Large variations in the thermosphere and ionosphere during minor geomagnetic disturbances in April 2002 and their association with IMF B y

Abstract

We investigate the variations in the thermosphere and ionosphere using multi-instrument observations during the April 2002 period, with a particular focus on periods during small geomagnetic disturbances. Large and long-lasting reductions in the daytime electron density were observed at midlatitudes by incoherent scatter radars, ionosondes, and GPS receivers. These reductions reached 30-50% and were observed over an extended longitudinal area. They propagated to middle latitudes (35-40°N) in the case of a weak geomagnetic disturbance (Kp = 3-) and to low latitudes (0-10°N) in the case of a stronger disturbance (Kp = 5-). Data from the GUVI instrument aboard the TIMED satellite reveal a reduction in the daytime O/N2 ratio in the coincident area. Similar decreases are also predicted by the TIMEGCM/ASPEN model in both O/N2 ratio and electron density, though the magnitude of the decrease from the model is smaller than observed. We suggest that these ionospheric and thermospheric disturbances result from high-latitude energy input and efficient transport of regions with reduced O/N2 to lower latitudes. We discuss the possible role of a strong positive By component of the interplanetary magnetic field in the transport of regions with reduced O/N2. Copyright 2006 by the American Geophysical Union.