Longitude variations in ion composition in the morning and evening topside equatorial ionosphere near solar minimum

Abstract

Ion composition data from the Defense Meteorological Satellite Program (DMSP) F10 have been averaged by geographic longitude and dip latitude for the months of June, September, and December 1993. The data were taken under near solar minimum conditions. Near 800 km at two fixed local times near 0920 hours and 2120 hours, and at all longitudes, significant variation in local time and season are found. Longitude variations are consistent with modulation of the F peak height by meridional and zonal neutral winds. The components of these winds parallel to the magnetic field lines act to raise and lower the height of the F peak and, additionally, at night, to modulate the plasma decay rate. Zonal winds were found to have significant effects in the longitude regions 150°E to 270°E and 300°E to 360°E, where the magnetic declination is significant. Under solstice conditions, the summer to winter meridional winds play a dominant role in regulating the F peak height, with the zonal winds enhancing or opposing the effects of the meridional winds at longitudes with significant magnetic declination. Zonal winds dominate the regulation of the F peak height near equinox, when the meridional winds are fairly symmetric about the dip equator. The longitude variations are most clearly seen in the O+ and H+ concentrations when O+ is the dominant ion and is in equilibrium with H+. These conditions were found during the daytime during all seasons. H+ is frequently the dominant ion near 800 km, and at night, the longitudinal variations clearly seen in the O+ concentrations were not as easily seen in the H+ concentrations due to the larger scale height of H+.