Phase variability of Sq (H) on normal quiet days in the equatorial electrojet region

Summary. From a study of 'abnormal quiet days' (AQDs) at equatorial latitudes it was found earlier (Sastri) that the occurrence of an abnormal Sq ( H ) phase confined to the equatorial electrojet belt is closely associated with the incidence of complete or partial counter-electrojet (CEJ) conditions (marked daytime depressions in the H field in the electrojet region) for about 5 hr around the normal time interval of the diurnal maximum of the H field. In this paper, we investigate the causative mechanism of the Sq ( H ) phase variability on 'normal quiet days' (NQDs), defined as days on which the diurnal maximum of the H field occurs in the time interval 0930-1230 LT, in the equatorial electrojet belt using published geomagnetic data of stations in the Indian equatorial region. It is found that much of the phase variability of Sq ( H ) on NQDs may be caused by the influence of southward (negative) perturbation fields in the H component, similar in nature to those associated with AQDs but of a much smaller amplitude, close to the usual time of the diurnal maximum of the H field. The perturbation fields are noticed to be essentially of the ionospheric dynamo region origin. Possible mechanisms that might give rise to the observed perturbation fields are discussed.