Abstract
Abstract. Coherent downstream oscillations of the magnetic field in shocks are produced due to the coherent ion gyration and quasiperiodic variations of the ion pressure. The amplitude and the positions of the pressure maxima and minima depend on the cross-shock potential and upstream ion temperature. Two critical cross-shock potentials are defined: the critical gyration potential (CGP), which separates the cases of increase or decrease in the component of the velocity of the distribution center along the shock normal, and the critical reflection potential (CRP), above which ion reflection becomes significant. In a weak, very low upstream kinetic-to-magnetic pressure ratio, β, the shocks' CRP exceeds the CGP. For potentials below the CGP, the first downstream maximum of the magnetic field is shifted farther downstream and is larger than the second maximum. For higher potentials, the first maximum occurs just behind the ramp and is lower than the second maximum. With the increase in the upstream temperature, the CGP exceeds the CRP. For potentials below the CRP, the effects of ion reflection are negligible and the shock profile is similar to that of very low-β shocks. If the potential exceeds the CRP, ion reflection is significant, the magnetic field increase toward the overshoot becomes steeper, and the largest peak occurs at the downstream edge of the ramp.
Highlights
Collisionless shocks (CSs) are one of the most efficient accelerators of charged particles in the universe
The latter occur within the scatter-free region; ion dynamics in the shock front is intimately related to the large-scale acceleration: while the diffusive acceleration occurs at scales much larger than the shock width, the spectrum of the accelerated particles is essentially determined by conservation laws at the scatterfree shock transition
In lowβi plasmas all ions are directly transmitted across the shock without reflection, and the above findings can be summarized as follows: (a) below the critical gyration potential (CGP) the first peak is the strongest, (b) with the increase in the potential toward the CGP the first peak moves closer to the ramp, (c) upon crossing the CGP the first peak is located at the downstream edge of the ramp and is no longer the strongest
Summary
Collisionless shocks (CSs) are one of the most efficient accelerators of charged particles in the universe. The oscillating trail behind the ramp exhibited all of the features expected for a supercritical shocks, such as the largest first peak, spatially periodical peaks, and the gradual decrease in the peak amplitude Such oscillations, albeit often less ordered, were found to be common in low-Mach number shocks (Russell et al, 2009; Kajdicet al., 2012). Albeit often less ordered, were found to be common in low-Mach number shocks (Russell et al, 2009; Kajdicet al., 2012) They were successfully explained as a result of coherent ion gyration upon crossing the shock ramp and subsequent collisionless relaxation due to gyrophase mixing (Balikhin et al, 2008; Ofman et al, 2009; Ofman and Gedalin, 2013; Gedalin, 2015; Gedalin et al, 2015, 2018). Amplitudes and positions of the first peaks, which are not yet distorted by gyrophase mixing, may provide information about the cross-shock potential as well about the ion transmission and reflection
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