Polar rain

spaceweb@oulu.fi - last update: 5 January 1999, 1310 UT (RR)


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).


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