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
• 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.
AIM has been selected for five previous extended missions following the 2-year Explorer baseline mission. As noted in the 2017 Senior Review (SR) proposal, AIM has evolved to become a global mission. That proposal began capitalizing on this transition with three Prioritized Science Goals (PSGs). PSG-1 examined GW origins, how dynamical variability in the lower atmosphere couples to geospace weather and how extraterrestrial phenomena propagate into the lower atmosphere. PSG-2 studied the roles of solar and anthropogenic forcing on PMCs and the structure of the mesosphere. PSG-3 investigated the geographic and temporal distributions of mid-latitude PMCs and how they are influenced by waves and tides, but this was exploratory in nature due to the low signal-to-noise for PMCs at mid-latitudes. For PSG-1, significant progress was made developing new GW products to identify global GW “hotspots”. For PSG-2 we discovered that surprisingly, the solar cycle in PMCs is weak or absent after ~2004, in stark contrast to predictions and observations of 20 years ago. We suggest that this is due to a reduced solar cycle response in water vapor based on analysis of SOFIE H2O and other satellite measurements. It has also been suggested that this phenomenon is due to a threshold effect whereby the recent solar cycle was too weak to trigger a variation. Either way, this conundrum is a key focus of our 2020 SR research. Finally, for PSG-3 we have begun expanding the CIPS PMC coverage to include mid-latitude NLCs. This effort paid off with CIPS observations of the unusual NLC outbreak in early June 2019 over Colorado and adjoining states. This event received public attention and has considerably increased interest in the question of mid-latitude NLC change. Ongoing studies show that the 2019 NLC ice mass at 30-50°N latitude in the first two weeks of June was the highest since 2002. Large uncertainty remains regarding whether mid-latitude NLCs are increasing or spreading to lower latitudes.
The extension from October 2020 through September 2023 will provide data to address key outstanding Science Objectives (SO) framed as questions:
Table 1 AIM Extended Mission Science Objectives (SOs)
Why More Data are Required
SO-1: What is the morphology of gravity waves entering the mesosphere and lower thermosphere?
Offers new opportunities to study GW impacts on the mesosphere and thermosphere with all-season, near global coverage and continuous multi-year GW data at ~50 km. Synergy with new GOLD data and the upcoming ISS/AWE mission.
SO-2: How does planetary wave activity influence PMCs and composition in the mesosphere?
Capitalize on the potential for synergy between the ICON low latitude wave data with AIM high latitude PW and auroral region observations. Extend statistics on key coupling events (such as SSWs) that may only occur once every two or three years.
SO-3: How does anthropogenic and extraterrestrial forcing impact the polar mesosphere?
New data are required to investigate the lack of a mesospheric solar cycle response in recent years, understand how rising greenhouse gases impact mesospheric conditions and PMCs, and quantify variations in meteoric influx.