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

Strong and localized thermospheric mass density events are observed in the polar cap region by the CHAMP satellites at about 400 km altitude during geomagnetic storms. During the 4 years considered (2002-2005) 29 storms with Dst < -100 nT occurred, in 90% of them polar cap density anomalies were detected. Based on the altogether 56 anomaly events a statistical analysis was performed. The anomalies are of medium scale (500-1500 km) and seem to have a short dwell-time (< 1.5 h) in the polar cap. The relative density enhancement is found to range around 2 in both hemispheres. The peak density is in the Northern Hemisphere by a factor of 1.4 larger than in the southern. Also the number of detected events in the north is twice as large as that in the south ( 37 vs. 19). Mass density anomalies in the polar cap occur under all interplanetary magnetic field ( IMF) directions. Numerous strong anomalies have been detected in positive and negative IMF B-z conditions when the magnetic field strength is above 5 nT. Rather few events occurred for small vertical bar B-z vertical bar (< 5 nT) or for positive B-z combined with vanishing B-y. Some of the density anomalies are accompanied by intensive small-scale field-aligned currents (FACs). But about as many show no relation to FACs. If FACs are present there, the current density is believed to be correlated with the strength of the IMF B-z. Although this paper concentrates on the presentation of the observations, we show for one event that the ion outflow mechanism could be responsible for the mass density anomalies in the polar cap.