Vlasov Plasma Turbulence in the Solar Wind at Proton Kinetic Scales

Abstract:

Solar-wind heating through turbulent dissipation at kinetic wavelengths represents one of the most studied and challenging problems in the field of space plasma physics. In this work, kinetic effects in the turbulent solar-wind plasma are investigated by means of multi-dimensional simulations of the hybrid Vlasov-Maxwell (HVM) model [1]. Using 5D (2D in space and 3D in velocity space) and full 6D simulations of plasma turbulence, it is found that kinetic effects manifest through the deformation of the proton distribution function (DF), with patterns of non-Maxwellian features being concentrated near regions of strong magnetic gradients.

Recent analyses [2] of solar-wind data from spacecraft aimed to quantify kinetic effects through the temperature anisotropy (T_⊥/T_//) on the proton velocity DF, with respect to the local magnetic field. Values of the anisotropy range broadly, with most values between 10^-1and 10. Moreover, the distribution of temperature anisotropy depends systematically on the ambient proton parallel beta (β_//), the ratio of parallel kinetic pressure to magnetic pressure, manifesting a characteristic rhomboidal shape.

In order to make contact with solar-wind observations, temperature anisotropy has been evaluated from an ensemble of HVM simulations [3], obtained by varying the global plasma beta and fluctuation level, in such a way to cover distinct regions of the parameter space defined by T_⊥/T_// and β_//. The HVM simulations presented here demonstrate that, when the DF is free to explore the entire velocity subspace, new features appear as complex interactions between the particles and the turbulent background.

In particular, our numerical results indicate that the main direction of the proper temperature anisotropy, calculated in the main reference frame of the DF [4], has a finite probability of being along or across the ambient magnetic field, and is associated with magnetic intermittent events and with gradient-type structures in the flow and in the density. Comparison of numerical results with solar-wind data shows remarkable quantitative agreement.

[1] B. A. Maruca et al., Phys. Rev. Lett. 107, 201101 (2011).

[2] F. Valentini et al., J. Comput. Phys. 225, 753 (2007).

[3] S. Servidio et al., AstroPhys. J. Lett. 781, L27 (2014).

[4] S. Servidio et al., Phys. Rev. Lett. 108, 045001 (2012).