| Summary: | We focus on total mass density enhancements in the vicinity of the polar cusp which have been frequently sampled by the CHAMP satellite in about 400 km altitude. In a statistical analysis these cusp-related density anomalies are found to be a continuous phenomenon in the dayside auroral regions of both hemispheres which is partly driven by the strength of the solar activity (P10.7), but more directly by the energy input of the solar wind (merging electric field). The atmospheric fountain effect in the polar cusp region causes the locally confined density anomaly. Its generation mechanisms are investigated in a combined CHAMP-EISCAT campaign, a model studies on soft particle precipitation, and a harmonic analysis of the density anomaly and of potential controlling parameters. The mechanism is suggested for the density anomaly: There is an energy input by the solar wind, which provides the power for the Joule heating of preferably neutral molecules. Simultaneous soft particle precipitation in the cusp increases the height of maximal Pedersen conductivity, thus lifting up the heated layer in the cusp. The density anomaly is then partly caused by a local composition change as a result of the enhanced heavier particle scale height.
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