Letter to the Editor

Dissipation-free ion cooling in plasma flows with frozen-in fields

Argelander Institut für Astronomie der Universität Bonn, Abteilung für Astrophysik und Extraterrestrische Forschung, Auf dem Huegel 71, 53121 Bonn, Germany e-mail: msiewert@astro.uni-bonn.de

Received: 20 December 2007
Accepted: 1 April 2008

Abstract

We reinvestigate the pick-up ion phasespace transport in the inner heliosphere, this time neglecting stochastic processes like quasilinear wave-particle interactions. In the conventional literature on this subject, adiabatic deceleration of ions at their co-motion with the solar wind bulk flow has been considered, but, as demonstrated here, it is an inadequate concept in the region of the supersonically expanding solar wind. Instead, one has to consider the effect on the ion distribution function by ions at their co-motion with the solar wind having to conserve the two magnetic CGL-invariants. Conservation of the magnetic particle moment leads to velocity drifts in the velocity plane perpendicular to the magnetic field , while conservation of the second CGL invariant leads to corresponding velocity drifts along the magnetic field. As we can show, the conservation of the second CGL invariant is therefore a mere consequence of field-parallel dispersive particle motions in diverging plasma flows, such as the solar wind, flowing into regions with differential bulk motion. Taking both effects of velocity drifts together, one can then calculate the resulting ion distribution functions from an adequate Boltzmann-Vlasov transport equation and find that these functions are general power laws with ubiquitous power index of . The same power index is also found for pick-up ions at higher energies, where they are subject to stochastic energy diffusion processes by wave-particle interactions. This is perhaps the explanation of the unbroken pick-up ion power laws from the 1 keV to the 100 keV energy level.