Ionospheric thermal radiation at radio frequencies II. Further observations

Abstract

The results of a series of observations of 2 Mc/s radio noise over a period of about a year at a very quiet site are described. The noise received during the midday hours is thought to have been due almost entirely to ionospheric thermal radiation. The derived ionospheric temperatures ranged between about 200° and 250° Kelvin, varied appreciably from day to day and were lower and less variable in winter. During SID's the ionospheric temperature increased by up to 40°K. This increase is probably due to a height change of the 2 Mc/s absorbing region. Polarization measurements showed that in September 1951 the intensity of the extraordinary component, absorbed at the lower ionospheric level, corresponded to a temperature about 20°K higher than the ordinary. This established that the absorbing regions were then located at a level where the temperature decreased with height. Information on the location of the absorbing regions was obtained from an auxiliary set of pulse reflection measurements. The experimental observations can be explained reasonably in terms of a fixed temperature-height model, similar to that determined by rocket measurements, but having a minimum of 217°Kat 80km.