The investigation of the high-latitude neutral thermosphere has greatly enhanced with the CHAMP mission. Using onboard-measured accelerometer data we can examine several characteristics of the high-latitude thermosphere. A prominent one is the thermospheric mass density anomaly in the dayside polar cusp region (Lühr et al., 2004). In order to characterize the high-latitude neutral density anomalies measured by CHAMP, we use the relative density variations, ρrel=ρ/ρback (Fig. 1), since this quantity is less dependent on sampling height (CHAMP orbit decayed from 460 to 320km) and from the seasonal scale height variations.
The anomaly feature is best visible in a geomagnetic MLat-MLT (Magnetic Latitude – Magnetic Local Time) coordinate system. Figure 2 shows the statistical properties of the density enhancements, ρrel, which covers 5 years of CHAMP measurements for the three local seasons: winter, combined equinoxes, and summer from left to right (Kervalishvili and Lühr, 2013). The density anomaly peaks appear in every season and their amplitudes show practically no seasonal dependence. However, the spatial distribution of the anomalies is strongly depends on local season and tends to expand from winter to summer more towards the equator. In local time it has a width of about ±2h centered at 12:00MLT. This confined dayside cusp signature of the density anomalies is clearly visible in all three seasons.
In order to investigate the internal relationship between the thermospheric density anomaly and other thermosphere - ionosphere – magnetosphere system variables, we use the superposed epoch analysis (SEA) technique (Kervalishvili and Lühr, 2014). Additionally, the dependence on the total magnetic field Bt and IMF Bx, By, and Bz components are considered using 1min average data downloaded from the OMNIWeb. Figure 3 presents the average latitude profiles of ρrel > ρrelthr = 1.2 values with respect to the reference latitude for 5 years of CHAMP observations in the Northern Hemisphere. The results are shown separately for local winter, combined equinoxes, and local summer, from left to right, for the IMF By positive (red) and negative (blue) values. The reference latitude, determined by the ρrelthr, is presented as zero. On this °MLat scale, negative values represent magnetic latitude in the equatorward direction and positive values towards the pole. The relative amplitude of the cusp density anomaly does not depend on the sign and amplitude of IMF By. Also, it is practically independent of the solar cycle phase and of season. This suggests a feedback on the mechanism causing the upwelling of air that is controlled by the ambient air density.
The neutral density enhancement is an important input concerning upper atmospheric modeling, fuel calculations for spacecraft missions, space debris calculations, and for achieving a better understanding of thermosphere - ionosphere - magnetosphere coupling processes.