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

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 ( K p = 3−) and to low latitudes (0–10°N) in the case of a stronger disturbance ( K p = 5−). Data from the GUVI instrument aboard the TIMED satellite reveal a reduction in the daytime O/N 2 ratio in the coincident area. Similar decreases are also predicted by the TIMEGCM/ASPEN model in both O/N 2 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/N 2 to lower latitudes. We discuss the possible role of a strong positive B y component of the interplanetary magnetic field in the transport of regions with reduced O/N 2 .