Ionization-induced heat flow in heliospheric hydrogen: Virtues and flaws of hydrodynamic treatments

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.