The Response of the Lower Ionosphere to the Great Solar Flare of August 7, 1972

Abstract

The response of the ionosphere in an altitudinal range of 50-200km to the great solar flare that occurred at about 1500UT on 7 August 1972 is studied. Sudden increases in electron concentration, Pedersen and Hall electric conductivities (SIEC), total electron content (SITEC) and sudden cosmic noise absorption (SCNA) due to solar X-ray flux increments at 1530, 1600, 1700 and 2100UT are calculated for the location of 40°N latitude and 75°W longitude, and some of the results are compared with observed values. The solar X-ray emissions concerned in this study are those with wavelengths of 1-100Å, of which emission fluxes measured from the SOLRAD 10 satellite are available for the 1-20Å band, while emission spectra for other wavelength bands are assumed. Two altitudinal profiles of increased electron concentrations at each time are first derived based on two assumed effective recombination coefficient profiles, then associated increments in 30MHz cosmic noise absorption and Pedersen and Hall electric conductivities are derived. It is found that the calculated 30MHz SCNA's (SCNA values) and SITEC's (50-200km) at 1530, 1600 and 1700UT for one of the increased electron concentration profiles are in reasonably good agreement with the observed ones, but the SITEC's roughly corrected for the entire ionosphere at 1530 and 1600UT seem to be greater by more than 20 and 30% than the observed ones, respectively. These results seem to imply that the derived electron concentration enhancements in the ionosphere above about 100km are overestimated.