Very-Low-frequency phase observations of solar flare ionization in the D region of the ionosphere

Sudden phase anomalies produced by solar flares have been observed simultaneously on several long VLF propagation paths. The paths providing useful data are NBA (18 kc/s) Panama to Boulder, Colorado; to Maui, Hawaii; to Frankfort, Germany; and to College, Alaska; and GBR (16 kc/s) Rugby, England, to Boulder; and NPG (18.6 kc/s) Seattle to College, Alaska. The observations show that during a solar flare the magnitude of the mean change of reflection height Δh on each sunlit path can apparently be related to the mean of log sec χ along the path, where χ is the solar zenith angle. Different flares, however, produce different curves of Δh versus the mean of log sec χ. These results are discussed in terms of a layer produced below the normal height of reflection, and the energy (wavelength) and intensity (flux density) of the X rays that produced the new layer are estimated.