Statistical analysis of thermospheric density response to solar wind sector structure

Abstract

AbstractThe global averaged thermospheric mass density data during 1967–2007 are analyzed to investigate thermospheric response to solar wind sector structure. Well‐defined solar wind sectors have polarities of interplanetary magnetic field (IMF) toward (+Bx, −By) or away (−Bx, +By) from the Sun for multiple days. In March, thermospheric mass densities increase from the away to toward polarities, with a maximum density perturbation of ~23% at 400 km with respect to the 11 day averages; they decline from the toward to away polarities, and the maximum reduction at 400 km can reach 12%, which is associated with a weakened heating effect of the geomagnetic activity. In September, thermospheric densities respond in an opposite way to the same sector structure as compared with the March results. In solstice seasons, thermospheric density variations in response to solar wind sector structure are typically smaller than 10% relative to the 11 day averages. Besides the seasonal dependence, relative density changes tend to become greater at higher altitude and lower solar activity. However, during solar minimum the density variations at 550 km are not substantially larger than those at 400 km due to the possible descending of the transition altitude between helium and atomic oxygen. Moreover, the corotating interaction region (CIR) has a high probability to occur around the sector boundary. Consequently, the CIR effects account for one third of the density enhancement at 400 km during ineffective‐effective sectors, whereas the density reduction associated with effective‐ineffective sectors can be impaired by about 50%, which is independent of altitude.