Generation of MHD turbulence by non-equilibrium ion velocity distributions in the outer heliosphere and the interstellar medium: magnetohydro-thermodynamic and kinetic views

Abstract

We show that energetic ions, appearing in plasmas due to injection of charge-exchange-induced new ions into the magnetized plasma background, flow due to their freely convertible energy drive turbulences of Alfvenic and magneto-acoustic types. This becomes important especially under conditions of strongly charge-exchanging fluids with supersonic differential bulk velocities, as is the condition in the inner heliosphere. These ion-driven waves lead to strongly enhanced fluctuation amplitudes. As a consequence, both low- and high-energy ion populations can partly reabsorb these wave energies, and thereby experience non-thermal heatings. As a quantitative example, we calculate the non-adiabatic behaviours of solar wind ions and pick-up ions expanding with the solar wind towards large solar distances. While pick-up ions behave nearly isothermal, normal solar wind ions behave pseudo-polytropically with negative effective polytropic indices that describe temperature increases at diverging plasma flows. Furthermore, we demonstrate that charge-exchanging plasmas in the inner and outer heliosheath, such as the solar wind and the interstellar ion and atom fluids, lead to unexpectedly enhanced turbulence amplitudes of low frequency Alfven and magneto-acoustic waves. Connected with that, lower periods for the relaxation of non-equilibrium ion distribution functions and lower coefficients for the spatial diffusion of galactic cosmic rays must be expected in the heliosheath. Finally, we study the interstellar turbulence generation caused by supernova shock fronts due to their generation of mirror-unstable anisotropic ion distribution functions downstream of the shock. We demonstrate that an unexpectedly high percentage of the shock-entropized thermal energy of the downstream ions is converted into magneto-acoustic turbulences. This not only characterizes the region immediately downstream of the SN shock as a high-turbulence region, but may, judged in a statistical view, also point to an increase of the average turbulence levels in the interstellar medium in general.

Keywords:

• Heliosphere
• MHD turbulence
• Plasma heating
• Ion distribution function

Acknowledgements

We very much appreciate the financial support through the DFG grant 436 RUS 113/110/12-3, and the Program ‘Plasma processes in the solar system’ of Russian Academy of Sciences. H.J.F. is also grateful to the financial support granted by the DFG in the frame of the project Fa-97/31-1.