Authors: John B. Sigwarth; Geoffrey Crowley
Affiliation: The University of Iowa; Southwest Research Institute
During the period of April 14-24, 2002, a series of solar disturbances began several chains of events that influenced geospace. Earth’s thermosphere is one endpoint for these chains. The Polar spacecraft was in a unique position to observe the end effects on the thermosphere. During April 2002, the 8 RE apogee altitude of the Polar orbit was near the equator at approximately 1000 local time. The ultraviolet sensitive Earth Camera of the Visible Imaging System on the Polar spacecraft acquired global images of Earth’s dayglow through a broadband filter at FUV wavelengths. For this filter, percentage variations relative to quiet times in dayglow intensity across the dayside hemisphere are related to variations in the O/N2 column density ratio of the thermosphere. An initial small decrease in the O/N2 ratio was observed on April 17, 2002. The O/N2 ratio reached a maximum depletion of 45% during the magnetospheric storm on April 19, 2002 at 1907 UT. Full recovery of the O/N2 ratio followed in less than 36 hours. The O/N2 depletions were located in the polar regions and extended to mid-latitudes in both the northern and southern hemispheres. These depletion regions were highly structured and differed significantly between the hemispheres. The variability of the solar EUV photon flux can be observed in the hemisphere-integrated dayglow intensities and must be taken into account in order to obtain accurate retrieval of the O/N2 ratios. Discussion will concentrate on the north-south asymmetry of the depletion regions and the variations of the observed O/N2 ratio in the thermosphere as compared with the model results from the TIMEGCM.