Effects of solar wind ultralow-frequency fluctuations on plasma sheet electron temperature: Regression analysis with support vector machine

To investigate whether ultralow-frequency (ULF) fluctuations from 0.5 to 8.3 mHz in the solar wind and interplanetary magnetic field (IMF) can affect the plasma sheet electron temperature (T-e) near geosynchronous distances, we use a support vector regression machine technique to decouple the effects from different solar wind parameters and their ULF fluctuation power. T-e in this region varies from similar to 0.1 to 10 keV with a median of 1.3 keV. We find that when the solar wind ULF power is weak, T-e increases with increasing southward IMF B-z and solar wind speed, while it varies weakly with solar wind density. As the ULF power becomes stronger during weak IMF B-z (similar to 0) or northward IMF, T-e becomes significantly enhanced, by a factor of up to 10. We also find that mesoscale disturbances in a time scale of a few to tens of minutes as indicated by AE during substorm expansion and recovery phases are more enhanced when the ULF power is stronger. The effect of ULF powers may be explained by stronger inward radial diffusion resulting from stronger mesoscale disturbances under higher ULF powers, which can bring high-energy plasma sheet electrons further toward geosynchronous distance. This effect of ULF powers is particularly important during weak southward IMF or northward IMF when convection electric drift is weak.