Low‐energy (<100 eV) ion pitch angle distributions in the magnetosphere by ISEE 1

Abstract

Low‐energy (<100 eV) ion data from the plasma composition experiment on ISEE 1 are examined statistically to study pitch angle distributions in all local times of the magnetosphere (L = 3–10). The pitch angle distributions in the data set used here can be classified into seven types; however, there are four major types, i.e., isotropic distribution, bi‐directional field‐aligned distribution, unidirectional field‐aligned distribution, and low flux. The isotropic distribution that consists of very low energy (typically <10 eV) ions is a persistent feature in the inner region. It is frequently observed with an accompanying loss cone‐like structure. The bi‐directional field‐aligned distribution consisting of warm ions (⩾10 eV) is a persistent feature on the outer dayside and it is seen just outside the isotropic distribution region of the nightside. It is noted that the loss cone‐like structure is also a common feature of this type of distribution in the noon sector. On the outer nightside the unidirectional field‐aligned distribution consisting of warm ions is the dominant signature, but in some cases only the low flux (no appreciable flux) is observed. The ‘sources’ of ions in various regions are discussed on the basis of these results and others.