Plasmaspheric depletion and refilling associated with the September 25 , 1998 magnetic storm observed by ground magnetometers

The plasmaspheric mass density at was monitored by two IGPP/LANL ground magnetometer stations during the magnetic storm on September 25, 1998. Even at this low latitude the plasma density dropped significantly to of the pre-storm value. The total electron content (TEC) inferred by GPS signals also shows a sizable decrease during the storm. The observations suggest that the convection caused by the strong electric field associated with the magnetic storm eroded the plasmasphere as low as , which is a much lower latitude than that expected from the estimated potential drop across the polar cap together with a simple model of the magnetospheric convection pattern. Introduction In the inner magnetosphere, magnetic flux tubes circulate on closed paths and form a region called the plasmasphere with dense cold plasma compared with the outer magnetosphere. The size of the plasmasphere is determined by a balance between the corotational electric field and that imposed by the solar wind [e.g., Nishida, 1966]. During magnetically disturbed times, the plasmapause moves closer to the Earth because of the enhanced dawn-dusk electric field. In observational aspects, our understanding of the plasmasphere is derived from both satellite measurements [e.g., Chappell, 1972] and whistler propagation [Carpenter, 1966]. More recently, the Faraday-rotation and dispersive-groupdelay techniques are also used to calculate the electron content in the ionosphere and the plasmasphere by receiving signals emitted from satellites. The observations from the Radio Beacon Experiment of the geostationary satellite ATS-6 show clear depletion of electrons in both the plasmasphere and the ionosphere during severe magnetic storms [Degenhardt et al., 1977]. However, since the electron content derived from this type of experiment is along the the slant path of the signal, it does not directly indicate the density at specific latitudes. In this letter, we present a new approach to study the dynamical behavior of the plasmasphere during magnetic storms. The gradient technique uses the principle that the phase difference of field line resonances observed by two closely spaced