Abstract
Abstract. The Imaging Proton Spectrometer (IPS) and the Imaging Electron Spectrometer (IES) on the Polar satellite have measured temporary deviations in the isotropy of the pitch angle distributions (PADs) of charged particles in the inner magnetosphere. As Polar passes through the nightside equatorial region, the IPS and IES observe dropouts of charged particles with pitch angles near 90°, known as butterfly distributions caused by the shadowing of the magnetopause. Additionally, Polar observes a lower energy (<60 keV) intensification of locally mirroring ions while simultaneously detecting butterfly PADs in both higher energy ions and electrons. While it is accepted that charged particles can be lost to the magnetopause due to shadowing effects, the modeling here can suggest that the magnetopause can also be a direct source for particles observed in magnetosphere, with a strong dependence upon both pitch angle and particle energy.
Highlights
The butterfly pitch angle distribution (PAD) was first described by West (1966) with observations of Soviet nuclear testing in the upper atmosphere on 28 October 1962
Many mechanisms have been proposed as the cause of the butterfly PAD, and through the modeling of ions and electrons, it can be supported that shadowing by the magnetopause can be at least one of the methods by which both 90◦ pitch angle electrons and ions are lost from the equatorial zone (West et al, 1973), and as reported the magnetopause is a source for ions of lower energy (< 60keV) as well
PADs are exemplified in the electrons at all energies measured by the Imaging Electron Spectrometer (IES) with high frequencies of occurrence in the nightside equatorial zone outside of 5.5RE
Summary
The butterfly pitch angle distribution (PAD) was first described by West (1966) with observations of Soviet nuclear testing in the upper atmosphere on 28 October 1962. In this region of quasitrapping, charged particles with pitch angles near 90◦ cannot orbit the earth in stable drift paths, but more field-aligned particles, which are not as affected by drift shell splitting, may remain in stable drift paths It can be shown through particle modeling that the outer regions of Polar’s observations of the nightside equatorial plane are subject to the differential drift paths created in the quasi-trapping region. This is done by using a fully three-dimensional particle tracer in simulated magnetic and electric fields
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