Ion thermalization in quasi‐perpendicular shocks involving reflected ions

Abstract

Ion thermalization mechanisms downstream of the quasi-perpendicular Earth's bow shock are examined by means of plasma and magnetic field data from the AMPT•/IaM spacecraft which include three-dimensional ion distributions, plasma fluid parameters derived every ~ 4.3 s, and spectra of transverse and parallel magnetic fluctuations up to 16 Hz. The objects studied in detail are low-Mach number, low-f/shocks in which reflected-gyrating ions are present and contribute to the downstream ion temperature but where processes beyond the ramp take place slowly, so that the basic phenomenology becomes apparent. In MHD terms, most of these shocks qualify as marginally critical. Downstream of the ramp, the initially separated core and ring ions slowly merge into a joint, less anisotropic distribution possessing a high-energy tail. The ion temperature ratio, Ti/Tii, is high not only in the shock foot and ramp but also within some distance downstream; its speed of decline rises and the residual level lessens with increasing f/. The ions diffuse about equally fast in energy and in pitch angle. An asymmetry of the distributions with respect to the field direction is present when the shock is slightly oblique. It decays only slowly, which might indicate that the pitch angle diffusion rate near zero pitch angle is reduced. Low-frequency electromagnetic waves are present below the proton gyrofrequency; they are characterized by strong left-hand-polarized emissions and a low level of parallel fluctuations except very close to the shock. The left-hand emissions are often concentrated into a narrow frequency band but sometimes they exhibit a double-humped structure. Waves and ion distributions approach a slowly varying equilibrium some distance downstream of the shock. After extending the analysis to one supercritical shock representing the majority of bow shock encounters, we conclude that our deductions are more generally valid, although the thermalization is faster, usually, and appears to involve nonlinear processes which tend to obscure most of the features noted. 1. sipated at the bow shock. In quasi-perpendicular shocks, the dominant dissipation process is ion heating. For shocks above a certain Mach number, a consistent model of the ion heating mechanism has evolved in the course of the last 20 years from the results of laboratory experiments, theoretical studies, computer simulations, and observations in space. But several details of the downstream thermalization still are unsupported by direct observations. It is the purpose of this paper to present and discuss such observations.The essential element of the ion heating mechanism in higher-Mach number, quasi-perpendicular shocks is the initial reflection of some of the incident ions off the shock surface. The majority of the ions (• 80 %) are directly transmitted through the shock. Their temperature rises by more than the adiabatic rate that corresponds to the magnetic field compression. A similar, or even higher contribution to the total ion temperature is provided by the refl...

Nongyrotropy in magnetoplasmas: simulation of wave excitation and phase-space diffusion

“…Important examples are collisionless shock waves and the interaction of the solar wind with comets. At collisionless shocks nongyrotropy has been detected in the electron population as well as in the ion components (Gosling et al,1982;Thomsen et al, 1985;Anderson et al, 1985;Sckopke et al, 1990;Fuselier et al, 1990). The interaction of the solar wind with cometary ions may also drive nongyrotropy in the cometary ion component (Motschmann and Glassmeier, 1993;Neubauer et al, 1993;Glassmeier and Neubauer, 1993;Coates et al, 1993).…”

Nongyrotropic particle distributions in space plasmas

“…These distributions were observed at pickup processes in space plasmas especially when the pickup is burst-like. Evidence for nongyrotropy was found at comets Glassmeier and Neubauer, 1993;Coates et al, 1993;Cao et al, 1998), at collisionless shocks (Gosling et al, 1982;Anderson et al, 1985;Thomsen et al, 1985;Sckopke et al, 1990;Fuselier et al, 1990), in the plasma environment of the shuttle (Cairns, 1990), at tangential discontinuities in the solar wind (Astudillo et al, 1996), in the plasma sheet boundary layer (BuÈ chner and Kuska, 1996), and in the geomagnetotail (Ashour-Abdalla et al, 1996).…”

A quasilinear theory of ion “thermalization” and wave excitation downstream of Earth's bow shock

“…1. In the events investigated by Sckopke et al [1990], in which V A tends to be large, b < 1. Simulations by Yoon [1992] show that, even if the mirror mode is excited, the ion cyclotron mode saturates at a much larger intensity compared with the mirror mode though initially the mirror mode may grow at a slightly faster rate.…”

“…In order to investigate the relaxation process in more detail, Sckopke et al [1990] selected several lowerMach-number marginally supercritical crossings of Earth's bow shock in the AMPTE/IRM data, which exhibit a slower evolution of the proton-wave interaction in a parcel of downstream plasma. Their data include time profiles of the ambient magnetic field, the magnetic fluctuation power, plasma fluid parameters, and the temperature anisotropy for each crossing.…”

“…The theory predicts the wave polarization, power and peak frequency, and the proton bulk velocity and temperature anisotropy, sufficiently far downstream of the shock that the ions and waves have relaxed to a quasiequilibrium. The results are compared with the AMPTE/IRM crossings of the marginally supercritical bow shock documented by Sckopke et al (1990), for which the number of ''reflected'' protons is small and the quasilinear approximation is expected to be valid. The theory starts with the trajectories of the specularly reflected protons and transmitted helium in the laminar electromagnetic fields of the shock transition.…”