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SH43C-06:
On the Radial Evolution of κ-Distributions of Pick-up Protons in the Supersonic Solar Wind

SH43C-06:

On the Radial Evolution of κ-Distributions of Pick-up Protons in the Supersonic Solar Wind

Thursday, 18 December 2014: 2:51 PM

##### Abstract:

It is well known that (pick-up) ions in the inner heliosphere do notmaintain Maxwellian distributions but tend to non-equilibrium 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 pick-up 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 adequate κ-parameter as function of the

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 value

of 3/2. This translates into values of 1.5 < κ < 2.2

in the upstream region of the (upwind) solar wind termination shock that defines

the outer validity range of the model.