Bibliography

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\textendash50\% and were observed over an extended longitudinal area. They propagated to middle latitudes (35\textendash40\textdegreeN) in the case of a weak geomagnetic disturbance (K_p = 3-) and to low latitudes (0\textendash10\textdegreeN) 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/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 B_y component of the interplanetary magnetic field in the transport of regions with reduced O/N₂.