A kinetic control of the heliospheric interface hydrodynamics of charge-exchanging fluids

Abstract

Abstract.It is well known that the Solar System is presently moving through a partially ionized local interstellar medium. This gives rise to a counter-flow situation requiring a consistent description of behaviour of the two fluids -ions and neutral atoms -which are dynamically coupled by mutual charge exchange processes. Solutions to this problem have been offered in the literature, all relying on the assumption that the proton fluid, even under evidently nonequilibrium conditions, can be expected to stay in a highly-relaxated distribution function given by mono-Maxwellians shifted by the local proton bulk velocity. Here we check the validity of this assumption, calculating on the basis of a Boltzmann-kinetic approach the actually occurring deviations. As we show, especially for low degrees of ionization, ξ ≤ 0.3, both the H-atoms and protons involved do generate in the heliospheric interface clearly pronounced deviations from shifted Maxwellians with asymmetrically shaped distribution functions giving rise to non-convective transport processes and heat conduction flows. Also in the inner heliosheath region and in the heliotail deviations of the proton distribution from the hydrodynamic one must be expected. This sheds new light on the correctness of current calculations of H-atom distribution functions prevailing in the inner heliosphere and also of the Lyman-α absorption features in stellar spectra due to the presence of the hydrogen wall atoms. Deviations from LTE-functions would be even more pronounced in magnetic interfaces, which via CGL-effects cause temperature anisotropies to arise.

The puzzling MILAGRO hot spots

“…The existence of such an effect has been directly confirmed by observations with the Ulysses spacecraft and indirectly by optical back-scatter observations [20,17]. Detailed analytical and numerical studies of this effect were first published by Fahr and co-workers back in the early 1970s [8,4].…”

Theoretical aspects of energetic neutral atoms as messengers from distant plasma sites with emphasis on the heliosphere

“…[28] On the other hand, only very recently, it was recognized and theoretically taken into account that under the influence of charge exchange interactions between countermoving neutral and ionized media not only do the hydrogen distribution functions attain a kinetic, non-Maxwellian character, but the proton distribution function does as well. Because of charge exchange -induced newly created protons outside of the bulk in velocity space of the other protons a permanent perturbation of the hydrodynamic proton distribution is locally maintained, which cannot relaxate rapidly enough by locally operating relaxation processes; for an illustration, see Figure 8 taken from Fahr and Bzowski [2004a]. These perturbations at many places in the heliospheric interface are of nonnegligible order of magnitude as estimated by Fahr and Bzowski [2004b] or Chashei and Fahr [2005].…”

“…As a result, non‐Maxwellian distribution functions f p and f H of both protons and H atoms appear, shown by black lines. Taken from Fahr and Bzowski [2004a].…”

“…In recent papers by Fahr and Bzowski [2004a] and Bzowski et al [2006], kinetic treatments based on the Boltzmann‐Vlasow equation have been carried out to describe perturbations in the HD distribution functions of neutral and ionized species in the heliospheric interface. In these papers the deviations of hydrogen and proton distribution functions become clearly eminent, while even more impressive nonequilibrium features appear at the passage of O atoms and O + ions of the local interstellar medium (LISM) over the outer heliospheric bow shock, where O + ions are converted into a highly anisotropic double‐ring distribution, which gradually also couples to the oxygen atom distribution (see Figure 9).…”

“…[29] In recent papers by Fahr and Bzowski [2004a] and Bzowski et al [2006], kinetic treatments based on the Boltzmann-Vlasow equation have been carried out to describe perturbations in the HD distribution functions of neutral and ionized species in the heliospheric interface.…”