Temporal variations and spatial extent of the electron density enhancements in the polar magnetosphere during geomagnetic storms

Abstract

In order to understand temporal variations and spatial distributions of plasma density enhancements in the polar magnetosphere during geomagnetic storms, nearly simultaneous observations of storm time electron densities in the polar magnetosphere by the Akebono satellite and ion upflows in the polar ionosphere by the DMSP satellites were investigated. Akebono observations show that the electron densities were highest (>100 cm−3 at ∼9000 km altitude) from the main to early recovery phases of geomagnetic storms. The regions of enhanced electron density were not localized but widely spread in the polar magnetosphere. Coordinated observations by the DMSP satellites detected ion upflows with large fluxes (∼1010/cm2/s) in and near the cusp, when the electron density enhancements were observed by the Akebono satellite. This result indicates that the storm time electron density enhancements are caused by cleft ion fountain mechanisms from the polar ionosphere. Very low‐energy component (<13 eV) of the cleft ion fountain drifted deep into the polar cap, and increased the plasma densities in a wide region of the polar cap and auroral zone. A large amount of the very low‐energy plasma may flow out through the polar magnetosphere during the main phase of geomagnetic storms.