ENTRY PROCESS OF LOW-ENERGY ELECTRONS INTO THE MAGNETOSPHERE ALONG OPEN-FIELD LINES - POLAR RAIN ELECTRONS AS FIELD LINE TRACERS

The strahl component of solar wind electrons, which constitutes field- aligned electron heat flux running away from the Sun, is a strong cand idate for the origin of the polar rain. We investigate the entry proce ss of the strahl electrons into the distant-tail magnetosphere and dis cuss topologies of Earth's field lines in this paper. The Geotail sate llite has often observed either gradual or abrupt transitions from the magnetosheath electrons to the bidirectional lobe electrons at the ma gnetopause. In some cases, the strahl flux of the sheath electrons flo wing tailward gradually turned near the magnetopause and finally becam e the earthward flux of the bidirectional lobe electrons. This transit ion is accompanied by the rotation of the magnetic field direction and indicates the direct entry of the strahl electrons along open field l ines. On the basis of the data of 38 magnetopause crossings by Geotail , we investigate variation of density of the strahl (polar rain) elect rons and that of ion density near the magnetopause. It is shown that t he strahl electrons decrease upon crossing the magnetopause, and the d ecrease is correlated with that of ions, although, quantitatively, the strahl electrons do not decrease so much as ions. It is suggested tha t the strahl electrons enter the magnetosphere along open field lines more freely than ions, but their entry is under the influence of charg e neutrality with ions. It remains as a problem why the Geotail data d o not show the presence of electrons escaping from the magnetosphere a long open field lines.