Abstract
Abstract. 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 Bz conditions when the magnetic field strength is above 5 nT. Rather few events occurred for small |Bz| (<5 nT) or for positive Bz combined with vanishing By. 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 Bz. 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.
Highlights
The thermospheric mass density and composition experience dramatic changes globally during magnetic storms (e.g. Burns et al, 1995; Forbes et al, 1996; Prolss, 1997)
Interplanetary magnetic field (IMF) and solar wind data are taken from the Advanced Composition Explorer (ACE) spacecraft, which was launched on 25 August 1997
We found one event where the ion upflow features observed by Defense Meteorological Satellites Program (DMSP) match very well the mass density enhancement detected by Challenging Minisatellite Payload (CHAMP)
Summary
The thermospheric mass density and composition experience dramatic changes globally during magnetic storms (e.g. Burns et al, 1995; Forbes et al, 1996; Prolss, 1997). Quite a number of studies have been devoted to the study of thermospheric storm effects (e.g. Liu and Luhr, 2005; Forbes et al, 2005; Sutton et al, 2005; Bruinsma et al, 2006; Burke et al, 2007). They have revealed important responses of the mass density on global scales. An extensive statistical analysis of the cusp-related density anomalies was performed by Rentz and Luhr (2008) They clearly identified the merging electric field as a suitable controlling parameter for the relative enhancement of the density at least during non-storm times. Owing to its low altitude and the coverage of all latitudes and local times, CHAMP is very suitable for a long-term monitoring of ionospheric and thermospheric characteristics
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