Influence of Solar Wind on Source of Ring Current Plasma

Abstract In the present study we aim to derive empirical equations relating source plasma of the ring current to the solar wind. We used the energy spectra at energies of 9-135 keV obtained by the suprathermal ion composition spectrometer (STICS) sensor of the energetic particle and ion composition (EPIC) instrument on the Geotail spacecraft. The plasma parameters (i.e., number density and temperature) of H + , O + , and He + were estimated by fitting the K-distribution function to the energy spectra in the region of a geocentric distance of 8.5–10.5 R e and magnetic local time of 2200-0200 hour. The results showed that the H + number density in the plasma sheet correlated with the solar wind density, while the O + and He + number density had no correlation with the solar wind parameters. Thus the origin of H + ions in the plasma sheet is thought to be the solar wind. O + and He + ions in the plasma sheet are expected to have different origin from the solar wind. It was also found that the temperature of H + , O + and He + has a good correlation with the solar wind velocity and that gradients of the derived empirical equations can be ordered by ion mass. This implies that ions are accelerated in a mass-dependent way.