Polar rain is spatially homogenous few hundred eV electron precipitation into the polar cap (Winningham and Heikkila, 1974). The open polar cap field lines are connected to the lobes of the magnetic tail, and polar rain electrons have been observed also there (Yeager and Frank, 1976). The origin of the polar rain electrons is the solar corona, i.e., they belong to the suprathermal (halo) portion of solar wind electrons (Fairfield and Scudder, 1985). In polar rain, there is typially little or no ion accompaniment.
Polar rain can be used as a diagnostic tool for open field lines (e.g., Shirai et al., 1998). However, although field lines with polar rain are certainly open, field lines without it are not necessarily closed.
Polar rain shows a strong hemispherical asymmetry, with the northern (southern) hemisphere favoured for an away (toward) interplanetary magnetic field (IMF) sector structure (Yeager and Frank, 1976). Also a dawn-dusk gradient controlled by IMF By has been observed (Meng and Kroehl, 1977). As the dayside merging affects the size of the polar cap, it is not surprising that also the IMF Bz component affects the polar rain (Gussenhoven et al., 1984; Sotirelis et al., 1997).
- Fairfield, D. H., and J. D. Scudder, Polar rain: Solar coronal electrons in the Earth's magnetosphere, J. Geophys. Res., 90, 4055-4068, 1985.
- Gussenhoven, M. S., D. A. Hardy, N. Heinemann, and R. K. Burkhardt, Morphology of the polar rain, J. Geophys. Res., 89, 9785, 1984.
- Meng, C.-I., and H. W. Kroehl, Intense uniform precipitation of low-energy electrons over the polar cap, J. Geophys. Res., 82, 2305-2313, 1977.
- Shirai, H., K. Maezawa, M. Fujimoto, T. Mukai, T. Yamamoto, Y. Saito, and S. Kokubun, Entry process of low-energy electrons into the magnetosphere along open field lines: Polar rain electrons as field line tracers, J. Geophys. Res., 103, 4379-4390, 1998.
- Sotirelis, T., P. T. Newell, and C.-I. Meng, Polar rain as a diagnosis of recent rapid dayside merging, J. Geophys. Res., 102, 7151-7157, 1997.
- Winnigham, J. D., and W. J. Heikkila, Polar cap auroral electron fluxes observed with ISIS 1, J. Geophys. Res., 79, 949-957, 1974.
- Yager, D. M., and L. A. Frank, Low-energy electron intensities at large distances over the earth's polar cap, J. Geophys. Res., 81, 3966-3976, 1976.