The Aeronomy of Ice in the Mesosphere Mission: Science results after five PMC seasons

The Aeronomy of Ice in the Mesosphere (AIM) mission was launched from Vandenberg Air Force Base in California at 1:26:03 PDT on April 25, 2007 becoming the first satellite mission dedicated to the study of Polar Mesospheric Clouds (PMCs). A Pegasus XL rocket launched the satellite into a near perfect 600 km, noon - midnight, sun synchronous orbit. The baseline two-year mission ended in May 2009 and the program is now in an extended mission phase currently approved for operations through September 2012. The AIM goal is to understand the fundamental mechanisms that cause PMCs to form and vary. Six objectives, stated in the form of questions, were established including: 1) What is the global morphology of PMC particle size, occurrence frequency, and dependence on H2O and temperature?; 2) What are the effects of gravity waves on PMC nucleation, growth, and/or dissipation?; 3) How does dynamical variability control the length of the cold summer mesopause season, its latitudinal extent and possible interhemispheric asymmetry?; 4) What are the relative roles of gas phase chemistry, surface chemistry, dynamics and condensation/sublimation in determining the abundance and variability of water vapor in the polar mesosphere?; 5) 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?; and 6) What is needed to establish a physical basis for the study of mesospheric climate change and its relationship to global change? AIM carries three instruments to address these objectives - a nadir imager, a solar occultation instrument and an in-situ cosmic dust detector. This paper will provide a brief mission overview, instrument descriptions and scientific findings. Results from the first five seasons of AIM observations show that the PMC season turns on and off like a “geophysical light bulb” transitioning at the season start from no clouds to 100% occurrence frequency in days and vice versa at the season end. CIPS images show that PMCs are highly variable from orbit-to-orbit and day-to-day with significant complex structure (Fig. 1). Data show that temperature change is a dominant factor in controlling season onset, variability during the season and season end. Rising water vapor levels at the beginning and falling values at the end also play a key role in season initiation and cessation. Structures seen in the clouds look very much like complex features seen in tropospheric clouds including large regions of near circular ice voids. Planetary waves modulate PMC occurrence and can effectively extend the period of PMC occurrence by providing several days of localized regions of saturated air in the trough of the wave. By contrast, gravity waves appear to locally diminish PMC frequency even though gravity wave drag on a global basis is acknowledged as the prime cause of the polar summer mesopause. Finally, AIM results provide new evidence that interhemispheric coupling, from the winter hemisphere to the summer hemisphere affects PMC variability. This paper will also describe the first satellite observations of cosmic smoke (micrometeoroid ablation residue) input to the atmosphere measured by the SOFIE instrument.