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Strong and localized thermospheric mass density events are observed in the\udpolar cap region by the CHAMP satellites at about 400 km altitude during\udgeomagnetic storms. During the 4 years considered (2002–2005) 29 storms with\udDst<−100 nT occurred, in 90% of them polar cap density anomalies were\uddetected. Based on the altogether 56 anomaly events a statistical analysis\udwas performed. The anomalies are of medium scale (500–1500 km) and seem to\udhave a short dwell-time (<1.5 h) in the polar cap. The relative density\udenhancement is found to range around 2 in both hemispheres. The peak density\udis in the Northern Hemisphere by a factor of 1.4 larger than in the southern.\udAlso the number of detected events in the north is twice as large as that in\udthe south (37 vs. 19). Mass density anomalies in the polar cap occur under\udall interplanetary magnetic field (IMF) directions. Numerous strong anomalies\udhave been detected in positive and negative IMF Bz conditions when the\udmagnetic field strength is above 5 nT. Rather few events occurred for small\ud|Bz| (<5 nT) or for positive Bz combined with vanishing By. Some\udof the density anomalies are accompanied by intensive small-scale\udfield-aligned currents (FACs). But about as many show no relation to FACs. If\udFACs are present there, the current density is believed to be correlated with\udthe strength of the IMF Bz. Although this paper concentrates on the\udpresentation of the observations, we show for one event that the ion outflow\udmechanism could be responsible for the mass density anomalies in the polar\udcap

Storm-time related mass density anomalies in the polar cap as observed by CHAMP