Solar wind dependence of electric conductances and currents in the auroral zone

Abstract

Based on 20 years-long data base of EISCAT incoherent scatter radar and IMAGE magnetometer observations in Scandinavia, we investigate statistically the ionospheric conductance variations in the dark nightside auroral zone. We focus on the relationship of precipitation-caused conductances with the variations of local equivalent current and global AL index, as well as on their dependence on the solar wind (SW) parameters. In terms of paired correlation, the main SW drivers for AL index and for the Pedersen and Hall conductances (ΣP and ΣH) are the SW merging electric field (characterized, e.g., with the Kan-Lee function, Ekl) and the solar wind velocity Vsw. The relative importance of these SW drivers varies. Whereas Ekl is the main driver of AL index, the role of Vsw increases for the conductances so that it outruns the Ekl as the main driver for the Hall conductance. Quantitatively this dependence is represented as ΣH=(7.7*V+1.75*V2)+(5.7*E−0.86*E2)−6.1 Siemens, where E and V are Ekl and Vsw normalized with <Ekl>=0.79 mV/m and <Vsw>=429 km/s. The strongest influence of Vsw is, however, observed for the Hall-to-Pedersen conductance ratio RHP=ΣH/ΣP, indicating solar wind velocity control of the electron acceleration. Physically the energization is a major factor which contributes to the large conductance values. On the nightside, local equivalent currents are significantly controlled by the local Hall conductance (CC = 0.78) and most of the equivalent current increase during active periods is due to the conductivity change. In that sense the AL index variations during active times are controlled by the Hall conductance variations which, to a large extent, are controlled by the processes of magnetospheric electron acceleration.