Effect of geomagnetic disturbances on the VHF nighttime scintillation activity at equatorial and low latitudes


  • R. S. Dabas,

  • D. R. Lakshmi,

  • B. M. Reddy


VHF nighttime scintillation observations from a chain of stations extending from the equator to 21°N magnetic latitude along 84°E longitude in the Indian zone and at Lunping (23.0°N; 122°E; magnetic latitude 14.9°N subionospheric) in the Far Eastern zone, have been examined during five individual geomagnetic storms of varying strength. The usual nighttime scintillation activity during the main phase of the storm was found to be unaffected. For cases in which recovery phase of the storm starts during midnight to dawn local time sector and where the Dst value goes below −100 nT, strong postmidnight scintillations extending well beyond sunrise hours were observed at locations from 84°E to 122°E longitudes and in a wide latitudinal belt (i.e., up to 21°N magnetic latitude) particularly in the Indian zone. Under similar circumstances ionosonde data, from the equatorial region of both the zones, show considerable postmidnight increase in h′F as well as in spread F. However, at all locations, in cases of those storms whose recovery phases start during local daytime hours, on the following night scintillation and spread F activities were found to be suppressed completely or partially depending upon the maximum negative excursion of Dst. The effect was found to be more pronounced for cases in which Dst reaches below −100 nT. The same is also true for the first two categories of storms for the following night after the recovery phase sets in. The above results were further confirmed by examining Lunping scintillation and Manila ionosonde data for another 52 cases of geomagnetic storms observed during 1977 to 1983. The results are interpreted in terms of the reversal of the equatorial electric field directions under magnetically disturbed conditions.