Bidirectional solar wind electron heat flux and hemispherically symmetric polar rain

Abstract

The nearly continuous precipitation of low‐energy electrons into the earth's polar caps is known as polar rain. Normally polar rain is much more intense in one polar cap than in the other. Recently Makita and Meng have shown that polar rain is occasionally (approximately once a month for a period of ∼10 hours at a time) observed at high intensity levels simultaneously in both polar caps. Such events are known as hemispherically symmetric polar rain. We have examined ISEE 3 solar wind electron data obtained concurrent with reported symmetric polar rain events and have found that a bidirectional solar wind electron heat flux is present whenever such polar cap events occur. We interpret this result as a demonstration that the solar wind heat flux is the prime source of polar rain. A bidirectional solar wind heat flux is evidence that the local interplanetary magnetic field is part of either a magnetic bottle rooted at both ends in the sun or a closed magnetic loop entirely disconnected from the sun. Thus, we infer that, in contrast to the normal situation when only one of the earth's polar caps is magnetically connected to the sun, during hemispherically symmetric polar rain events either both of the earth's polar caps are magnetically connected to the sun, or else both are connected to a magnetic loop which is entirely disconnected from the sun. The relative timing between bidirectional solar wind heat flux and symmetrical polar rain events can be utilized to determine certain magnetospheric quantities such as the cross‐tail convection speed.