Ionospheric Electron Density and Conductance Changes in the Auroral Zone During Substorms

Enhanced precipitation of magnetospheric energetic electrons during substorms increases ionospheric electron density and conductance. Such enhancements, which have timescales of a few hours, are not reproduced by the existing ionospheric models. We use the linear prediction filter (LPF) method to reconstruct the substorm-related response of electron densities and integral conductances from long-term ionospheric observations made by the European Incoherent SCATer radar located at Tromso. To characterize the intensity of substorm dipolarization at a 5 min time step, we use the midlatitude positive bay index. We build response functions (LP filters) as a function of substorm time between T-0-1 h and T-0 + 4 h (T-0 is a substorm onset time) in different magnetic local time (MLT) sectors to estimate the magnitude and delays of the ionospheric density response at different altitudes. Systematic and large relative changes are mostly observed in the D- and E regions. The duration of the response is about 3 h. It starts and reaches maximum magnitude near midnight, propagating from there toward the east and decaying after passing into the noon-evening sector. The reliability of LPF results is confirmed by the consistency of D-region response with independently derived response of the auroral absorption. Whereas strong ionization increases are seen in both E- and D-regions on the nightside, the D-region response is stronger in the morning-dayside sector. Such MLT variation corresponds to the drift motion and precipitation of the high-energy electrons injected in the nightside magnetosphere during substorm dipolarization. The inferred ionization changes result in strong enhancements of integral Hall (and Pedersen) conductance in the nightside auroral zone, where intense auroral currents are known to occur during substorms