Response of the topside ionosphere to recurrent geomagnetic activity

In the present study we investigate the solar activity, local time, and latitudinal dependence of the topside ionosphere response to recurrent geomagnetic activity, using 8 years (1998-2005) of data on total ion density (Ni) retrieved from Defense Meteorological Satellites Program observations at about 840 km altitude. It is the first attempt to explore the presence of oscillations in the topside ionosphere in response to recurrent geomagnetic activity. Results indicate that striking periodic oscillations around 9 days in Ni are present during the latter part of the declining phase of solar cycle 23. The percentage of the magnitude of 9 day oscillations relative to background values in Ni at 2130 LT tends to be 5%-10% larger than at three other local times (0930, 0510, and 1710 LT). Moreover, latitudinal profiles of bandpass-filtered 9 day perturbations in Ni show multiple peaks as a function of local time and day of the year, which is distinctly different from previous results at or around the F2 layer peak. There are in-phase correlations between Kp and the Ni responses in the winter hemisphere and at middle and low latitudes, and approximately anti-phase at high latitude in the summer hemisphere. Additionally, the 9 day oscillations in ion temperature are is mainly restricted to high latitudes. The combined effects of neutral winds, E x B drift, auroral ionization precipitation, and migration of subauroral trough region are possible candidates to explain the latitudinal profiles of the 9 day variations in Ni.