On the identification and occurrence of ion frictional heating events in the high-latitude ionosphere

Abstract

Enhancements in the F-region ion temperature, caused by frictional heating, are a notable manifestation of the interaction between the ionized and neutral atmospheres. Heating of this type arises whenever the ion and neutral populations move with different velocities. A velocity difference of order 1 km s −1 provides sufficient heat input to double the unperturbed ion temperature. This paper presents a synoptic study of ion heating events observed by the EISCAT UHF facility during the 2-year period from January 1985 to December 1986. The data set consists of observations obtained during the ‘Common Programme’ operation of EISCAT in the CP-1 and CP-2 modes. Only ion temperatures measured in the field-aligned direction have been considered, since in this direction the line-of-sight ion temperature can be obtained by assuming that the ion thermal velocity distribution remains Maxwellian. This assumption is known to break down progressively as the difference between the ion and neutral velocities increases, but the resulting discrepancies in the field-parallel ion temperature are smaller than those for other observing directions and do not significantly affect the conclusions presented here. A criterion previously applied to the examination of satellite data has been adopted to identify intervals of F-region frictional heating and employed to derive their diurnal distribution in 1000 h of EISCAT measurements. Almost equal numbers of heating events were encountered in both the dawn and dusk sectors. While this result contrasts with some earlier findings, the present study makes use of a much larger data set than previous investigations and is concerned with somewhat lower latitudes. The magnitudes of the ion temperature enhancements are similar for both the pre-midnight and post-midnight sectors, although the longest intervals of ion heating tended to be concentrated in the early morning hours. Many of the ion heating events occurred in association with intervals of intense and structured particle precipitation, with F-region ion temperatures and velocities being enhanced at the boundaries of auroral forms, but depressed above the regions of precipitation themselves. It is suggested that this behaviour can be explained by the presence of a polarization electric field within the auroral form, whose direction opposes that of the convection electric field. Similar observations of reduced electric fields above regions of particle precipitation have been reported by previous workers, who have classified such events as ‘anti-correlation’ or ‘asymmetric’ auroral arcs.