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We report on the unexpected detection of considerable structure in highlatitude thermospheric densities, as derived from an accelerometer onboardthe CHAMP satellite. The width of the structures, which can either be maximaor minima, varies between a few hundred km and 2000 km. The amplitudes ofthese density extrema can reach 50% of ambient. Maxima cluster around75° (N and S), while minima are found closer to the poles. In a magneticlatitude-magnetic local time frame the maxima are found mainly around thecusp region. Overall, the observed structures somewhat resemble so-calleddensity cells previously found in model calculations. However the modelsgenerate their cells around 140–300 km altitude and show little, if anyremnant at 400 km or above. This has to be contrasted with the fact that theCHAMP observations were obtained near 430 km altitude. We have exploredJoule heating as a possible mechanism for the generation of the structures,at least in density enhancement regions, using Hall currents measured onCHAMP and simultaneous incoherent scatter measurements with EISCAT. However,the electric fields were usually quite small during the period ofobservation, making the quest for an explanation for the structures all themore challenging.  <b>Keywords.</b> Meteorology and atmospheric dynamics (Thermosphericdynamics) – Magnetospheric physics (Polar capphenomena) – Atmospheric composition and structure (Pressure,density, and temperature

Thermospheric density structures over the polar regions observed with CHAMP