Pitch angle properties of magnetospheric thermal protons and satellite sheath interference in their observation

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Abstract
Cold proton observations from the geosynchronous GEOS 2 satellite are presented to show the pitch angle distribution of the refilling cold ion population found beyond the plasmapause. This refilling cold ion population flows into the depleted equatorial regions of the flux tubes from the topside ionosphere. This refilling is observed both day and night. The ions have energies between 0.5 and 3 eV. These energy characteristics are consistent with other recent observations. Although the observed angular distribution of these ions is highly complex, when allowance is made for the satellite sheath several conclusions can be made concerning the ion pitch angle distribution. The angular distribution is very field aligned with a characteristic source cone angle ranging from 10° to 20°. This range of angles is consistent with ions originating below 20,000 km in the exosphere rather than in the heavy‐ion ionosphere. As the flux tube density increases, the degree of field alignment decreases; however, based on the limited pitch angle coverage it is not possible to determine if the field‐aligned component is simply being swamped by the local isotropic component or if the field‐aligned component changes its pitch angle characteristics. A crucial aspect of the study was the availability of a unique data base of sheath‐plasma observations. The observations were obtained by using the boom mounting and voltage biasing capabilities of the detector package in relation to the satellite. These data enabled us to describe how the satellite sheath affects the observation of cold (<5 eV) protons. This qualitative understanding enabled the highly anisotropic data to be interpreted in terms of magnetospheric pitch angle distributions. The overall implications of this sheath‐plasma coupling is that a technique to make satellite sheath observations has been demonstrated.

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