The dependence of plasma density in the topside ionosphere on the solar activity level

Abstract. In this paper, the ten-year (1996–2005) total ion density N_i measurements from the Defense Meteorological Satellite Program (DMSP) spacecraft in the morning and evening (09:30 and 21:30 LT) sectors have been analyzed to explore the dependence of plasma densities in the topside ionosphere at middle and low latitudes on the solar activity level. Results indicate that there is a strong solar activity dependence of DMSP N_i at 848 km altitude, which has latitudinal and seasonal features. The plasma density in the topside ionosphere has an approximately linear dependence on daily F107 and a strongly nonlinear dependence on SEM/SOHO EUV, such that the change rate of N_i becomes greater with increasing solar EUV. This is quite different from the dependence of N_i near the F-Region peak (NmF2), at which the rate of change of NmF2 decreases with increasing solar EUV. The rate of change of N_i at the DMSP altitude is greatest in the latitude range where N_i is greatest during high solar activity. We suggest that this greater rate of change (or amplification effect) of N_i at the DMSP altitude is mainly a consequence of the solar activity variations of the topside scale height. The changes in the height of the F-Region peak (hmF₂) and the density NmF2 play a secondary role.