Satgen 628 Auroral Questions by GM4IHJ (BID SGEN628) 2001-04-07 Seven radio auroras in ten days, culminating in a massive solar storm on 30/31st of March 2001. Has produced a large crop of email question as correspondents ponder the strange quirks of auroral events. Several people asked why a beacon on HF and a beacon on VHF, both at the same site, had peak signals on different azimuth bearings? This is very clearly seen if you follow two satellites on roughly the same track. One (say RS12 on HF). the other a Russian Meteor weather sat on VHF. The signals rarely peak anywhere near the same azimuths. If you follow RS12 near an active auroral oval , you often get a direct tone 6 to 9 signal plus two auroral tone signals, and all these signals peak on different azimuth bearings. Clearly the back scatter is coming from geographically separated parts of the auroral oval. With a general preference for HF signal to be on azimuths west of VHF signals, when they come from northern auroral zone targets. At times there is a sudden marked change in prefered peak signal azimuth. Which on an FFTDSP display, is accompanied by a jump in the central frequency of the broad auroral tone 1 or 2 signal. Equally noteworthy . As the satellite overflies the near edge of the auroral oval or leaves the auroral oval, the FFT display can reveal a series of rapid changes in signal frequency . Perhaps because the signal is scattering off a moving auroral curtain producing one doppler shift one moment , then a different part of the auroral curtain with a different motion with respect to your station and hence different doppler shift a quarter minute later. On RS12 this results in an FFT display showing a sequence where the received signal gradually drops 200 Hz in 30 seconds then jumps back suddenly to the higher frequency and immediately starts another gradually descent for 30 seconds. Producing a pattern of sloping stripes which can at times last for several minutes before the signal returns to a normal slow doppler shift. All of which hints at very rapid changes taking place in the auroral curtain, where you get the FFT stripes. Far less easy to explain are some of the other features of recent radio auroras. Such as why is there a prevalence for auroral activity at the time of the equinox ( March, and again in October ).Year after year as the Sun rides high over the Earths equator at the equinox, we get lots of auroras. It is very difficult to see how the Sun above the equator can influence the Coronal Mass Expulsions on the Sun which cause many of our big auroras. We now have lots of satellites studying the Earths magnetosphere, where most of the action takes place ( Ace and Geotail are perhaps the best). And, they are begining to tell us how relatively benign magnetosphere events suddenly produce explosive magnetic storms. Citing Whistler waves as the feature which alters the nozzle size of the magnetic gun. But lots more information is needed before we will get a clear picture. Meanwhile, please remember that radio aurora and visual aurora are not one and the same thing. We get 20 radio auroras at 58N MagLat for every visual event. Stations further south hear less radio aurora but see a higher % of visuals, because big events which get down to low latitudes often feature both types.