particle size and size distribution. CIPS is providing:
216 • Panoramic nadir imaging at 265 nm with a 120º x 80º field-of-view and 5 km X 5 km
217 spatial resolution
218 • Scattered radiances from Polar Mesospheric Clouds near 83 km altitude to derive PMC
219 morphology and cloud particle size information.
220 • Multiple exposures of individual clouds at various scattering angles to derive PMC
221 scattering phase functions.
222 • High-spatial resolution measurements of the variation of the Rayleigh scattered
223 background that provides information on gravity wave activity in the 55-km region and
224 small scale variations in lower mesospheric ozone.
225 CIPS Instrument Design
226 The CIPS instrument consists of an array of four cameras (Figure 4) operating with a 15 nm
227 passband centered at 265 nm. Each camera has an overlapping FOV and a pixel size at the nadir
228 of ~ 2 km. The FOV of the camera system is 80º x 120º, centered at the sub-satellite point, with
229 the 120º axis along the orbit track (Fig. 5). Because of slant viewing, the spatial resolution
230 increases to ~6.4 km near the edge of the FOV of the forward and aft cameras. The combination
231 of images from the four cameras is referred to as a scene. CIPS records scenes of atmospheric
232 and cloud radiance in the summer hemisphere from the terminator to ~ 40o latitude along the
233 sunlit portion of the orbit. The near-polar orbit and cross-track FOV will cause the observation
10 234 swaths to overlap at latitudes higher than about 70o, so that nearly the entire polar cap will be
235 mapped daily by the 15-orbit per day coverage (Fig. 6). The CIPS images cover up to about 85
236 degrees latitude in each hemisphere.
237 Each camera has a focal ratio of 1.12, a focal length of 28 mm, a 25 mm lens diameter and
238 includes an interference filter and a Charge Coupled Device (CCD) detector system. The
239 throughput of the optical elements and their sizes are designed for a ±1% measurement precision
240 of the background sunlit Earth. The custom UV filters were manufactured by Barr associates and
241 centered at 265 nm. The CCD detectors are coupled with Hamamatsu V5181U-03 image
242 intensifiers (40 mm diameter active area) and have 2048 x 2048 useful pixels that are
243 electronically binned in 4 x 8 combinations for an effective 340 (cross track) x 170 (along track)
244 pixel images. The signal in each pixel is digitized to 12 bit resolution. On average, 26 images are
245 produced per orbit in the summer polar region with special ‘first light’ images just beyond the
246 terminator.
247 Imaging is achieved with this body-fixed camera assembly using an exposure time of 1 second,
248 which, when combined with the field-of-view, yields the nadir spatial resolution of ~2 km.
249 Between four and seven exposures of the same cloud volume are made during a satellite
250 overpass, at a rate of one scene every 46 sec. Each CCD is equipped with a Digital Signal
251 Processing (DSP) interface that incorporates a lossless Huffman compression algorithm,
252 reducing data volume by about a factor of two. Therefore each scene produces 523 Kbytes of
253 data yielding approximately 18 Mbytes per orbit. The CIPS requirements and measured
254 performance characteristics are listed in Table 3. As the AIM satellite moves in orbit, the CIPS
255 instrument provides seven images of the same region of the atmosphere at seven different
256 scattering angles from which the PMC scattering phase function is derived.
257
11 258 CIPS PMC High Resolution Structure
259 PMCs are identified as enhancements of brightness against the Rayleigh-scattered background
260 coming from the lower atmosphere. Thomas et al. [1991] proved the feasibility of this detection
261 method using 273.5 nm data from the Solar Backscattered Ultraviolet (SBUV) nadir-viewing
262 spectrometer on board the NIMBUS 7 spacecraft. They showed that the brightest PMCs could be
263 distinguished against the background despite the clouds under-filling the 175 x 175 km FOV.
