Climatology of the cusp-related thermospheric mass density anomaly, as derived from CHAMP observations

摘要

We report on the thermospheric mass density anomaly in the vicinity of theionospheric cusp. A systematic survey of the anomalies is presented, based ona statistical analysis of 4 years of data (2002–2005) obtained by theaccelerometer onboard CHAMP. The anomalies are detected during all years andseasons in both hemispheres but with stronger signatures in the NorthernHemisphere. For the same geophysical conditions, solar flux and geomagneticactivity the anomalies in the north are larger by a factor of about 1.35.Over the course of the survey period the amplitude decreases by more than afactor of 5 while the level of solar flux reduces by a factor of 2. Theanomaly strength also depends on the solar wind input. The merging electricfield, E_merg, is generally enhanced for about an hour before the anomalydetection. There is a quadratic response of the anomaly amplitude toE_merg. For geophysical conditions of P10.7<150 and E_merg<1 mV/mhardly any events are detected. Their amplitudes are found to becontrolled by an additive effect of P10.7 and E_merg, where the weight ofE_merg, in mV/m, is by about 50 times higher than that of the solarflux level. The solar zenith angle and the influence of particleprecipitation are found to play a minor role as a controlling parameter ofseasonal variation. The well-known annual variation of the thermosphericdensity with a minimum around June also influences the formation of the cuspanomalies. This leads to a clear hemispheric asymmetry with very weakanomalies in the south during June solstice, which is supposed to be acombined effect of the minimum in annual variation and the seasonal decreaseof solar insolation in this region.