Equatorial scintillations during the major magnetic storm of April 1981


  • J. Aarons,

  • A. DasGupta


The geomagnetic storm of April 11–14, 1981, was the most severe storm reported since the August 1972 events. Observations of equatorial scintillations in the VHF through microwave range of frequencies (250 MHz to 1.5 GHz) by two stations, Huancayo, Peru, situated near the magnetic equator, and Ascension island, close to the equatorial anomaly crest, are presented. Unusually high levels of activity were recorded at both the stations even around sunrise. At Huancayo, triggered by a reversal of electric field and therefore a rise in the height of the F layer, the 1.5-GHz transmissions from the satellite MARISAT (15°W) exhibited a scintillation index of 9 dB, which is one of the highest levels recorded at Huancayo over a period of several years. At Ascension island, severe depletions in ionospheric electron content associated with scintillations were detected for the first time in our observations near sunrise. The dual-station multisatellite observations are discussed in terms of plasma processes at the equator. The reversal of direction after a deep magnetic bay caused the reversal of direction of the electric field, produced an upward bubble velocity, and generated F layer irregularities. This bay, however, was unable to produce an upwelling after sunrise because of E layer conductivity. The postmidnight and sunrise behavior of the F layer irregularity region has been verified by observations of additional severe magnetic storms during years of high sunspot activity. One example, observations on March 5, 1981, at Manila, is shown. The localized generation of patches during periods of magnetic activity indicates the possibility of bubble formation during magnetically quiet periods without a triggering wave. A hypothesis is proposed for instabilities forming from longitudinal (east-west) electron density gradients existing in this case during the magnetic bay; in the case of magnetically quiet postsunset activity, steep and irregular gradients in electron density may exist across the longitudinal plane at the magnetic equator, producing conditions for the generation of instabilities.