Premium
On the radial evolution of κ distributions of pickup protons in the supersonic solar wind
Author(s) -
Fahr HansJörg,
Fichtner Horst,
Scherer Klaus
Publication year - 2014
Publication title -
journal of geophysical research: space physics
Language(s) - English
Resource type - Journals
eISSN - 2169-9402
pISSN - 2169-9380
DOI - 10.1002/2014ja020431
Subject(s) - heliosphere , physics , solar wind , convection–diffusion equation , phase space , diffusion , distribution function , shock (circulatory) , supersonic speed , diffusion equation , ambipolar diffusion , mechanics , plasma , thermodynamics , nuclear physics , medicine , economy , economics , service (business)
It is well known that (pickup) ions in the inner heliosphere do not maintain Maxwellian distributions but tend to nonequilibrium distributions with extended suprathermal tails. Such states have been classified as quasi‐equilibria which in many cases can well be described by so‐called κ distributions. With the present study we start out from a phase space transport equation for pickup ions in the inner heliosphere that adequately describes the most important processes such as injection, convection, cooling, and diffusion in velocity space. Assuming that the underlying distribution functions are κ distributions, we proceed from this transport equation to a second‐order moment, i.e., pressure equation which represents an ordinary differential equation for the κ parameter as function of heliocentric distance. This strategy allows one to describe the transition from an initial Vasyliunas‐Siscoe distribution to κ distributions with gradually more pronounced suprathermal tails. While the velocity dependence of the velocity diffusion coefficient determines the systematic reduction of the parameter κ , the latter always has the (formal) asymptotic valueκ ∞ = 3 / 2 . This translates into values of 1.5≤ κ TS ≤2.2 in the upstream region of the (upwind) solar wind termination shock that defines the outer validity range of the model.