Abstract
VLF phase measurements were made at three locations along the same great circle path of propagation during a solar flare which occurred on 29 June 1973. The termination for the solar flare effect on the ionosphere at VLF reflecting heights occurred between solar zenith angles 84° and 87°, considerably less than the sunset zenith angle of 94° for this height. Ionization production is considered for the various ionization sources as a function of zenith angle. The measurement is consistent with the electron density at VLF reflecting heights being primarily controlled by solar photo-detachment but at zenith angles greater than 76° the ionization available for photo-detachment is produced by scattered Lyman-α as the direct Lyman-α becomes too heavily absorbed.
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