Abstract
Abstract. We start our considerations from two more recent findings in heliospheric physics: One is the fact that the primary solar wind protons do not cool off adiabatically with distance, but appear to be heated. The other one is that secondary protons, embedded in the solar wind as pick-up ions, behave quasi-isothermal at their motion to the outer heliosphere. These two phenomena must be physically closely connected with each other. To demonstrate this we solve a coupled set of enthalpy flow conservation equations for the two-fluid solar wind system consisting of primary and secondary protons. The coupling of these equations comes by the heat sources that are relevant, namely the dissipation of MHD turbulence power to the respective protons at the relevant dissipation scales. Hereby we consider both the dissipation of convected turbulences and the dissipation of turbulences locally driven by the injection of new pick-up ions into an unstable mode of the ion distribution function. Conversion of free kinetic energy of freshly injected secondary ions into turbulence power is finally followed by partial reabsorption of this energy both by primary and secondary ions. We show solutions of simultaneous integrations of the coupled set of differential thermodynamic two-fluid equations and can draw interesting conclusions from the solutions obtained. We can show that the secondary proton temperature with increasing radial distance asymptotically attains a constant value with a magnitude essentially determined by the actual solar wind velocity. Furthermore, we study the primary proton temperature within this two-fluid context and find a polytropic behaviour with radially and latitudinally variable polytropic indices determined by the local heat sources due to dissipated turbulent wave energy. Considering latitudinally variable solar wind conditions, as published by McComas et al. (2000), we also predict latitudinal variations of primary proton temperatures at large solar distances.Key words. Interplanetary physics (interstellar gas, plasma waves and turbulence; solar wind plasma)
Highlights
We can show that the secondary proton temperature with increasing radial distance asymptotically attains a constant value with a magnitude essentially determined by the actual solar wind velocity
For many years it has been recognized that the solar wind dynamics and thermodynamics at larger distances are influenced by the creation and incorporation of pick-up ions, secondary ions which are produced from the ionization of neutral interstellar H-atoms and move simultaneously with the solar wind bulk
We first formulate the coupled system of enthalpy flow conservation equations to describe the thermodynamics of the two-fluid solar wind plasma, consisting of primary and secondary protons in terms of pressures P1 and P2, taking into account the effects of adiabatic cooling and of heating both by the diffusive energy flux k(k = k0, r) in the convected turbulence spectral power and by locally generated turbulent energy Q1,2 locally pumped into the wave field by freshly injected secondary ions
Summary
For many years it has been recognized that the solar wind dynamics and thermodynamics at larger distances are influenced by the creation and incorporation of pick-up ions, secondary ions which are produced from the ionization of neutral interstellar H-atoms and move simultaneously with the solar wind bulk. As shown in VOYAGER1/2 and PIONEER-11 data primary solar wind ion temperatures T1 fall off with distance in a non-adiabatic manner inside of 20 AU and stay rather constant at distances beyond, though their temperatures there definitively remain much below those of the secondary ions, i.e. T1 ≤ T2 This clearly demonstrates that heat sources are operating at the expansion of both fluids, which, are different in magnitude for the two different ion species, with the energy injection rate to secondary ions obviously being greater. In the following part of the paper we study these different heat sources and want to show that the different behaviours with solar distance r of the two temperatures T1(r) and T2(r) can thereby be explained
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