Electron density enhancements in theEandFregions of the ionosphere during solar flares

Using the method of Thomson scatter, it has been possible to observe detailed ionospheric effects due to solar flares. Observations have been limited to four 2B flares, and two of these events have been examined intensively. The two events that have been studied exhibit very similar characteristics, i.e., a strong and long-lasting E-region electron density enhancement, a moderate and shorter duration F1-region enhancement, and a moderate F2-region enhancement characterized by a long-duration ‘tail.’ Flare X-ray data, available in the 2- to 12-angstrom band, shows excellent agreement with electron density fluctuations in the lower E region. The F1-region enhancement is believed to follow the time history of the EUV ionizing radiations. The F2-region tail is attributed to the much smaller recombination rate at these heights. Plasma-line observations, permitting unequivocal measurement of electron density, have been possible within a limited height range. Comparison of electron density, as deduced from the plasma lines, with backscattered power measurements corrected for the effects of absorption, has resulted in estimates of flare-associated changes in the ratio of electron to ion temperature.