Effects on VLF propagation during ionospheric substorms

Quantitative study of substorm‐associated VLF phase anomalies and precipitating energetic electrons on November 13, 1979

“…VLF radio waves propagating through the auroral region often suffer short-term (0.5 ---3 hours) disturbances in correlation with auroral absorption and geomagnetic disturbances [Reder et al, 1964;Egeland and Naustvik, 1967;Egeland et al, 1969;Svennesson, 1973;Westerlund and Reder, 1973;Kikuchi, 1981]. In spite of the generally good correlation between the VLF disturbances and the ionospheric absorption of cosmic radio noise, some cases have been reported where there is little correlation between them, especially during the day [Svennesson, 1973;Westerlund and Reder, 1973].…”

“…VLF radio waves propagating through the auroral region often suffer short-term (0.5 ---3 hours) disturbances in correlation with auroral absorption and geomagnetic disturbances [Reder et al, 1964;Egeland and Naustvik, 1967;Egeland et al, 1969;Svennesson, 1973;Westerlund and Reder, 1973;Kikuchi, 1981]. In spite of the generally good correlation between the VLF disturbances and the ionospheric absorption of cosmic radio noise, some cases have been reported where there is little correlation between them, especially during the day [Svennesson, 1973;Westerlund and Reder, 1973]. The poor correlation between VLF phase and absorption anomalies may indicate that the precipitating electron spectrum is so hard that only VLF signals are disturbed [Westerlund and Reder, 1973], although it may be possible that when VLF phase anomalies are observed, absorption anomalies take place somewhere along the VLF propagation path [Svennesson, 1973].…”

“…In spite of the generally good correlation between the VLF disturbances and the ionospheric absorption of cosmic radio noise, some cases have been reported where there is little correlation between them, especially during the day [Svennesson, 1973;Westerlund and Reder, 1973]. The poor correlation between VLF phase and absorption anomalies may indicate that the precipitating electron spectrum is so hard that only VLF signals are disturbed [Westerlund and Reder, 1973], although it may be possible that when VLF phase anomalies are observed, absorption anomalies take place somewhere along the VLF propagation path [Svennesson, 1973]. Kikuchi [1981], on the basis of a statistical analysis of the VLF phase anomalies associated with magnetic bays, showed that frequent precipitation of energetic electrons responsible for VLF phase anomalies takes place at all local times, but the precipitation is predominant in the afternoon to evening sector as well as in the late morning and premidnight sector.…”

“…These results suggest that VLF phase anomalies are caused by harder electrons compared to the absorption anomalies. The energy of the precipitating electrons responsible for VLF propagation disturbances was estimated by several workers as E > 200 keV for the daytime condition and E > 60 keV for the nighttime condition [Svennesson, 1973;Westerlund and Reder, 1973]. On the other hand, Kiklwhi…”

“…In spite of the great advantages of VLF wave propagation for detection of precipitating energetic electrons, as mentioned above, investigations of a relation between VLF wave propagation disturbances and magnetospheric substorms are quite sparce. It has been shown that there is a close relation between VLF phase anomalies and occurrence of geomagnetic bays [Westerlund and Svennesson, 1971;Svennesson, 1973;Westerlund and Reder, 1973]. In addition, the flux of precipitating electrons with E > 40 keV has been estimated from phase anomalies on a VLF signal propagating along a mid-latitude path during the night [Potemra and Rosenberg, 1973].…”