Solar and seasonal dependence of ion frictional heating

Ion frictional heating constitutes one of the principal mechanisms whereby energy, originating in the solar wind, is deposited into the Earth's ionosp here and ultimately the neutral atmosphere. Common programme observations b y the EISCAT UHF radar system, spanning the years 1984 to 1995, provide the basis for a comprehensive statistical study of ion frictional heating, res ults of which are documented in this and a previous paper by the authors. I n the present work, the authors demonstrate the solar and seasonal dependen ce of the universal time distribution of frictional heating, and explain th ese results with reference to corresponding dependences of the ion velocity . Although EISCAT observes a significant increase in the occurrence of enha nced ion velocities associated with increased solar activity, the latter ch aracterised according to the prevailing 10.7 cm solar flux, this is not ref lected to such an extent in the occurrence of frictional heating. It is sug gested that this is a consequence of the decreased neutral atmosphere respo nse times associated with active solar conditions, resulting from the highe r ionospheric plasma densities present. Seasonal effects on the diurnal dis tribution of ion frictional heating are well explained by corresponding var iations in ionospheric convection, the latter principally a result of geome trical factors. It is noted that, over the entire dataset, the variations i n the unperturbed F-region ion temperature, required to implement the ident ification criterion for ion heating, are highly correlated with model value s of thermospheric temperature.