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The AIM goal can be characterized by six specific scientific questions. The first five of these deal with mechanisms for PMC formation, i.e., when and where they occur and how they respond to changes in their thermal, chemical and dynamical environments. The AIM mission will answer these five questions directly. The sixth question links PMCs to the larger question of mesospheric climate change. The models we will develop and validate to answer the first five questions will be used to address this last question.

The six objectives are:

1. PMC Microphysics: What is the global morphology of PMC particle size, occurrence frequency and dependence upon H2O and temperature?

2. Gravity Wave Effects: Do gravity waves (GWs) enhance PMC formation by perturbing the required temperature for condensation and nucleation?

3. Temperature Variability: How does dynamical variability control the length of the cold summer mesopause season, its latitudinal extent and possible interhemispheric asymmetry?

4. Hydrogen Chemistry: What are the relative roles of gas phase chemistry, surface chemistry, condensation/sublimation and dynamics in determining the variability of water vapor in the polar mesosphere?

5. PMC Nucleation Environment: Is PMC formation controlled solely by changes in the frost point or do extraterrestrial forcings such as cosmic dust influx or ionization sources play a role?

6. Long-Term Mesospheric Change: What is needed to establish a physical basis for the study of mesospheric climate change and its relationship to global change?


AIM 2011 Highlights:

Outcome 3B.2: Progress in understanding how human society, technological systems, and the habitability of planets are affected by solar variability and planetary magnetic fields.

AIM satellite helps to illuminate why Noctilucent Clouds form and vary
The Aeronomy of Ice in the Mesosphere (AIM) is the first satellite mission dedicated to the study of noctilucent or "night-shining" clouds (NLCs). Since AIM launched in April of 2007, it has provided a detailed view of these clouds over four Northern Hemisphere and four Southern Hemisphere seasons with an unprecedented horizontal resolution of 3 miles by 3 miles. NLCs, also referred to as Polar Mesospheric clouds (PMCs), are of great scientific and public interest because of their possible role as an indicator of global climate change [Thomas, 2003]; yet how they form and why they vary has proven to be difficult to determine.

Dramatic changes occurred in ice clouds near the edge of space
AIM instruments have observed dramatic differences in the Southern summer cloud seasons in 2010 and 2011. The 2011 cloud season began one month later than in 2010 and the atmospheric temperature was about ten degrees warmer.  Also, the ice content at the peak of the season was six times lower in 2011 than in the previous year, and the lowest for all years observed by AIM. The approximate one month delay in the start of the 2011 season appears to be caused by 50 km (30 miles) directly below the clouds, where an enormous system of winds swirling rapidly around the pole lingered longer than usual. The presence of the system prevented the air above, where the clouds form, from cooling down and triggering the start of the season.  The cause of the dramatically low ice content is apparently a pulse of warm air in the upper atmosphere in January about a month and a half after the clouds appeared that engulfed the entire polar cap, causing ice clouds near 83 km to nearly vanish. AIM scientists speculate that the warm temperatures were triggered by changing weather conditions in the opposite (Northern) hemisphere 20,000 km (13,000 miles) away.  This process, known as “teleconnection”, is a relatively recent discovery that is believed to be caused by changes in the intensity of the pole-to-pole atmospheric circulation.  These two anomalies - the one month delay and the low ice content - occurred within two months of one another and both appear to be connected to episodic events far below NLC altitudes in the stratosphere and below, and even across the equator.  These singular events highlight the importance of coupling of physical processes throughout the atmosphere from pole-to-pole and as high as 50 miles.

Karlsson, B., C. E. Randall, T. G. Shepherd, J. Lumpe, K. Nielsen, S. M. Bailey, M. Hervig, J. M. Russell III, On the relationship between southern hemisphere polar mesospheric clouds and the breakdown of the stratospheric polar vortex, Submitted to the J. Geophys. Res., April, 2011.
Thomas, G. E. (2003), Are noctilucent clouds harbingers of global change in the middle atmosphere? Advances in Space Research, Vol. 32, 9, p. 1737-1746. doi:10.1016/S0273-1177(03)90470-4.


Wave motions in the atmosphere extend the length of the length of the season of ice clouds on the edge of space
Noctilucent clouds in the northern hemisphere usually occur between May 15th and August 15th, but sometimes they extend longer than this and past the time when the average atmospheric temperatures around a latitudinal belt are cold enough to allow them to form. Studies were recently completed by AIM science team members that investigated wave motions in the atmosphere that might create cold spots in the troughs of the waves which would permit ice clouds to form locally even though the average temperature is too warm. Studies carried out using AIM cloud data and the Navy Operational Global Atmospheric Prediction System–Advanced Level Physics High Altitude (NOGAPS-ALPHA) analysis fields for the summer 2007 showed that significant amplitudes in the temperature wave of up to ~ 6K occurred, which was more than sufficient to permit the growth of ice clouds.  At the same time, wave motion induced changes in water vapor, another key ingredient for cloud formation, were very small throughout the season. These studies were materially aided by another Great Observatory experiment, called SABER on the TIMED satellite, which was used along with the MLS instrument on the Aura satellite to drive the model.  The model temperatures and AIM data showed a clear anti-correlation, with bright PMCs forming in the trough of the temperature wave. The late season extension of cloud occurrence due to these wave motions may explain previous ground-based reports of bright noctilucent cloud displays in August when the season was expected to be over.

Nielsen,K., D. E. Siskind, S. D. Eckermann, K. W. Hoppel,3 L. Coy, J. P. McCormack, S. Benze, C. E. Randall, and M. E. Hervig, Seasonal variation of the quasi 5 day planetary wave: Causes and consequences for polar mesospheric cloud variability in 2007, J. Geophys.  Res., , VOL. 115, D18111, doi:10.1029/2009JD012676, 2010.


Ice clouds on the edge of space appear to be similar to clouds in the lower atmosphere
AIM CIPS data have recently been used to show that polar mesospheric clouds are fractal in nature. That is, just like snowflakes, cauliflower, or many kaleidoscope displays, PMCs have geometric shapes that repeat on smaller and smaller scales. From a purely aesthetic viewpoint, their fractal nature can perhaps explain why PMCs are so visually appealing and intriguing. From a more technical viewpoint, understanding the precise mathematical description of the PMC fractals can tell us about the forces that control their growth and decay. von Savigny et al. (2010) found that the fractal nature of PMCs seems to be very similar to that of tropospheric clouds and rain fields, possibly suggesting that similar physical processes govern the distribution and shape of both tropospheric clouds and PMCs. This finding represents an important advance in the quest to understand why NLCs form and vary.

von Savigny, C., L. A. Brinkhoff, S. M. Bailey, C. E. Randall, and J. M. Russell III, First determination of the fractal perimeter dimension of noctilucent clouds, Geophys. Res. Lett., 38, L02806, doi:10.1029/ 2010GL045834, 2011.


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