Abstract
Eight major magnetic storm periods occurring under sunspot maximum conditions (1968–1969) have been investigated using total electron content N T , maximum electron density N m and magnetic field data taken at low latitudes. Near Hong Kong (lat. 22.2°N, dip 30°) the main phase onset (MPO) of a storm occurring during the daytime was sometimes preceded by a slight increase and often followed by a sharp decrease in N m and N t , changes in N m being usually the most significant. The earlier the occurrence of the MPO after local midnight the more pronounced was an observed irregular build-up of ionisation after sunrise. Abrupt changes of the horizontal component of the geomagnetic field near the magnetic equator were often followed 2–3 hr later by similar sign changes in N m near Hong Kong and opposite sign changes in N m at Manila (10° lat. nearer the Equator). These results imply that during storms there are current variations induced in the electrojet region and the corresponding eastward electric field which uplifts ionisation at the magnetic equator also changes. It appears that during the main phases of the storms there were often reductions in the intensity of this eastward electric field (even reversals on some occasions) and the development of the equatorial anomaly was consequently retarded. During the northern summer a large day-to-day variability has been observed, even on magnetically quiet days, of the effect of the electrojet at the magnetic equator and of the development of the equatorial anomaly. Storms occurring during this summer period have not been included.
Published Version
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