Aeronomy of Ice In the Mesosphere

Project Description
AIM is the first satellite mission dedicated to the study of noctilucent or “night-shining” clouds (NLCs) also called Polar Mesospheric clouds (PMCs). It has provided the first global-scale view of the clouds over the entire 2007 Northern Hemisphere season with an unprecedented resolution of 5 km by 5 km and is nearing completion of observations in the Southern Hemisphere season. Despite a significant increase in PMC research in recent years, relatively little is known about the basic physics of these clouds at ”the edge of space” and why they are changing. They have increased in brightness over time, are being seen more often and appear to be occurring at lower latitudes than ever before. The overall goal of the baseline mission is to determine why PMCs form and vary. Since the launch of AIM on April 25, 2007, significant progress has been made in achieving this goal and that progress continues at a rapid rate. The AIM data is of very high quality and has changed our view of PMCs and their environment after only one northern hemisphere (NH) season of observations. The startling similarity between the PMC structure observed by CIPS and that seen in tropospheric clouds suggests that the mesosphere may share some of the same dynamical processes responsible for weather near Earth’s surface. If this similarity holds up in further analysis, it introduces an entirely different view of potential mechanisms responsible for PMC formation and variability.

AIM has provided the most detailed picture of NH clouds ever collected:

- The clouds appear every day, are widespread and are highly variable on hourly to daily time scales.
- PMC brightness varies over horizontal scales of a few kilometers, and because of the AIM high horizontal resolution, we now know that over small regions the clouds are ten times brighter than measured by previous space-based instruments.
- A previously suspected, but never before seen, population of very small ice particles was measured that is believed to be responsible for strong radar echoes from the summertime mesosphere.
- Mesospheric ice occurs in one continuous layer extending from below the main peak at 83 km up to around 90 km.
- Mesospheric cloud structures, resolved for the first time by the CIPS imager, exhibit complex features present in normal tropospheric clouds.