Low Latitude and High Altitude PMCs Detected by Odin/OSIRIS in 2005-2006

The Optical Spectrograph and InfraRed Imager System (OSIRIS) instrument on the Odin satellite has observed Polar Mesospheric Clouds (PMCs) in both hemispheres since November, 2001. The latitudinal coverage of Odin is typically from 82.2 N to 82.2 S. At PMC altitudes, OSIRIS is sensitive to even very small radiance enhancements, typically 40-50% larger than the corresponding Rayleigh background. OSIRIS PMCs are detected automatically and also manually; this results in the finding of a large number of faint clouds that are disregarded by automatic detection methods. Several faint PMCs observed by OSIRIS between 88 and 90 km suggest either a very strong dynamic activity at those locations and times, or a possibility for formation of 10-15 nm particles at altitudes several kilometers higher than expected. The analysis of all NH low latitude, equatorward of 50 degrees, PMCs measured in 2002 - 2005 suggests that there is an anticorrelation between the PMC formation at low latitudes and annual mean PMC brightness at higher latitudes. In addition, the number of low latitude, equatorward of 50 degrees, PMC detections in the SH during the 2005/2006 season is comparable to that for the NH-2005 season. This contradicts the previously observed and modeled asymmetry of mesospheric properties between the two hemispheres and encourages related theoretical and experimental studies.