Electron density and conductance in the auroral ionosphere during substorms. Influence of solar wind and plasma sheet state.

Enhanced precipitation of magnetospheric energetic particles during substorms increases ionospheric electron density and conductance. Such enhancements, which have timescales of a few hours, are not reproduced by the current ionospheric models. Using EISCAT (Tromso) measurements we reconstruct the substorm related response of electron densities and conductances in the ionosphere with respect to the intensity of substorm injections. We also investigate how the intensity of the response is influenced by the variations of the plasma sheet high energy (tens keV) fluxes and solar wind state. To characterise the intensity of substorm injection at a 5min time step we use the midlatitude positive bay (MPB) index which basically responds to the substorm current wedge variations. We build response functions (LPF filters) between T0-1h and T0+4hrs (T0 is a substorm onset time) in different MLT sectors to estimate the magnitude and delays of the ionospheric density response at different altitudes. The systematic and largest relative substorm related changes are mostly observed in the lowest part of E and in D regions. It starts and reaches maximum magnitude near midnight, from which it mainly propagates toward east, where it decays when passing into the noon-evening sector. Such MLT structure corresponds to the drift motion of the injected high energy electron cloud in the magnetosphere. Besides the injection intensity, we look at how the magnitude of the response depends on the energetic (tens keV) fluxes level in the plasma sheet before the substorm onset. We use a previously developed empirical model of the plasma sheet fluxes with solar wind parameters as inputs to count the plasma sheet fluxes with energy 10, 31 and 93 keV in the reference point of transition region (6 Re, 270o SM Long). We found that during enhanced high energy fluxes in the plasma sheet before the substorm (fast solar wind, high solar wind reconnection electric field) the background ionisation in the ionosphere, as well as the peak ionisation value during the substorm, are higher. This implies that together with substorm intensity the prehistory of the plasma sheet/solar wind state forms the magnitude development of substorm related ionospheric response. Research was supported by Russian Ministry of Science and Higher Education grant № 075-15-2021-583