Equatorially Trapped Plasmas - SCATHA
Here are some spectrograms to go with the early line graphics (this paper emerged in the interval of time where it was hard to get really good spectrograms for the SCATHA satellite).
There is a bit of a story to go with these images. McIlwain had obtained an odd sort of grey-scale graphics printer (~1978) to make spectrograms. It worked fairly well, though the graphics weren't suitable for publication. I got the spectrogram for day 136 off, and thought the machine had broken. There was a big black spot. I showed it to McIlwain, who immediately said - "that must be the magnetic equator", or words to that effect. He was right, of course.
In the figures here, the flux of ions in the equatorially trapped population is so large, the grey scale saturates, and cycles back to black. Here is a pair of spectrograms for day 81. You can click on these to pop-up the larger version. In the 24 hour figure, the satellite is passing inwards towards perigee (5.6 Re) as it comes into the afternoon - so it moves into the outer plasmasphere at about 1100. You can see there is a significant ring current injection of ions at about 1200; the higher energy ions disperse towards lower energy. The spacecraft crosses into the plasma sheet at ~1430 UT, 5.3 Re, 1900 LT. The equatorially trapped plasmas appear at ~1230 UT, as the satellite crosses the magnetic equator.
Somewhat more detail is visible in the 2 hour version. No electrons to speak of. This was odd, since Gordon Wrenn had put a small paper out showing the GEOS observations of electrons. I could not figure out why he had not seen the ions, which were sort of hard to miss. One difference is that the ions are a plasmaspheric phenomenon, and the electrons observed at geosynchronous orbit don't occur when the background electron density is too high (fce stuff), that is they are primarily outside the plasmasphere. In retrospect, I think there were also magnetic latitude issues. SCATHA, not really being geosynchronous, passed through the magnetic equator, while geosynchronous satellites sort of wander within a few degrees of the equator, depending on longitude.
One of the things that always fascinated me was the apparent relationship between equatorially trapped plasmas, and the so-called, equatorial noise. Don Gurnett did the first major article on the topic, and it was just complete enough that none of our subsequent work seemed significant enough to publish. Jim Roeder and I did a lot of analysis on the SCATHA field & wave sensors, which is briefly illustrated here.
Power Point (recreated from slides I used 20 years ago). ref: Olsen, R. C., Recent results on wave-particle interactions as inferred from SCATHA, invited talk, presented a the XXIst General Assembly of International Union of Radio Science, Florence, Italy, August 28 - September 5, 1984.