Polar rain: Solar coronal electrons in the Earth's magnetosphere

ISEE 1 low‐energy electron measurements have revealed the frequent presence of field‐aligned fluxes of a few hundred eV electrons in the geomagnetic tail lobes. In the northern tail lobe these electrons are most prominent when the interplanetary magnetic field is directed away from the sun. This characteristic helps identify the electrons as polar rain electrons that are usually identified by their uniform precipitation pattern over the earth's polar caps. Although tail lobe electrons have been seen previously at high altitudes, the ISEE data provide the first demonstration of their field‐aligned nature, and this characteristic places a stringent constraint on their origin. By mapping the tail lobe velocity distribution function into the solar wind, we have confirmed previous suggestions that the polar rain is indeed of solar wind origin and is due to the access of electrons to the magnetotail lobe. More specifically, however, it is demonstrated that the more energetic component of the polar rain is composed of electrons from the solar wind “strahl,” a field‐aligned component of the solar wind which is difficult to measure but which is thought to be caused by the collisionless transit of hundred eV electrons from the inner solar corona to 1 AU. The preferential entry of the strahl into that polar cap most directly connected magnetically to the sun explains a known north‐south asymmetry in polar rain intensity. Furthermore, it is suggested that cases of higher‐energy polar rain that have been thought to require a magnetospheric field‐aligned acceleration mechanism are more likely due to an enhanced intensity of the solar wind strahl caused by easier access of solar corona electrons to 1 AU and the polar caps. This idea is supported by the observation that previously reported cases of intense polar rain are invariably associated with tenuous or high‐speed solar wind that is expected to produce enhanced strahls. It is shown how polar rain measurements may provide a good measure of (1) the temperature and the state of statistical equilibrium in the solar corona and (2) the degree to which interplanetary field lines connect with those of the earth.