The "step feature" of suprathermal ion distributions: a discriminator between acceleration processes?

Abstract

Abstract. The discussion of exactly which process is causing the preferred build-up of v −5 -power law tails of the velocity distribution of suprathermal particles in the solar wind is still ongoing. Criteria allowing one to discriminate between the various suggestions that have been made would be useful in order to clarify the physics behind these tails. With this study, we draw the attention to the so-called "step feature" of the velocity distributions and offer a criterion that allows one to distinguish between those scenarios that employ velocity diffusion, i.e. second-order Fermi processes, which are prime candidates in the present debate. With an analytical approximation to the self-consistently obtained velocity diffusion coefficient, we solve the transport equation for suprathermal particles. The numerical simulation reveals that this form of the diffusion coefficient naturally leads to the step feature of the velocity distributions. This finding favours -at least in regions of the appearance of the step feature (i.e. for heliocentric distances up to about 11 AU and at lower energies) -the standard velocity diffusion as a consequence of the particle's interactions with the plasma wave turbulence as opposed to that caused by velocity fluctuation-induced compressions and rarefactions.

On the radial evolution of κ distributions of pickup protons in the supersonic solar wind

“…From the observational side, it has long been recognized that PUIs exhibit a core distribution below the injection velocity and an extended tail at higher velocities often characterized by a so-called (−5) velocity power law [Gloeckler, 2003;Fisk et al, 2010;Hill et al, 2009]. In consecutive papers by, e.g., Fahr and Fichtner [2011], Fahr and Fichtner [2012], and Zhang and Schlickeiser [2012], it has been attempted to describe this velocity space feature of PUIs as a phase-space transport phenomenon resulting from a joint action of injection, cooling, and velocity diffusion. It turned out that with appropriately shaped, but physically motivated velocity diffusion coefficients, one can convincingly reproduce the observed features of a core distribution and a tail distribution of PUIs.…”