Effects of the 13-14 March 1989 geomagnetic storm on the E region Tidal Ion Layer structure at Arecibo during AIDA

The formation and vertical motion of intermediate layers and so-called sporadic-E layers (80-150 km altitude) are thought to be predominantly under tidal control at middle and low latitudes. We refer to these layers as Tidal Ion Layers (TILs). Study of the time height trajectories of TILs, derived from Incoherent Scatter Radar (ISR) data obtained during the AIDA (Arecibo Initiative in Dynamics of the Atmosphere) campaign, reveals that layer trajectories are greatly influenced by geomagnetic activity when the Kp index rises above about 6 for periods greater than 3 h during the night-time. In particular, the large geomagnetic storm of 13-14 March 1989 resulted in the complete disruption of the TIL structure in the entire 80-150 km altitude range. The time of disruption of the layer structure coincided almost exactly with the time of the magnetic field disturbance recorded by mid-latitude magnetometers. We hypothesize that the layer disruption is caused by an electric field with a magnitude sufficient to overwhelm the usual V × B ion convergence mechanism (where V is the ion velocity and B is the geomagnetic field intensity). Computer simulation of ion motion, based on an ion momentum equation which incorporates both tidal wind and electric field effects, allows an estimate of the strength of the electric field for the observed disturbances. The model electric field strengths required to cause TIL disruption are in the range of 0.5 3 mV/m, perpendicular to the B field, and directed anywhere between eastward and southward.