M. Bzowski et al., Ionization-induced heat flow in heliospheric hydrogen: Virtues and flaws of hydrodynamic treatments, ASTROPHYS J, 544(1), 2000, pp. 496-507
The interaction of the inflowing interstellar neutral hydrogen gas with the
radially expanding solar wind plasma has recently been treated by use of l
owest-moments hydrodynamic approaches. Though a Boltzmann-kinetic treatment
of the neutral hydrogen how is advised here, a hydrodynamic treatment is m
uch simpler and has some interesting descriptive virtues. We check the regi
on of applicability of a simple hydrodynamic model by comparing its results
with results of a full three-dimensional kinetic approach and show that ex
cellent agreement is found in the density and bulk velocity distributions e
ven at small heliocentric distances of a few astronomical units on the upwi
nd side. We do show, however, that differential elimination of hydrogen ato
ms by charge exchange with salar wind protons and by solar EUV photoionizat
ion induces a squeezed, non-Maxwellian shape of the distribution function.
Because of this asymmetry, a local heat flux appears in the hydrogen gas. A
t this level, no comparison of a kinetic modeling with simple hydrodynamic
approaches is possible anymore. We check the significance of the ionization
-invoked heat flux in the inner heliosphere and show that during solar mini
mum it does not exceed 4% of the thermal energy flux carried with the how o
f the gas within similar or equal to 10 AU. For demonstration purposes, we
develop an analytic one-dimensional kinetic representation of the hydrogen
distribution function and test its accuracy. We show that this approach can
be used to calculate the main features of the local asymmetric H-atom dist
ribution. The model is suitable to calculate the bulk velocity of atoms for
solar minimum conditions (mu similar or equal to 1) and to check the signi
ficance of heat flux with respect to the thermal energy flux carried by the
hydrogen gas in the upwind hemisphere.