264 One key to detecting relatively weak PMCs is to observe at wavelengths where the
265 Earth's albedo is at or near a minimum due to ozone absorption. The 265 nm central wavelength
266 of CIPS was chosen because it represents a peak in ozone absorption and therefore a minimum in
267 the background originating from scattering of sunlight from air molecules. This observed
268 background radiance is generally limited by ozone to scattering in the vicinity of 55-km and
269 above. A second key feature is the measurement of cloud brightness at small (< 35º) scattering
270 angles that greatly enhances the cloud contrast with respect to the Rayleigh background because
271 of the forward-scattering property of ice particles in the Mie regime.
272 CIPS observations at solar zenith angles (SZA) from 87 deg to about 94 deg (the 'shadow band')
273 experience a reduction in background signal by at least a factor of 10 from full daylight
274 brightness while the higher altitude PMCs remain at least 95% illuminated relative to an
275 overhead sun condition. This results in greatly enhanced PMC contrast because the background
276 is mostly in darkness. The shadow band is roughly 700 km wide and centered on the location of
277 SOFIE coincident observations. We refer to that location as the 'Common Volume' because both
278 CIPS and SOFIE observe the same volume of atmosphere within a period of six minutes.
279 The bright PMCs are confined to a summertime period about 70 days long, and mostly to a
280 region poleward of the Arctic and Antarctic circles [Thomas et al., 1989]. The CIPS observing
281 strategy allows for adequate collection of data from cloud free-regions, both before and after the
12 282 cloud 'season’, and at lower latitudes during the season (<55o). These regions are being used to
283 characterize signatures of small scale wave transport of energy from the lower atmosphere –
284 gravity waves, as well as planetary scale waves.
285 Gravity wave effects on background CIPS signals result from the dependence of ozone
286 photochemistry on temperature. Temperature fluctuations associated with waves can alter the
287 ozone density. The resulting effect on the UV albedo in the highly-absorbing Hartley bands of
288 ozone is easily found from a single-scattering calculation to be linearly dependent upon the
289 ozone perturbation. Multiple scattering is negligible at 265 nm. Wave amplitudes as small as 1-2
290 K at 55 km will cause approximately a 3% perturbation in background signal, a change easily
291 detectable by CIPS above the otherwise smoothly-varying background due to Rayleigh
292 scattering. This capability has led to identification of gravity wave signatures in the CIPS data
293 [Chandran et al., 2008].
294 Clouds are identified by their large angular-dependent scattering signature, a well-established
295 property of PMC particles [Thomas and McKay, 1986; Gumbel and Witt, 1998]. Cloud ice
296 particles are strongly forward scattering. This effect is more pronounced for brighter clouds, or
297 more specifically clouds having larger particle size. The background Rayleigh scattered sunlight
298 follows a well known variation with scattering angle that is symmetric about 90° scattering
299 angle. Any brightness enhancement that shows forward scattering behavior can be identified as a
300 PMC, and not an underlying background irregularity. This leads to a methodology for separation
301 of the signature of the cloud albedo from that of the underlying background. The scene recorded
302 by CIPS includes a Rayleigh scattered sunlight background at altitudes near 50 km and Mie
303 scattered sunlight by PMC particles at ~ 83 km. The magnitude of the background is controlled
304 by ozone which absorbs radiation along the path. For solar zenith angles away from the
305 terminator, ozone limits contributions to the observed CIPS radiance to altitudes only above 50
13 306 km. The crux of CIPS measurement interpretation is determining the PMC signal by subtracting
307 the Rayleigh background. CIPS determines the scattering phase function by imaging the same
308 point in space at multiple angles. In one approach, the background signal can be inferred using
309 the difference between cloud and molecular scattering phase functions. Alternately, the
310 background can be calculated given profiles of temperature, pressure, and ozone taken from
311 SOFIE measurements within the common volume.
312 Given the water-ice composition [Hervig et al., 2001] and the shape factor of the irregular ice
313 particles [see Rapp et al., 2007 for further discussion of the shape issue], the CIPS angular
314 distributions of brightness at a single wavelength yield column mass, surface area, and mean
315 particle size. These results allow correlation of PMC properties with PMC extinction,
316 temperature, water vapor, and other atmospheric parameters. The particle size distribution can be
317 more constrained by combining the CIPS observations with SOFIE measurements of PMC
318 optical depths in the common volume rather than using the data from one instrument alone.
