Source of magnetospheric plasma

spaceweb@oulu.fi - last update: 23 December 1998, 1320 UT (RR)

There is no question about the source of ionospheric plasma, since it is created from the ambient neutral atmosphere by ionization. However, the source of magnetospheric plasma is a much more complicated question: although it seems obvious that both solar wind and ionosphere feed it, the relative importance of these two sources is unclear. Furthermore, it seems also that plasmasphere can provide part of the plasma in plasma sheet; although it originates, in the end, from ionosphere, the transport mechanism is quite different than in direct ionospheric outflow.

When studying the plasma source, separating different ions is important. Protons (H⁺) are the primary ions in solar wind and one of the major components in ionospheric outflows, and thus not good indicators of their source. Also He⁺ has a mixed origin (Kremser et al., 1993). However, O⁺ (and O⁺⁺) come from ionosphere, and He⁺⁺ and highly charged oxygen from solar wind (Kremser et al., 1987). The original idea that magnetospheric plasma would invariably be of solar wind origin was first seriously questioned when precipitating O⁺ was discovered in early 1970s. First observations of direct ion outflows were made few years later, and ionospheric ions were observed in the magnetosphere by GEOS-1 satellite (Geiss et al., 1978). Today we know that O⁺ can be found from ring current, inner plasma sheet, and far magnetotail. Especially high geomagnetic activity enhances ionospheric plasma presence in the magnetosphere. On the other hand, quiet time plasma sheet plasma seems to be at least partly controlled by solar wind (see, e.g., Winglee, 1998).

The effect of the geomagnetic activity is obvious in long time scales, and magnetospheric O⁺ content displays clear solar cycle variation. In addition, it has been suggested that ionosphere response could be fast enough to control the evolution of dynamic geospace processes like substorms via transient and localized dominance (Daglis and Axford, 1996). See also magnetosphere - ionosphere coupling.

Although the (partial) ionospheric source is well established, there are no reliable quantitative estimates of the outflow and its spatial extent, and the speed of the transport is not known. Finally one should be able to show what mechanisms are doing the energization from cold ionospheric source plasma to relatively high energy plasma sheet plasma.

References

Daglis, I. A., and W. I. Axford, Fast ionospheric response to enhanced activity in geospace: Ion feeding of inner magnetotail, J. Geophys. Res., 101, 5047-5065, 1996.
Geiss, J., H. Balsiger, P. Eberhardt, H. P. Walker, L. Weber, D. T. Young, and H. Rosenbauer, Dynamics of magnetospheric ion composition as observed by the GEOS mass spectrometer, Space Sci. Rev., 22, 537-566, 1978.
Kremser, G., W. Stüdemann, B. Wilken, G. Gloeckler, D. C. Hamilton, and F. M. Ipavich, Average spatial distribution of energetic O⁺, O²⁺, O⁶⁺, and C⁶⁺ ions in the magnetosphere observed by AMPTE/CCE, J. Geophys. Res., 92, 4459-4466, 1987.
Kremser, G., B. Wilken, G. Gloeckler, D. C. Hamilton, F. M. Ipavich, L. M. Kistler, and P. Tanskanen, Origin, transport, and losses of energetic He⁺ and He⁺⁺ ions in the magnetosphere of the Earth: AMPTE/CCE observations, Ann. Geophys. 11, 354-365, 1993.
Winglee, R. M., Multi-fluid simulations of the magnetosphere: The identification of the geopause and its variation with IMF, Geophys. Res. Lett., 25, 4441-4444, 1998.