Imprints of a heliospheric bowshock on interstellar oxygen populations

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 over-shoot 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. © Arbeitsgemeinschaft extraterrestrische Forschung 2006.