319 The Cosmic Dust Experiment (CDE)
320 CDE monitors the variability of cosmic dust influx into our atmosphere. It is an in situ dust
321 impact experiment, however the time for an incoming micrometeoroid from low Earth orbit to
322 reach the altitude region of 80 to 85 km is very short ( < 1 min). Hence CDE measurements
323 directly indicate the influx of cosmic matter to the mesosphere. CDE observations integrated
324 over several days are expected to show the temporal variability of the cosmic dust influx that
325 could influence the formation and evolution of PMCs, and results will be used to establish
326 possible correlations with remote sensing observations of PMC brightness and coverage, for
327 example. CDE is an impact dust detector based on thin (28 µ m) permanently polarized
328 polyvinylidine fluoride (PVDF) foil sensors [Simpson and Tuzzolino,1985]. PVDF dust
329 detectors were flown on the VEGA 1 and 2 missions to comet Halley [Simpson et al., 1986], on
14 330 the STARDUST mission to comet Wild 2 [Tuzzolino et al., 2004], on the Cassini mission to
331 Saturn [Srama Et al., 2004], and the Earth observing ARGOS satellite [Tuzzolino Et al., 2005].
332 A copy of the CDE instrument is on the New Horizons mission to Pluto [Horanyi et al., 2007].
333 PVDF sensors have many advantages: they require no bias or high accelerating voltages, they are
334 electrically, mechanically and thermally stable, radiation resistant, do not respond to charged
335 particles, and can measure very high rates of dust impacts. The disadvantage of this sensor is
336 that the charge signal generated by an impact is a function of both the mass and the velocity of
337 the dust grain. Hence, in the analysis of the CDE data one has to assume a characteristic
338 meteorite speed and average flight angle with respect to the local zenith in order to infer
339 meteorite mass. A similar approach was used in the analysis of the impact craters on the Long
340 Duration Exposure Facility (LDEF) [Love and Brownlee, 1993]. The charge Q (in units of
341 electron charge) generated by a particle with mass m, (in units of grams), and relative velocity v
342 (measured in km/s) is: Qmv=×3. 8 1 017 1.3 3 343
344 The size of an individual PVDF detector ultimately determines the mass threshold as well as the
345 expected impact rate. The capacitance of the PVDF film is proportional to its surface area and, in
346 turn, the electronic noise level is set by the capacitance. The required mass (charge) threshold
347 ultimately sets the maximum size of an individual PVDF patch. The CDE science goal of
348 measuring an expected impact rate of ≥ 100 hits/week requires a mass threshold of ≤ 4x10-12 g
349 and a total sensitive surface area of ≥ 0.1 m2. To meet this requirement CDE (Figure 7) consists
350 of 12 active PVDF patches with surface areas of 85 cm2 each. In addition, there are two
351 additional sensors (identical to the front side patches), on the back side of CDE that cannot be hit
352 by dust. These reference detectors will be used to measure the noise background. The CDE
353 dynamic range provides mass resolutions within a factor of ≤ 3 in the mass range of 4x10-12g ≤
15 354 m ≤ 4x10-9g covering an approximate size range in particle radius of 0.8 µ m ≤ a ≤ 8
355 µ m.
356 Each of the 14 sensors has an adjustable threshold to optimize CDE operations. To follow the
357 possible degradation of its performance due to ageing, CDE has onboard calibration capabilities
358 for its electronics. Internal signals can be injected in each of the 14 channels with amplitudes
359 covering its entire dynamical range. Each dust hit generates a science event, where the time,
360 channel number and the impact charge are recorded, in addition to all relevant housekeeping
361 data. Using appropriate averages this can be turned into a time-dependent global map to show the
362 possible spatial and temporal variability of the amount of cosmic dust entering the atmosphere.
