Climatology of zonal wind and large-scale FAC with respect to the density anomaly in the cusp region: seasonal, solar cycle, and IMF B_y dependence

Abstract. We investigate the relationship of the thermospheric density anomaly (ρ_rel) with the neutral zonal wind velocity (U_zonal), large-scale field-aligned current (FAC), small-scale FAC, and electron temperature (T_e) using the superposed epoch analysis (SEA) method in the cusp region. The dependence of these variables on the sign of the interplanetary magnetic field (IMF) B_y component and local season is of particular interest. Also, the conditions that lead to larger relative density enhancements are investigated. Our results are based on CHAMP satellite data and OMNI online data of IMF for solar maximum (March 2002–March 2007) and minimum (March 2004–March 2009) conditions in the Northern Hemisphere. In the cusp region the SEA technique uses the time and location of the mass density anomaly peaks as reference parameters. On average, the amplitude of the relative density anomaly, ρ_rel, does not depend on the solar cycle phase, local season, and IMF B_y sign. Also, it is apparent that the amplitude of IMF B_y does not have a large influence on ρ_rel, while the negative IMF B_z amplitude prevailing about half an hour earlier is in good correlation with ρ_rel. Both the zonal wind velocity and the large-scale FAC (LSFAC) distribution exhibit a clear dependence on the IMF B_y sign. U_zonal is directed towards dawn for both positive and negative IMF B_y at all local seasons and for solar maximum and minimum conditions. There is a systematic imbalance between downward (upward) and upward (downward) large-scale FACs peaks equatorward and poleward of the reference point, respectively, for the IMF B_y⁺ (B_y⁻) case. Relative density enhancements appear halfway between region 1 and region 0 currents in closer proximity to the upward FAC region. FAC densities and mass density anomaly amplitudes are not well correlated, but it is apparent that there is a close spatial relationship between ρ_rel and LSFAC. At this point we cannot offer any simple functional relation between these two variables, because there seem to be additional quantities controlling this relation.