Geomagnetic disturbances at high latitudes during very low solar wind density event

Abstract

In this letter, we report geomagnetic observations over Greenland and Antarctica during May 11, 1999, when the solar wind almost disappeared. Greenland magnetometers show no magnetic activity in the nominal auroral zone but disturbances reached ∼200 nT at higher latitudes; however, conjugate observations in Antarctica show much weaker activity ∼40 nT. Analyzed data provide evidence that the cusp current system could be extended far to the dusk (dawn) in the northern (southern) polar region. This is consistent with the Ørsted satellite observations, where field‐aligned currents were detected near cusp, over the northern geomagnetic pole, and in the nightside polar cap; however, almost no were detected over the southern polar cap. We conclude that the low‐density solar wind quieted activity at auroral latitudes (”loading‐unloading” processes); but the “directly‐driven” convection observed over both polar caps was mainly unaffected by the significant inflation of the magnetosphere.