363 3. AIM Observation Scenario
364 Occultation and imaging measurements have inherent benefits, but also present unique
365 challenges to measurement interpretation. For example, CIPS offers excellent horizontal
366 resolution but little height information, while SOFIE provides excellent vertical resolution but
367 coarse horizontal resolution. Combining observations allows mitigation of certain measurement
368 limitations and offers opportunities to increase the information provided by each data set. Thus,
369 the AIM observation scenario provides space coincident CIPS and SOFIE observations on every
370 orbit. The SOFIE tangent point location and footprint defines the AIM Common Volume (CV).
371 CIPS will view the CV roughly six minutes before or after SOFIE, which is well under the 20-30
372 minute time scale of PMC variability [see e.g. Gadsden, 1982]. The SOFIE horizontal footprint
373 occupies 280 km along the limb by 5 km across-limb. Limb retrievals traditionally assume that
374 all atmospheric absorbers are distributed in a spherically symmetric fashion. While spherical
375 symmetry is an appropriate assumption when measuring gaseous species, PMCs can vary over
376 smaller scales than accommodated by the limb path geometry [Fritz, et al., 1993]. If ignored,
377 cloud inhomogeneity can result in fundamental errors, although the sign of the errors is at least
16 378 predictable. Variable cloud cover will always result in an underestimate of the instantaneous
379 cloud extinction because the measurement inversion assumes that the absorber occupies the
380 entire tangent path length. A cloud within the line of sight but not at the tangent point will
381 always be assigned an erroneously low altitude because its position is assumed to be at the
382 tangent point. SOFIE measurement interpretation will address these issues using CIPS images to
383 provide the detailed horizontal distribution of PMCs.
384 The AIM spacecraft will accommodate CV measurements by transitioning from sun pointing
385 during the SOFIE occultation to an Earth inertial pointing mode for the CIPS imaging period.
386 This orientation also allows SOFIE and CIPS to view though the same 2-D plane, with CIPS
387 viewing multiple angles as it passes over the occultation tangent point. Earth rotation (~1°,
388 typically < 50 km) will prevent perfectly coincident planar viewing, but this is unimportant
389 considering the natural horizontal resolution of the SOFIE sample volume (~280 km).
390 4. Spacecraft Status
391 All AIM spacecraft systems have been functioning nominally since launch on April 25, 2007
392 except for the command receiver. The receiver has had periods of intermittent command signal
393 rejection, but the AIM Flight Operations team has been able to successfully work around these
394 difficulties. Full science operations began on May 22, 2007. Routine science data processing
395 started in February 2008. All data products are available to the public via the internet at the
396 main AIM webpage at aim.hamptonu.edu.
397 5. Early Science Results
398 Brief highlights of early science findings are presented in this section and more results as well as
399 additional details are contained in ten other AIM papers in this special issue. AIM is providing
400 the first global scale view of PMCs in the polar cap region with a spatial resolution that is
401 unprecedented. Results show that the clouds are highly variable from orbit to orbit and day to
17 402 day. Figures 8 and 9 show daily mean CIPS images on two successive days in late June and
403 early July 2007. As discussed above, these images have had the Rayleigh background signal
404 removed so that what remains is only PMC signatures. Further details of how the background
405 removal is accomplished are provided in papers by Rusch et al. [2008] and Bailey et al. [2008] in
406 this issue. Note the significant increase in the number of clouds from June 30 to July 1st. The
407 clouds on June 30th are not as bright and cover a much smaller area than on the next day. Also
408 note the very pronounced void in the cloud field in the first quadrant of Fig. 9. We refer to this
409 feature as an ice void. These features are currently unexplained but are common occurrences in
410 the CIPS images. They could be caused by energy deposition (heating) through gravity wave
411 breaking, but they may also occur due to turbulent mixing from vertically displaced air (which
412 can also be due to gravity wave breaking). Ice voids appear in circular or oval shapes with
413 diameters that vary from ten to several hundred kilometers or greater. Rusch et al., [2008]
414 discuss this feature in more detail. These structures bear a startling similarity to those seen in
415 tropospheric clouds and suggest that the mesosphere may share some of the same dynamical
416 processes responsible for weather near Earth’s surface. If this similarity holds up under further
417 analysis, it introduces an entirely different view of potential mechanisms responsible for PMC
418 formation and variability than existed before AIM was launched.
