Ring currents and internal plasma sources

The discovery of terrestrial O+ and other heavy ions in magnetospheric hot plasmas, combined with the association of energetic ionospheric outflows wi th geomagnetic activity, led to the conclusion that increasing geomagnetic activity is responsible for filling the magnetosphere with ionospheric plas ma. Recently it has been discovered that a major source of ionospheric heav y ion plasma outflow is responsive to the earliest impact of coronal mass e jecta upon the dayside ionosphere. Thus a large increase in ionospheric out flows begins promptly during the initial phase of geomagnetic storms, and i s already present during the main phase development of such storms. We hypo thesize that enhancement of the internal source of plasma actually supports the transition from substorm enhancements of aurora to storm-time ring cur rent development in the inner magnetosphere. Other planets known to have ri ng current-like plasmas also have substantial internal sources of plasma, n otably Jupiter and Saturn. One planet having a small magnetosphere, but ver y little internal source of plasma, is Mercury. Observations suggest that M ercury has substorms, but are ambiguous with regard to the possibility of m agnetic storms of the planet. The Messenger mission to Mercury should provi de an interesting test of our hypothesis. Mercury should support at most a modest ring current if its internal plasma source is as small as is current ly believed. If substantiated, this hypothesis would support a general conc lusion that the magnetospheric inflationary response is a characteristic of magnetospheres with substantial internal plasma sources. We quantitatively define this hypothesis and pose it as a problem in comparative magnetosphe res.