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Abstract

Field-aligned currents (FACs) are an important mechanism for transporting energy and momentum from the solar wind into the upper atmosphere at high latitudes. The efficiency of energy dissipation depends on the ionospheric conductivity and the transverse scale size of the field-aligned current circuits. According to Vogt (2002) the conversion of electric current into heat should be most effective for scales of the order of some 10 km when typical ionospheric conductivities are considered. In order to test this conclusion we have performed a statistical analysis with FACs of various scale sizes. This study is based on observations that are derived from CHAMP magnetic field measurements taken during the years 2002-2005. FAC densities within the medium-scale wavelength band of 15-150 km and large-scale FACs (>150 km) are considered separately and their distributions are compared. The thermospheric mass density recorded simultaneously by CHAMP is used as a measure for the heat deposited in the ionospheric E-layer. The relation between current strength of the two FAC types and thermospheric heating is determined in a superposed epoch analysis. The analysis is performed separately for each season and covers all local time sectors.