No abstract

Thermal (≲100 electron volts) ion observations made with the plasma composition experiment on ISEE 1 are combined with plasma density profiles obtained from plasma frequency measurements made with the plasma wave experiment to conduct an investigation of thermal plasma behavior in the vicinity of the plasmasphere during periods of quieting magnetic activity. Normally, the principal thermal ion population in the plasmasphere consists of cold (kT ≲ 1 eV), isotropic distributions with ion species in the order of dominance H+ :He+ :O+, while outside the plasmapause, the observed E ≲ 100 eV ion distributions usually are field‐aligned in structure, have characteristic energies E ≲ 10 eV and H+ :O+ :He+ order of dominance in fluxes. During periods in which the magnetic activity quiets, the above two regions are separated by a new region in which, at times, low‐energy (∼1–2 eV) H+ and He+ are found flowing along the magnetic field lines. On other occasions following quieting magnetic activity, pancake distributions (peak fluxes at 90° pitch angle) are observed in this region. Other complex distributions have been seen, and these complexities and the limitations of the data coverage preclude a satisfactory simple interpretation. It seems plausible to identify this region as the site of plasmasphere refilling. However, the data presumably also contain evidence of the quiet time rotation of the plasmasphere bulge region into the morning sector.

Observations of low-energy ionospheric ions by the plasma composition experiment aboard ISEE 1 often show conical pitch angle distributions, that is, peak fluxes between 0 ø and 90 ø to the directions parallel or antiparallel to the magnetic field. Frequently, all three primary ionospheric ion species (H+, He +, and O +) simultaneously exhibit conical distributions with peak fluxes at essentially the same pitch angle. A distinction is made here between unidirectional, or streaming, distributions, in which ions are traveling essentially from only one hemisphere, and symmetrical distributions, in which significant fluxes are observed traveling from both hemispheres. The orbital coverage for this survey was largely restricted to the night sector, approximately 2100-0600 LT, and moderate geomagnetic latitudes of 200-40 ø . Also, lack of complete pitch angle coverage at all times may have reduced detection for conics with small cone angles. However, we may conclude that the unidirectional conical distributions observed in the northern hemisphere are always observed to be traveling from the northern hemisphere and that they exhibit the following characteristics relative to the symmetric distributions, in that they (1) are typically observed on higher L shells (that is, higher geomagnetic latitudes or larger geocentric distances or both), (2) tend to have significantly larger cone angles, and (3) are associated with higher magnetic activity levels. range of 0-17 keV/e with mass discrimination and is described by Shelley et al. [1978]. In this report we have concentrated on data taken when the instrument was in its thermal plasma mode, sampling ions in the energy per charge

Observations of pancake (peak flux near 90° pitch angle) distributions of low‐energy (≲100 eV) ions are reported. Pancake distributions occur often in H+ and He+ simultaneously while O+ fluxes are either undetectable or field‐aligned. These H+ and He+ pancake distributions typically display characteristic energies of the order of 10 eV and are frequently mixed with higher density, colder (κT≲3 eV), isotropic, quasi‐Maxwellian components. They appear often within the outer regions of the plasmasphere, and seem to occur most frequently on the dayside and near the magnetic equator.

The initial measurements of magnetospheric thermal ions by the Plasma Composition Experiment on ISEE‐1 are presented to demonstrate the surprising variety in this plasma population. The data provide evidence that the adiabatic mapping of the high latitude ionosphere to the equatorial plasma trough provides an insufficient description of the origin, transport, and accumulation processes which supply low energy ions to the outer plasmasphere and plasma trough.