THE POLAR RADIO BLACK-OUT OF THE IONOSPHERE OF 7 JULY, 1958

Abstract

Ionospheric sounding station data, particularly f-plots, were used to study the ionospheric storm of 7 July, 1958, which is shown to have consisted clearly of two stages. Polar cap absorption began shortly after an importance 3+ flare was observed at 0039 U.T., and a magnetic storm and auroral absorption event followed at 0749 U.T. on July 8.The polar cap absorption spread towards the geomagnetic equator, but the use of higher than first order magnetic dipole terms was found necessary in order to arrange the data properly in latitude. The absorption began some hours earlier at northern stations than at most southern stations, the main cause of the differences in time of onset being thought to be due to the presence or absence of sunlight.The inference is drawn that ionospheric absorption measured at about 3 Mc/s is caused by ionization at 50- to 80-km heights, in which case the data suggest that electrons first precipitated to the earth above 83° corrected geomagnetic latitude, and then protons and helium ions to lower latitudes. The cutoff latitude was 59°, corresponding to a particle magnetic rigidity of 9.6 × 108 ev/c. This storm was compared with the more severe event of 23 February, 1956, and showed to be of a similar nature.