419 The multi-wavelength SOFIE observations are shedding new light on PMCs because of their
420 extremely high sensitivity and signal-to-noise. While previous instruments suggested that ice is
421 present about half the time near 70°N, SOFIE now shows that ice is almost always present, i.e.
422 100% of the time, at these latitudes during the PMC season. Hervig et al. [2008b] compare a time
423 series of SOFIE ice occurrence frequency to SME and HALOE results. They check their results
424 by downgrading SOFIE sensitivity to be consistent with HALOE and SME and find good
425 agreement with results from these experiments, thus confirming that the lower ice frequencies
18 426 recorded by the older satellites were a result of reduced sensitivity. SOFIE is 15 - 200 times
427 more sensitive to PMC particles than previous instruments, including lidars. In addition, it
428 contains channels located at the peak in the ice absorption spectrum (~3 µm wavelength) where
429 PMC extinction is ~100 times greater than for surrounding NIR and IR wavelengths.
430 The time versus height cross section of SOFIE H2O in Figure 10 illustrates the development of
431 water vapor in the PMC environment during the 2007 northern cloud season. Water vapor, an
432 essential gas for PMC formation, is measured at a vertical resolution of 1.6 km and precision of
433 better than 21 ppbv. The seasonal evolution of water vapor is characterized by a rapid mixing
434 ratio rise in the beginning of the season and a steady increase after that time which is most likely
435 caused by vertical transport due to upwelling. SOFIE simultaneous O3 observations shown in
436 Fig. 10 indicate a mesospheric ozone minimum at about 81 km altitude at the beginning and end
437 of the season and near 79 km throughout most of the season, with a peak observed near 90 km.
438 O3 enhancement is observed at altitudes from roughly 80 to 95 km during times that ice is
439 present. For example, at 92 km the mixing ratio increases from 0.8 ppmv on 22 May to ~1.1
440 ppmv by 11 July, returning to ~0.8 ppmv by 15 August. This change in ozone seems to be
441 consistent with the suggestion by Sisknd et al [2007] that ozone should be enhanced above the
442 ice layer due to dehydration resulting in lowered HOx. These observations of highly transient
443 features, combined with the other SOFIE products are giving a first-time detailed look at the
444 physics of PMCs.
445 An example of the quality of the underlying H2O profiles is demonstrated in Figure 11 which
446 shows a set of retrieved water vapor profiles for August 12, 2007. The various profiles indicate
447 where PMCs have formed as evidenced by reduced water vapor near 85 km, and where PMCs
448 have sublimated resulting in enhanced water vapor just above 80 km. The abrupt decrease in
449 H2O above PMC altitudes (83 km) is consistent with a loss of water vapor to ice growth, and also
19 450 with destruction of H2O by increasing photolysis at higher altitudes. These findings point to a
451 fundamental connection between ice particle characteristics and water vapor.
452 SOFIE has also demonstrated that mesospheric ice exists as one continuous layer extending from
453 ~80 km altitude to the mesopause (~87 km) and often higher, as shown in Figure 12. This view
454 is supported by model predictions [von Zahn and Berger, 2003; Rapp and Thomas, 2006], but is
455 in contrast with previous measurements that were not sensitive enough to detect the most tenuous
456 ice populations. The SOFIE results now confirm a previously suspected but never before
457 directly measured layer of small ice particles believed to be responsible for the high latitude
458 summer phenomenon called Polar Mesosphere Summer Echoes or PMSEs [see e.g. Rapp and
