Abstract
Abstract. Two distinct populations of reflected and accelerated ions are known to originate from quasi-perpendicular shocks, gyrating ions and reflected ion beams. Recent observations under such bow shock conditions with Cluster have shown strong evidence that both particle distributions appear to emerge from the same reflection process. In this paper the basic production mechanism of field-aligned beams has been investigated by using CLUSTER multi-spacecraft measurements. We have analyzed several quasi-perpendicular shocks with the Cluster Ion Spectrometry experiment (CIS) and followed the spatial and temporal evolution of the reflected and transmitted ion populations across the shock. These observations show that the field-aligned beams most likely result from effective scattering in pitch angle during reflection in the shock ramp. Investigating a low Mach number shock, leakage of a fraction of the thermalized ion distribution in the downstream region does not appear to be the source as the volume in phase space occupied by beam ions is empty downstream of the shock ramp.
Highlights
IntroductionThere have been a number of proposals to produce the field-aligned ion beams. Sonnerup (1969) demonstrated that solar wind protons could be energized if the bow shock could manage to turn them around in such a way that they left the shock reasonably well field-aligned
A very prominent and well-known feature of the Earth’s bow shock is the presence of ions backstreaming into the foreshock region (e.g. Lin et al, 1974)
While we have concluded from the sequence of observations of ion distributions that leakage from far downstream cannot be the main process for the formation of the ion beam, the observations at these large θBn suggest that direct reflection of solar wind ions at the bow shock is most likely not the basic formation process of the field-aligned ion beams
Summary
There have been a number of proposals to produce the field-aligned ion beams. Sonnerup (1969) demonstrated that solar wind protons could be energized if the bow shock could manage to turn them around in such a way that they left the shock reasonably well field-aligned. Kucharek et al.: On the origin of field-aligned beams at the quasi-perpendicular bow shock the particle statistics in this part of phase space He found that the density of the reflected ions decreases with increasing Bn, as well as with increasing Mach number. Solar wind ions incident on a quasi-perpendicular shock are specularly reflected, accelerated by the upstream v×B motional electric field to ∼2 v1, where v1 is the upstream flow speed These ions are transmitted downstream and constitute after sufficient pitch angle scattering an approximately isotropic high energy shell in the downstream rest frame. Burgess and Luhmann (1986) have tested one part of the scenario proposed by Tanaka et al (1983) They studied the propagation of low energy (but suprathermal) protons through the magnetosheath in a model obtained from gas-dynamic simulations. Since it turned out that the protons in the beams originate from the wings of the incident Maxwellian distribution, he used a test particle technique, combined with the hybrid simulation, to enhance
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