Distortions of the nightside ionosphere during magnetospheric substorms

Ionosonde records from widely spaced ground stations show that during magnetospheric substorms the F layer is pushed upward in the premidnight local time sector and downward in the postmidnight sector. The area affected probably depends on the substorm intensity, but it may be as large as ∼20°–60° in geomagnetic latitude and ∼9 hours in local time. These distortions are believed to be caused by eastward electric fields in the evening side and westward fields in the morning side. The inferred ionospheric electric fields do not show close correlation with magnetic perturbations at middle latitudes, thus suggesting that the latter are caused primarily by magnetospheric currents. The F-layer concentrations change as a result of vertical motions of the layer and influx of thermal plasma from the plasmasphere. In one of the cases reported, a thick nocturnal E layer with ƒ0E ∼ 2.5 MHz was observed near 60° geomagnetic latitude. This effect is attributed to substorm-induced precipitation of energetic particles.