459 Lubken, 2004].
460 These results represent only a few of the rich findings about the properties of PMCs that have
461 come from the first northern hemisphere season of observations. AIM has also just completed
462 observations of the first southern hemisphere season and already conventional ideas about inter-
463 hemispheric differences in cloud properties are being challenged. The mission is providing high
464 quality data on particle size, size variation with altitude, particle shape and its altitude
465 dependence, characteristics that describe the onset and end of the PMC season, the interplay
466 between particles, water vapor and temperature, brightness variability over the entire polar cap
467 region and space and time variability. The unprecedented sensitivity and spatial resolution
468 provided by the AIM instruments is providing a rich data base to address all six AIM objectives
469 and to answer the question of why these clouds form and vary.
470 6. Summary
471 Much has been learned about PMCs from a number of satellite missions [see e.g. Deland et al.,
472 2006]. These studies were all serendipitous in the sense that none were conceived or designed to
20 473 address PMC phenomena. AIM is the first mission dedicated to the exploration of PMCs. All
474 systems designed to run the satellite are functioning as planned except for the command receiver
475 and work arounds have been established for this subsystem. All instruments are functioning
476 nominally. The appropriate mix of measurement capabilities and orbit parameters has been
477 brought together to answer the fundamental question of why these clouds form and vary. The
478 three science instruments are providing data needed to address a series of six objectives
479 formulated by the AIM science team. Initial science results from the mission have significantly
480 advanced our understanding of PMC properties, variability, and spatial distribution after one
481 northern hemisphere season. The presence of voids in the ice layer observed by CIPS appears to
482 be similar to a tropospheric weather phemenon and if true, it adds a potentially new mechanism
483 that must be understood in addressing the question of why these clouds form and vary.
484 Acknowledgements
485 The authors acknowledge the many contributions of expertise, time, talent and dedication given
486 to the mission by the team of engineers, scientists, and managers of organizations supporting
487 AIM at Hampton University, the University of Colorado Laboratory for Atmospheric and Space
488 Physics, the Space Dynamics Laboratory of Utah State University, Orbital Sciences Corporation,
489 GATS, Incorporated, the Goddard Space Flight Center and the Kennedy Space Center. Special
490 thanks is given to the NASA Headquarters AIM Program Executive, Victoria Elsbernd and the
491 AIM Program Scientist, Mary Mellott for their exceptional contributions to AIM through their
492 sincere and extraordinary dedication to the success of this mission right from its inception. We
493 also thank the AIM flight operations team at LASP, Orbital Sciences Corporation and GATS,
494 Inc. for their commitment to excellence and their duty beyond the reasonable at times in order to
495 make the mission succeed. It would not have been possible to present the exceptional data in this
496 and other AIM papers in this special issue without the support of these individuals and
21 497 organizations. Funding for the AIM mission was provided by NASA’s Small Explorers Program
498 under contract NAS5-03132.
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579
580 Tables
581 Table 1. SOFIE Channel Characteristics
582 Table 2. SOFIE Retrieval Characteristics.
583 Table 3. CIPS Requirements and Performance
25 584 Figures
585 Figure 1. Noctilucent cloud photographed by Tom Eklund, July 28, 2001, Valkeakoski,
586 Finland.
587 Figure 2. SOFIE instrument
588 Figure 3. Calculated transmission spectra for key absorbers near the PMC height within the
589 SOFIE water vapor channel wavelength region.
590 Figure 4. CIPS Instrument
591
592 Figure 5. CIPS Field-of-View projected to 83 km. The satellite velocity vector direction is to
593 the right.
594
595 Figure 6. CIPS daily mean image showing coverage for a full day of CIPS images on July 17,
596 2007. FOV overlap from one orbit to the next occurs at ~70N.
597 Figure 7. The 12 active PVDF sensors of CDE. Each patch has a surface area of about 85
598 cm2.The panel is mounted to point towards the local zenith direction at all times,
599 minimizing the impact rates from orbital debris.
