A morphological study of substorm-associated disturbances in the ionosphere

The whistler technique was used in an earlier study to observe a rapid depletion of the day side plasmasphere during isolated substorm activity on June 25, 1965 (Park, 1973). It was inferred that this depletion was due to dumping of plasma into the underlying ionosphere. The average downward flux was estimated to be of the order of 109 el/cm² s across the 1000-km level, large enough to cause significant enhancements in electron concentrations in the ionospheric F region. In this paper the ionospheric behavior during the same event is examined by using ionosonde data from some 40 stations in the northern hemisphere. The results show that the F2 layer critical frequency ƒoF2 was enhanced up to 25% above the monthly median level in the same general area where downward fluxes were inferred from whistlers. This was followed by ∼20% depressions in ƒoF2 the next day, when geomagnetic conditions were quiet. These substorm effects were observed in a wide range of latitude but were limited in local time and longitude. In the area affected, the ionospheric disturbance caused by this moderate substorm activity (∼500 γ maximum AE index and ∼6-hour duration) was similar in many respects to major worldwide ionospheric storms. These results support the view that ionospheric storms can be understood in terms of the superposed effects of many substorms.