Diurnal and seasonal variations of exospheric heat flux at a mid-latitude station

Abstract

Meaesurments made with the pulsed incoherent scatter radar at Malvern between 1968 and 1971 are used to calculate the downwards conductive heat flux at 500 km altitude. This flux is about 10 μW m −2 at noon, and less than 1 μW m −2 at midnight and shows little seasonal variation. The changes between daytime and night-time conditions at dawn and sunset are rapid, being virtually complete between solar zenith angles of 90° and 98°. When local dawn or sunset is well separated in time from that at the magnetic conjugate point, the flux is observed to change in two steps, showing that local and conjugate point photoelectrons contribute roughly equally to the available flux. Abnormal fluxes may occur on magnetically disturbed days, but significant short term variations also occur on quiet days. The long term mean values agree well with results for St Santin, France, and Millstone Hill, U.S.A., suggesting that the flux varies little with magnetic latitude.