600 Figure 8. CIPS daily mean image for June 30, 2007.
601
602 Figure 9. CIPS daily mean image for July 1, 2007.
603
604 Figure 10. Time – height cross section of SOFIE H2O and O3 observations for the 2007
605 northern polar summer. Mesopause altitude (solid line) and the altitude of peak ice
606 extinction (dashed line) are also indicated.
607 Figure 11. SOFIE mesospheric water profiles on August 12, 2007.
26 608
609 Figure 12. SOFIE altitude versus time cross section of ice mass density showing that the
610 mesospheric ice extends over a broad altitude range. The solid line is the
611 mesopause and the dashed line is the location of peak PMC extinction.
Tables
Table 1. SOFIE Channel Characteristics.
Chann Band Target* Center λ G∆V el (µm) 1 O s 0.292 1 3 30 2 O3 w 0.330 3 PMC s 0.867 2 300 4 PMC w 1.037 5 H O w 2.46 3 2 96 6 H2O s 2.618 7 CO s 2.785 4 2 110 8 CO2 w 2.939 9 PMC s 3.064 5 120 10 PMC w 3.186
11 CH4 s 3.384 6 202 12 CH4 w 3.479
13 CO2 s 4.324 7 110 14 CO2 w 4.646 15 NO w 5.006 8 300 16 NO s 5.316 *s indicates strongly absorbing band, w denotes weakly absorbing band
27
Table 2. SOFIE Retrieval Characteristics. Altitude Retrieval Measurement Range Bands 7, Channels 4 T 15 - 110 and 13
Sun sensor, 701 nm T 50 - 100 refraction angle O3 Band 1 (Channel 1) 15 - 105 H2O Band 6 (Channel 3) 15 – 105 CO2 Channels 4 and 7 15 - 95 CH4 Band 11 (Channel 6) 15 – 90 NO Channel 8 80 - 110
PMC Bands 2, 3, 4, 5, 8, 9, cloud extinction 10, 12, 14, and 15
Table 3. CIPS Requirements and Performance Parameter Reqmt. Design Measured Spatial Resolution (nadir) 3.0 km 2.1 km 2.1 km Absolute Accuracy 15% 10% 10% Precision (SNR for Earth 2.0 4.7 5.2 albedo=2x10-5 sr -1)1 1SNR=Signal-to-Noise-Ratio
28 Figures
Figure 1. Noctilucent cloud photographed by Tom Eklund, July 28, 2001, Valkeakoski, Finland.
Figure 3. Calculated transmission spectra for key absorbers near the PMC height within the SOFIE
Figure 2. SOFIE instrument water vapor channel wavelength region.
29
2000 1500
1000
500
0
km Crosstrack, -500
-1000
-1500
-2000 -2500 -2000 -1500 -1000 -500 0 500 1000 1500 2000 2500 Alongtrack, km
Figure 5. CIPS Field-of-View projected to 83 km. The satellite velocity vector direction is to the right. Figure 4 CIPS Instrument
Figure 7. The 12 active PVDF sensors of CDE. Each patch has a surface area of about 85 cm2.The panel is mounted
to point towards the local zenith direction at all times, minimizing the Figure 6. CIPS daily mean image showing impact rates from orbital debris. coverage for a full day of CIPS images on July 17, 2007. FOV overlap from one orbit to the next occurs at ~70N.
30
Figure 8. CIPS daily mean image for June 30, 2007
Figure 10. Time – height cross section of
SOFIE H2O and O3 observations for the 2007 northern polar summer. Mesopause altitude (solid line) and the altitude of peak ice extinction (dashed line) are also indicated.
Figure 9. CIPS daily mean image for July 1, 2007
Figure 11. SOFIE mesospheric water profiles on August 12, 2007
31
Figure 12. SOFIE altitude versus time cross section of ice mass density showing that the mesospheric ice extends over a broad altitude range. The solid line is the mesopause and the dashed line is the location of peak PMC extinction.
32