![]() Available evidence points to variability of the order of 10–40 per cent in the intensity of the solar Lyman-α radiation as the most likely cause of these unusual, though infrequent, enhancements in absorption.The influence of geomagnetic storms on equatorial absorption is found to be insignificant, both at solar minimum and at solar maximum.In the diurnal variation of absorption, the maximum absorption occurs about 20–30 min after the time of minimum solar zenith angle. Attention is drawn to the incidence of days of high absorption even in the absence of solar X-ray activity. at a solar zenith angle of 10°, is approximately 0.5 × 10−3 erg cm−2 sec−1. It is shown that the threshold flux for D-region modification. A quantitative empirical relationship is derived between absorption and 1–8 Å solar flux for moderate levels of solar activity. An extensive series of vertical incidence absorption measurements made at an equatorial station is analyzed in detail to provide information which will lead to a better understanding of the lower ionosphere.
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