Abstract
Abstract. We report on the thermospheric mass density anomaly in the vicinity of the ionospheric cusp. A systematic survey of the anomalies is presented, based on a statistical analysis of 4 years of data (2002–2005) obtained by the accelerometer onboard CHAMP. The anomalies are detected during all years and seasons in both hemispheres but with stronger signatures in the Northern Hemisphere. For the same geophysical conditions, solar flux and geomagnetic activity 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 a factor of 5 while the level of solar flux reduces by a factor of 2. The anomaly strength also depends on the solar wind input. The merging electric field, Emerg, is generally enhanced for about an hour before the anomaly detection. There is a quadratic response of the anomaly amplitude to Emerg. For geophysical conditions of P10.7<150 and Emerg<1 mV/m hardly any events are detected. Their amplitudes are found to be controlled by an additive effect of P10.7 and Emerg, where the weight of Emerg, in mV/m, is by about 50 times higher than that of the solar flux level. The solar zenith angle and the influence of particle precipitation are found to play a minor role as a controlling parameter of seasonal variation. The well-known annual variation of the thermospheric density with a minimum around June also influences the formation of the cusp anomalies. This leads to a clear hemispheric asymmetry with very weak anomalies in the south during June solstice, which is supposed to be a combined effect of the minimum in annual variation and the seasonal decrease of solar insolation in this region.
Highlights
The polar upper atmosphere in the altitude range 100– 1000 km is affected by many different processes
Our aim is to determine the dependence of the cusp-related density anomaly on various parameters which are briefly described in the following
The transit time of we provide an estimate of this effet in the cusp region based ach readingNofrrtohmernth(SeoAutCheErn)loHceamtiiosnphaetreL. 1Dupeoitnotthtoe ltahregemcahgan-ge ofnulml loatditeuldlinnagl wreisduthltast ohaflMf milalxwimaurdme(tFaWl.H, M(1,9c9f.9T)aibnlese1)ction 4.2
Summary
The polar upper atmosphere in the altitude range 100– 1000 km is affected by many different processes. In a case study of 25 September 2000, Luhr et al (2004) showed that the air drag measured along the CHAMP orbit sometimes contains superimposed small-scale features, which can reach almost a factor of 2 above the ambient density under solar maximum conditions These drag peaks coincide well with cusp crossings. According to Luhr et al (2004), these incoming particles are supposed to be associated with field-aligned currents (FACs) These currents may fuel local cusp heating, which can be responsible for air up-welling, leading to density enhancements at higher altitudes. Our approach for making progress in the investigation of the cusp density anomaly is to take advantage of 4 years of continuous CHAMP air drag measurements Within this period, the above mentioned controlling parameters vary considerably and a correlation analysis is expected to reveal significant results.
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