The local time, substorm, and seasonal dependence of electron precipitation at L ≈ 4 inferred from riometer measurements
We have examined the variations of electron precipitation at L ≈ 4 as inferred from riometer measurements of cosmic radio noise absorption made during 1975 at Siple Station and Halley Bay, Antarctica. The results are presented in the form of annual and seasonal averages of ½‐hourly values for two geomagnetic activity subsets, AE > 140 nT (disturbed) and AE ≦ 140 nT (quiet). Monthly quiet day curves were used to remove the diurnal and seasonal variations in the background noise levels. Generally, the local time characteristics of the absorption were the same at both stations; the highest absorption occurred in the 0400–1600 MLT sector during disturbed conditions and in the 1200–2000 MLT sector during quiet conditions. For high AE, the highest correlation was obtained at a lag equal to the magnetic local time difference (1.5 hours) between the two stations. On the other hand, for low AE, the highest correlation occurred for a lag of 3.0 hours, nearer the local solar time difference (3.8 hours). Consistently higher absorption was measured at Halley on the average during both levels of magnetic disturbance and in all seasons. At both locations, and for both geomagnetic activity subsets, more absorption was observed in summer and equinox than in winter. This is in contrast to earlier studies for L ≧ 6, and suggests that a meridional reversal of seasonal behavior occurs between L = 4 and L = 6. In a further comparison with Siple ELF/VLF data, the 2‐ to 4‐kHz emission intensity was found to maximize in the morning hours during disturbed conditions, as was found for the absorption. For quiet conditions, the emission intensity was near minimum levels during the afternoon absorption peak. The VLF intensity is maximum in winter in contrast to the riometer result. This is consistent with increased attenuation in summer as a result of increased ionospheric opacity at that time. The local time characteristics of the absorption and wave data can be explained by a model of electron drift and precipitation under quiet and disturbed conditions that was first suggested by Thorne et al. (1974) to account for the local time variation of ELF emission intensity with substorm activity. Thus, the afternoon‐evening absorption peak during quiet conditions is attributed to cyclotron resonance interactions of drifting energetic electrons with plasmaspheric hiss, an emission largely unobservable from the ground. We associate the correlation time delay of ∼3 hours with the time difference between encounters of the Halley and Siple flux tubes with the plasmasphere bulge. The morning maximum of ionospheric absorption and 2‐ to 4‐kHz wave intensity during disturbed conditions is consistent with cyclotron resonance interactions between substorm‐injected electrons and chorus emissions occurring outside the plasmapause. For these conditions the absorption data suggest that maximum precipitation is ordered in MLT.