Imprints of a heliospheric bowshock on interstellar oxygen populations

Abstract

Abstract.It is well known that the nearby interstellar medium represents a partially ionized gas composed by different chemical species, amongst them hydrogen, helium and oxygen as the most abundant elements. While the passage of the interstellar protons and H-atoms over the solar system has been satisfactorily well modelled meanwhile, the entrance into the heliosphere of the other interstellar chemical species needs some additional care. Here we especially follow with Boltzmann-kinetic treatments the phase-space history of interstellar oxygen ions and atoms at their passage over the outer heliospheric bowshock and at their further approach towards the inner heliosphere. We describe the nose region of the bowshock as a mild MHD shock of a nearly perpendicular type. While the O-atoms pass over this shock with no velocity-space imprints, by contrast the O + -ions run over the shock structure with a typical overshoot velocity and on the downstream region are then picked up by the frozen-in magnetic fields that comove with the plasma bulk flow, i.e. the proton bulk flow. This leads to a strongly non-relaxated O-ion velocity distribution which substantially differs both from the local O-atom and proton distribution functions. Due to a strong charge exchange coupling between O-ions/atoms and H-atoms/ions the distributions downstream from the bowshock undergo permanent changes which we estimate in this study here. Even though relaxation processes operate by Coulomb collisions, wave-particle interactions and elastic atom-atom collisions, the bowshock imprints on the O-ion and atoms distribution functions may be conserved throughout the whole heliospheric interface, and, as we are going to suggest in this paper, in fact as such may be used as tracers to the physics and the nature of the bowshock. One of the main conclusions is that the population of O-atoms created in the vicinity of the bow-shock should carry a signature of the effective shock strength that could be detectable if the LIC magnetic field is about 2 microgauss or stronger.

Kinetic study of the ion passage over the solar wind MHD termination shock

“…There are other instances, though, when the thickness of the shock does matter. For example, if the shock is thicker than the average ion gyroradius, heavy ions in an astrophysical plasma will conserve their magnetic moments at the passage, while for an abrupt shock, these ions cannot adiabatically adapt to the change in the B-field magnitude and direction (Bzowski et al 2006).…”

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

“…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).…”

“…Dashed line shows the corresponding normalized distribution function of neutral O atoms immediately downstream of the shock. Taken from Bzowski et al [2006].…”

Upstream ions converting into downstream pick-up ions: the effect of shock-decelerated frozen-in fields

“…A comparable system related to the heliospheric bowshock and astrophysical boundary layers was studied by Bzowski et al (2006), who e.g., described overshooting of oxygen ions at the outer bow shock of the solar system. Therefore, if the shock represents a sufficiently abrupt spatial structure, there appears a need to conserve the -what one could call -"total magnetic moment", μ tot , connected with the total perpendicular ion motion, adequately including overshoot velocities (see also Bzowski et al 2006). As we consider further below, this could perhaps best be respected by requiring that (see also Bzowski et al 2006) …”

“…Therefore, if the shock represents a sufficiently abrupt spatial structure, there appears a need to conserve the -what one could call -"total magnetic moment", μ tot , connected with the total perpendicular ion motion, adequately including overshoot velocities (see also Bzowski et al 2006). As we consider further below, this could perhaps best be respected by requiring that (see also Bzowski et al 2006) …”