Electron density distribution in the magnetosphere

We use 8 months of differential potential (ΔV) measurements from POLAR to study the electron density distribution in the magnetosphere and its dependence on the level of geomagnetic activity identified by the Kp index. The ΔV measurement is directly proportional to the electron density, and this technique can be used for detecting fast electron density variations in low-density plasmas with a good accuracy. The inner magnetospheric regions are particularly investigated in this study. The cusp is found to be denser during low Kp, and it moves equator-ward with increasing Kp. The average density in the cusp is approximately 10 cm^-3, which is a little less than the magnetosheath density. The plasmapause is quite asymmetric, as expected. In particular on the nightside, the plasmapause is compressed closer to the Earth with increasing Kp. While the density gradients at the dayside plasmapause are usually not very steep, they can be quite large at other local time sectors. A particularly pronounced sharpening of the plasmapause occurs at the dusk sector with increasing Kp. The density in the region between the dayside plasmapause and magnetopause is relatively high during all Kp levels; the average densities are several electrons per cm³. During disturbed periods, the density in the near-Earth plasma sheet near midnight increases and becomes higher than the densities towards the flanks of the plasma sheet.