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Ann. Geophys., 17, 682-691, 1999
www.ann-geophys.net/17/682/1999/
© European Geosciences Union 1999
Solar and seasonal dependence of ion frictional heating
J. A. Davies1, M. Lester1, and I. W. McCrea2
1Radio and Space Plasma Physics Group, Department of Physics and Astronomy, University of Leicester, University Road, Leicester, LE1 7RH, UK
2EISCAT Group, Space Science Division, Rutherford Appleton Laboratory, Chilton, Didcot, Oxfordshire, OX11 0QX, UK
Abstract. Ion frictional heating constitutes one of the
principal mechanisms whereby energy, originating in the solar wind, is deposited
into the Earth's ionosphere and ultimately the neutral atmosphere. Common
programme observations by the EISCAT UHF radar system, spanning the years 1984
to 1995, provide the basis for a comprehensive statistical study of ion
frictional heating, results of which are documented in this and a previous paper
by the authors. In the present work, the authors demonstrate the solar and
seasonal dependence of the universal time distribution of frictional heating,
and explain these results with reference to corresponding dependences of the ion
velocity. Although EISCAT observes a significant increase in the occurrence of
enhanced ion velocities associated with increased solar activity, the latter
characterised according to the prevailing 10.7 cm solar flux, this is not
reflected to such an extent in the occurrence of frictional heating. It is
suggested that this is a consequence of the decreased neutral atmosphere
response times associated with active solar conditions, resulting from the
higher ionospheric plasma densities present. Seasonal effects on the diurnal
distribution of ion frictional heating are well explained by corresponding
variations in ionospheric convection, the latter principally a result of
geometrical factors. It is noted that, over the entire dataset, the variations
in the unperturbed F-region ion temperature, required to implement the
identification criterion for ion heating, are highly correlated with model
values of thermospheric temperature.
Keywords. Ionosphere (auroral ionosphere;
ionosphere-atmosphere interactions; plasma temperature and density)
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