Sounding the upper mesosphere using broadband solar occultation: The SOFIE experiment Larry L. Gordley a , Mark E. Hervig* a , James M. Russell b Chad Fish c , Gregory J. Paxton a , John C. Burton a , Martin J. McHugh a a GATS, Inc., 11864 Canon Blvd., Suite 101, Newport News, VA 23606 b Hampton University, 23 Tyler St., Hampton, VA 23668 c Space Dynamics Laboratory, Utah State University, Logan, UT ABSTRACT The Solar Occultation For Ice Experiment (SOFIE) is scheduled for launch onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite in March 2007. SOFIE is designed to measure polar mesospheric clouds (PMCs) and the environment in which they form. SOFIE will conduct solar occultation measurements in 16 spectral bands that are used to retrieve vertical profiles of temperature, O 3 , H 2 O, CO 2 , CH 4 , NO, and PMC extinction at 10 wavelengths. Thirty occultations are observed each day covering latitudes from 65° - 85°S and 65° - 85°N. The PMC measurements are simultaneous with temperature and gas measurements that are unaffected by PMC signal. This data set will be the first of its kind, and allow new advancements in the understanding of the upper mesosphere. Keywords: Solar Occultation, SOFIE, AIM, Mesosphere, PMC. 1. INTRODUCTION The Solar Occultation For Ice Experiment (SOFIE) is one of three science instruments onboard the Aeronomy of Ice in the Mesosphere (AIM) satellite. The AIM goal is to characterize polar mesospheric clouds (PMCs) and the environment in which they form. PMCs, also known as noctilucent clouds (NLCs), exist near 83 km altitude during polar summer and are visible with the naked eye. PMCs are observed to vary over latitude, between hemispheres, and on seasonal and decadal time scales. Even more intriguing is the mounting evidence for long-term increases in PMC activity. Composed of ice particles 1 , PMCs respond to atmospheric temperature, humidity, and the presence of ice nuclei. The observed increase in PMC frequency and brightness towards the poles is well correlated with decreasing temperatures as the poles are approached. Many observations indicate that PMCs are brighter and more numerous in the northern hemisphere than in the south 2 . Hemispheric PMC differences are attributed primarily to colder temperatures in the north, since mesospheric humidity appears to be nearly identical in the north and south 3 . Decadal variability in PMCs has been correlated with the 11-year solar cycle 4 , during which solar intensity at certain wavelengths can change by a factor of two. Because water vapor is destroyed by solar lyman alpha radiation and increased solar intensity enhances diabatic heating, a warmer and drier mesosphere at solar maximum should result in fewer and dimmer PMCs. These expected relationships were recently quantified using measurements from the Halogen Occultation Experiment (HALOE) 3 . Finally, long-term changes in PMC characteristics are expected to result from anthropogenic climate forcing 5 . Increasing atmospheric carbon dioxide and methane have been linked to decreasing temperatures and higher humidity in the mesosphere. Since these changes are qualitatively consistent with increased PMC activity, PMCs were proposed as a visible indicator of atmospheric change 5 . PMCs are occurring more often, becoming brighter, and are now being sighted at middle latitudes for the first time 6 . Documenting long-term PMC changes and substantiating the connections between PMCs and climate change and has been challenging due to limitations in the observations of PMCs and their environment. While compelling evidence for long-term change exists 7 , open questions concerning the drivers behind PMC formation and variability have roused debate over the interpretation of these findings 8,9 . *m.e.hervig@gats-inc.com; phone 208-354-8887; gats-inc.com Invited Paper Infrared Spaceborne Remote Sensing XIV, edited by Marija Strojnik, Proc. of SPIE Vol. 6297, 62970G, (2006) · 0277-786X/06/$15 · doi: 10.1117/12.682050 Proc. of SPIE Vol. 6297 62970G-1