Ionization‐induced Heat Flow in Heliospheric Hydrogen: Virtues and Flaws of Hydrodynamic Treatments

Abstract

The interaction of the inflowing interstellar neutral hydrogen gas with the radially expanding solar wind plasma has recently been treated by use of lowest-moments hydrodynamic approaches. Though a Boltzmann-kinetic treatment of the neutral hydrogen flow is advised here, a hydrodynamic treatment is much simpler and has some interesting descriptive virtues. We check the region of applicability of a simple hydrodynamic model by comparing its results with results of a full three-dimensional kinetic approach and show that excellent agreement is found in the density and bulk velocity distributions even at small heliocentric distances of a few astronomical units on the upwind side. We do show, however, that differential elimination of hydrogen atoms by charge exchange with solar wind protons and by solar EUV photoionization induces a squeezed, non-Maxwellian shape of the distribution function. Because of this asymmetry, a local heat flux appears in the hydrogen gas. At 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 minimum it does not exceed 4% of the thermal energy flux carried with the flow of the gas within ≃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 distribution. The model is suitable to calculate the bulk velocity of atoms for solar minimum conditions (μ ≃ 1) and to check the significance of heat flux with respect to the thermal energy flux carried by the hydrogen gas in the upwind hemisphere.