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

Abstract

[1] A quasilinear theory is presented for the relaxation of the proton and helium distribution functions and the associated excitation of ion cyclotron waves, downstream of the quasi-perpendicular Earth's bow shock. 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. In a simplified treatment of the downstream relaxation process, neglecting wave dispersion and assuming that the downstream reflected proton speed v 0 is large compared with the Alfven speed V A , the wave and proton quasi-equilibrium predictions are qualitatively in agreement with the observations. In a more precise treatment limited to a perpendicular configuration, in which dispersion is included and v 0 may be comparable with V A , the agreement with the observations is generally very good if the contribution of the transmitted core protons is included. One interesting feature is that the minor ion helium may contribute to the wave power, and in return change the proton and helium distribution functions. The helium contribution is estimated but requires further investigation.

Case studies of mirror-mode structures observed by THEMIS in the near-Earth tail during substorms

“…The compression of magnetic field accompanied with the dipolarizations is obvious at each dipolarization event, which can heat the ions preferentially in the direction perpendicular to the magnetic field and produce the perpendicular anisotropy of ions. This adiabatic acceleration of ions in the plasma sheet is different from the mechanism for the magnetosheath plasma anisotropy, which comes from the heating of the bow shock [e.g., Liu et al, 2005].…”

“…The magnetic holes have been well studied in the solar wind [Stevens and Kasper, 2007, and references therein] and the magnetosheaths [Joy et al, 2006;Soucek et al, 2008, and references therein]. In the planetary magnetosheaths, the pressure anisotropy is generated as the plasma flow moves through the bow shock where the ion temperature increases with a larger proportion of heating preferentially going to the perpendicular component [Pokhotelov et al, 2001;Liu et al, 2005]. Close to the bow shock, mirror modes are generated and appear as quasi-sinusoidal waves in the early stages of development.…”

Physical Processes in the Outer Heliosphere

“…Similar behavior is observed at Earth's bow shock (Sckopke et al 1990). However, these distributions are unstable (Liu et al 2005; and relax to nearly isotropic distributions further downstream of the TS. For the ENA intensities observed by IBEX, which averages over the heliosheath, we need to consider the broader shock transition between states in which the proton distributions are nearly isotropic.…”

“…At supercritical quasi-perpendicular shocks the ion heating occurs by ion reflection in the shock "ramp" due to a combination of electric and magnetic fields (Leroy et al 1982;Gosling and Robson 1985;Liu et al 2005). A fraction of the incoming upstream ions are reflected back into the upstream plasma where they gyrate back to the shock ramp while they gain energy in the motional (E = −c −1 V × B 0 ) electric field.…”

Coherent wave activity in Mercury's magnetosheath

“…However, in comparison to the left-hand polarized waves, the right-hand detections are relatively rare and both less coherent and less transverse than the left-hand polarized observations. This may either be a consequence of the negatively Doppler shifted waves being more difficult to observe, as lower frequency waves require longer periods of undisturbed conditions which makes them more prone to low-frequency noise and changes in the magnetosheath magnetic field, or it could be an effect of the driving instability, since at the quasi-perpendicular shock the reflected-gyrating downstream ions gain a velocity component parallel to the magnetic field, so that the distribution function is not perfectly symmetrical in the field-parallel direction [Liu et al, 2005]. The solar wind ion distribution often includes secondary proton and alpha particle streams that propagate faster than the bulk plasma, which may also contribute to anisotropic ion distributions downstream [e.g., Marsch et al, 1982aMarsch et al, , 1982b.…”