Abstract
Abstract. We report occasional observations of two simultaneously distinct ion foreshock components recorded by the Cluster spacecraft upstream of the Earth's bow shock. In most occurrences, the lower-energy population originates as a field-aligned beam (FAB) associated with quasi-perpendicular regions, which loses energy as the IMF rotates into oblique geometries. A second beam, with energies in excess of ~10 keV, appears sometimes in association with the onset of ultra-low frequency (ULF) waves, and sometimes ahead of the appearance of the latter. Measurements from the mass spectrometer indicate that both beams consist of protons. While the lower-speed beam is well-accounted for by a known reflection mechanism, the non-radial IMF orientations as well as other arguments seem to rule out magnetosheath or magnetospheric sources for the higher energy component. The wave characteristics are typical of the oblique foreshock and we have found that they are in cyclotron-resonance with the low speed beam (FAB). These observations constitute a theoretical challenge since conventional mechanisms described in the literature cannot account for the production of beams at two different energies.
Highlights
It is well recognized that the characteristics of regions upstream of collisionless shocks are strongly controlled by shock geometry, which is determined by the angle θBn between the upstream magnetic field and the shock normal direction
The authors argued that the tail is associated with ions emanating from the quasi-parallel shock region while it is known that field-aligned beam (FAB) are seen on interplanetary magnetic field (IMF) lines connected to quasiperpendicular geometries
This possibilty may be observed when the spacecraft is located near a boundary between the FAB and the diffuse ion populations The results presented below are of particular interest, because we report for the first time the occurrence of distinct, simultaneous field-aligned beam populations
Summary
It is well recognized that the characteristics of regions upstream of collisionless shocks are strongly controlled by shock geometry, which is determined by the angle θBn between the upstream magnetic field and the shock normal direction. FABs are produced by shock drift acceleration, in which the particle energization results from gradient drift motions along the shock surface parallel to the tangential component of the motional electric field (Paschmann et al, 1980) Where this mechanism satisfactorily accounts for the beam bulk speed, it suffers from the injection problem, as only a tiny portion from the tail of the incident solar wind beam distribution has pitch angles that should result in mirroring in the shock ramp (Meziane et al, 2005). The authors argued that the tail is associated with ions emanating from the quasi-parallel shock region while it is known that FABs are seen on IMF lines connected to quasiperpendicular geometries This possibilty may be observed when the spacecraft is located near a boundary between the FAB and the diffuse ion populations The results presented below are of particular interest, because we report for the first time the occurrence of distinct, simultaneous field-aligned beam populations. In another study, Kis et al (2007)
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