Coordinated observations of electron density enhancements in the polar magnetosphere and ion upflows in the ionosphere during geomagnetic storms

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 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 (~10^10 /cm^2/s) in and near the cusp, when the electron density enhancements were observed by the Akebono satellite. The regions of the ion upflows with large fluxes almost coincided with those of intense soft electron precipitations. In the polar cap far from the cusp, ion upflows were much weaker and its flows were occasionally directed downward. The coordinated observations by the DMSP and Akebono satellites indicate that the enhanced plasma was provided by cleft ion fountain mechanisms from the polar ionosphere. The thermal energy component 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 thermal plasma may flow out through the polar magnetosphere during the main phase of geomagnetic storms.