On the criterion of geomagnetic field at the time of disappearance of equatorial Es-q layer

The condition of equatorial electrojet currents (or electrostatic field) as derived from geomagnetic field (H) variation at the time of sudden disappearance of the equatorial sporadic E layer is discussed. This association between the reversed electrojet and disappearance of Es is confined to only the q type of Es. The estimate of electron drift velocity to be between 100 and 250 m/sec at the time of disappearance of Es-q by some workers seems to be in error due to the inaccuracies in defining the base level for delta-H. The disappearance of Es-q occurs precisely when the latitudinal profile of delta-H is reversed and not necessarily when delta-H at an equatorial station decreases below the nighttime level. The present criterion is not affected by the choice of the time at which the H field is taken as the base level.