Correlated satellite measurements of proton precipitation and plasma density

Abstract

The main experimental findings relating plasma densities in the equatorial plane (Ogo 5) to proton precipitation at high latitudes (Esro 1A), obtained from nearly simultaneous observations in the evening/midnight local time sector, can be summarized as follows: When the plasma density is high out to large L values, most common during quiet geomagnetic conditions, the greater than 100-keV proton precipitation has no sharp equatorward boundary. The change from an anisotropic pitch angle distribution peaked at 90° with the magnetic field lines to an isotropic one is gradual with increasing L. When a sharp plasmapause is detected and the plasma density outside the plasmapause is low, there is a region outside the plasmapause where the proton flux is highly anisotropic. A rather abrupt transition to an isotropic pitch angle distribution takes place at approximately 1 RE outside the plasmapause. There is thus a region outside the plasmapause where the proton population is stable to precipitation losses. The high-energy protons reach their peak precipitation rate at lower L values than the low-energy protons do. An example where the precipitation pattern consists of two regions, one in the auroral zone and one at lower L values, both having increased plasma density, is presented. The spatial and energy dependence of the proton precipitation pattern shows that the proton population is stable to precipitation losses in regions where the magnetic energy per particle roughly exceeds the energy of the observed protons. In regions where the magnetic energy per particle is less than the energy of the protons, the protons can precipitate with either an isotropic or an anisotropic pitch angle distribution. The on-off switch thus seems to be controlled by the relation of particle energy to magnetic energy per particle, whereas the strength of the precipitation in the on state is otherwise determined.