The variability of Joule heating, and its effects on the ionosphere and thermosphere

A considerable fraction of the solar wind energy that crosses the magnetopa use ends up in the high-latitude thermo sphere-io no sphere system as a res ult of Joule heating, the consequences of which are very significant and gl obal in nature. Often Joule heating calculations use hourly averages of the electric field, rather than the time-varying electric field. This leads to an underestimation of the heating. In this paper, we determine the magnitu de of the underestimation of Joule heating by analysing electric field data from the EISCAT Incoherent Scatter Radar, situated at the 67 degrees E mag netic latitude. We find that the underestimation, using hourly-averaged ele ctric field values, is normally similar to 20%, with an upper value of abou t 65%. We find that these values are insensitive to changes in solar flux, magnetic activity and magnetic local time, implying that the electric field fluctuations are linear related to the amplitude of the electric field. As suming that these changes are representative of the entire auroral oval, we then use a coupled ionosphere-thermosphere model to calculate the local ch anges these underestimations in the heating rate cause to the neutral tempe rature, mean molecular mass and meridional wind. The changes in each parame ter are of the order of a few percent but they result in a reduction in the peak- F-region concentration of similar to 20% in the summer hemisphere at high latitudes, and about half of this level in the winter hemisphere. We suggest that these calculations could be used to add corrections to modelle d values of Joule heating.