Charge-exchange Induced Modulation of the Heliosheath Ion Distribution Downstream of the Termination Shock

Abstract:

We consider the evolution of the solar wind ion distribution function along
the plasma flow downstream from the termination shock induced by charge
exchange processes with cold interstellar H-atoms. We start from a kinetic
phase space transport equation valid in the bulk frame of the plasma flow
that takes into account convective changes, cooling processes, energy
diffusion and ion injection, and describes solar wind and pick-up ions
as a co-moving, isotropic, joint ion population. From this kinetic transport
equation one can ascend to an equation for the pressure moment of the ion
distribution function, a so-called pressure transport equation, describing
the evolution of the ion pressure in the comoving rest frame. Assuming that
the local ion distribution can be represented by an adequate kappa function
with a kappa parameter that varies with the streamline coordinate, we
obtain an ordinary differential equation for kappa as function of the
streamline coordinate s. With this result then we gain the heliosheath ion
distribution function downstream of the termination shock. The latter then
can be used to predict the Voyager-2 measured moments of the distribution
function like ion density and ion temperature, and it can also be used to
predict spectral fluxes of ENA`s originating from these ions and registered
by IBEX-Hi and IBEX-Lo.We especially analyse the solar wind ion temperature decrease
measured by Voyager-2 between the years 2008 to 2011 and try to explain it
as a charge-exchange induced cooling of the ion distribution function during
the associated ion convection period.