Locating the magnetospheric ring current

Protons are studied in the global depression of the earth's horizontal magnetic field. It is shown that 10 to 100 keV protons dominate ring current energetics in two preferred regions of cyclotron instability, which serve as stable trapping boundaries for ring current protons. The only apparent means of removing this stably trapped belt of particles are considered to be by charge exchange interactions, or by outward expansion of the plasmapause to erode the ring current. Both of these processes require about two days, which is the characteristic decay period of the main phase depression. Questions whose answers are necessary to formulate a quantitative theory of geomagnetic storms which relates main phase depression to solar wind parameters are included.