Plasmaspheric depletion and refilling associated with the September 25, 1998 magnetic storm observed by ground magnetometers at L = 2

The plasmaspheric mass density at L ≃ 2 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 ≃ 1/4 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 L = 2, 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.