On the development of abnormally large postsunset upward drift of equatorial F region under quiet geomagnetic conditions

The regular postsunset enhancement of upward drift of the equatorial F region is observed to be abnormally large on certain quiet days (Ap less than or equal to 5) as manifested by an anomalous dusktime increase of F region height. We explored the origin of this extreme form of the quiet-time variability of postsunset vertical drift/height through case studies using data from the ionosonde and magnetometer networks in the Indian equatorial region, It is found that on the days with all unusually large postsunset increase of F region height near the magnetic equator, the diurnal profile of the equatorial electrojet (EEJ) strength is severely distorted (with a shift, in some cases, of Sq(H) phase from the usual time interval, characteristic of the abnormal quiet days) with enhanced EEJ conditions in the postnoon period (1300-1600 LT). This is accompanied, near the magnetic equator, by higher values of F layer peak height (hpF2) and lower values of peak electron density (f0F2) in the early evening period (1600-1800 LT), compared with the monthly median/quiet day mean values. These changes in EEJ and hpF2/f0F2 are consistently seen in all eight cases studied. It is suggested that the perturbations in plasma density distribution of equatorial F region increase the thermospheric zonal wind and its local time gradient and the ratio of flux-tube-integrated Pedersen conductivity of the F to E region. These modifications just prior to sunset of the properties of the equatorial thermosphere-ionosphere prompt an efficient F region dynamo action, resulting in the observed abnormally large dusktime increase of F region height.