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

Abstract

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.

A synoptic study of VLF sudden phase anomalies recorded at Visakhapatnam

“…The duration of SPA event, i.e., the time from the beginning of the event to recovery normally varies from 30 minutes to 2-3 hours (Chilton et al, 1963). In the present investigation, though the duration varied from 16 minutes to 100 minutes, it is observed that over 65% of the SPAs lasted between half an hour to one hour as seen in Fig.…”

Synoptic report of VLF sudden phase anomalies observed at Toyokawa, Japan

“…This results in a lowering of the reflection height of VLF signals and SPA's occur (CHILTON et al, 1963(CHILTON et al, , 1965. So, it becomes necessary to know phase changes during solar flares for the detection of SPA's.…”

“…The distribution of the duration of SPA events is already reported as from 30 minutes to 2 or 3 hours (CHILTON et al, 1963). Using the data of 682 SPA events observed on NWC-Toyokawa circuit from 1971 to 1974, a histogram of the duration of SPA events is obtained as shown in Fig.…”

“…Some interesting work along this line has already been done by many workers (CROMBIE et al, 1958(CROMBIE et al, , 1965CHILTON et al, 1963CHILTON et al, , 1965 but much remains to be done.…”

Analysis of daytime ionosphere behavior between 2004 and 2008 in Antarctica

“…These ionization enhancements are detected as abrupt phase variations of the VLF signals, the called sudden phase anomalies (SPAs). The size of the SPAs depends on the XͲray flare fluxes (Chilton et al, 1963;Crombie, 1965;Grubor et al, 2005;Kaufmann et al, 2002;McRae and Thomson, 2004). SPA studies have shown that the DͲregion conductivity and reference height present different characteristics as a function of the solar activity level.…”