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.

Characteristics of low‐energy plasma in the plasmasphere and plasma trough

“…Early measurements with the Dynamics Explorer of the midqatitude plasmapause crossings in the bulge region show that the crossings occur at lower L values than the equatorial plasmapause crossings determined with OGO 5 data [Chappell, 1982]. The presence of trapped plasma at the equator as shown in this work and by Horwitz and Chappell [1979] and the absence of trapped plasma in ISEE 1 observations at higher latitudes as reported by Nagai et al [ 1983 ] suggest that the ionosphere and the equatorial bulge region, or parts of it, may actually be disconnected rather than just topologically separated. This would suggest at the least a topological separation of ionospheric plasma and the equatorial bulge region plasma.…”

Electron conic distributions produced by solar ionizing radiation in planetary atmospheres

“…There are, however, reports of bi-directional ion conic distributions on closed field lines at Earth. Nagai et al (1983) and Sagawa et al (1987) reported bi-directional ion distributions on closed field lines near and equtorward of the auroral zone with morphology similar to the bi-directional electron distributions seen at Mars. These ion distributions are seen in both H + and O + ions primarily on the dawn side of the magnetosphere on auroral field lines.…”

Observations of the warm plasma cloak and an explanation of its formation in the magnetosphere

“…Note that the occurrence probabilities in the morning to afternoon sector are in the 50–100% range with the routine occurrence level of >80% in the prenoon region. Nagai et al [1983] found that the bidirectional distributions' occurrence probability increased with decreasing Kp in contrast to the unidirectional field‐aligned distributions seen on the nightside at higher L shells. Even though its occurrence probability varies with magnetic activity this distribution is a persistent feature of the outer prenoon region in both quiet and active times.…”

“…Magnetic activity was low during this survey, with over 88% of the data acquired while Kp was less than or equal to 3. Since the ISEE survey (see Figure 1) took place 22 years earlier, our observations fell during the same portion of the solar cycle as those published by Nagai et al [1983].…”

“…Occurrence probabilities of low‐energy (<100 eV) bidirectional, field‐aligned ions plotted in L shell versus local time, observed by the plasma composition experiment on board ISEE 1 [ Nagai et al , 1983]. …”