Correlation of PMC relative brightness and altitudes observed by Odin/OSIRIS in the Northern Hemisphere in 2002–2003

The Optical Spectrograph and Infrared Imager System (OSIRIS) instrument onboard the Odin satellite detects polar mesospheric clouds (PMCs) using limb scattered sunlight and is able to routinely measure the relative brightness and altitude of each detected cloud. OSIRIS has created a significant database of PMCs detections and cloud parameters collected during two intensive campaigns in the northern hemisphere summer months of 2002 and 2003 (days 9–24 from solstice). These observations have revealed a definite anti-correlation between the cloud's relative brightness and altitude. We have measured that, on average, the PMCs descend by 22.5 m for every unit increase in relative brightness. No systematic difference has been observed between the tangent altitudes of the morning (∼07:30 local time) and afternoon (∼17:30 local time) PMCs but the morning clouds were found to be at least 30% brighter than those detected in the afternoon. We have also found a strong 10–14 day oscillation in the global PMC relative brightness in both 2002 and 2003 that is possibly related to a dynamical activity within the mesosphere.