The effects of magnetic storm phases on F layer irregularities below the auroral oval

Through the study of two periods in September and October 1981 it was possible to observe F layer irregularity development and intensity primarily over subauroral latitudes in the area of the plasmapause. Using sets of data which include in situ measurements of low-energy (< 12 eV) electron precipitation, ionosonde measurements of spread F, and a series of scintillation measurements, we observed the following phenomena: (1) the descent of the auroral irregularity region to include subauroral latitudes in the general area of the plasmapause during the main phases of a series of magnetic storms, and (2) the “subauroral or plasmapause storm” case when irregularities were noted primarily at lower latitudes, which took place during the recovery phase of magnetic storms when local magnetic activity was low or moderate. Irregularities at these latitudes appearing during both magnetic quiet and magnetically disturbed conditions make statistical modeling of intense F layer irregularities for latitudes in this region very difficult. Dynamic modeling, which would follow the phases of a magnetic storm from commencement through injection to recovery, would more likely be successful in predicting periods of spread F, scintillations, and other manifestations of F layer irregularities at subauroral latitudes. We suggest that the subauroral irregularities in the recovery phase of magnetic storms are the result of energy stored in the ring current which then is slowly released. The appearance of intense F layer irregularities at subauroral latitudes over several days in the recovery phase of magnetic storms is similar to the recovery pattern of Dst, the measure of ring current energy density.