Abstract
Turbulence in space plasmas usually exhibits two regimes separated by a spectral break that divides the so called inertial and kinetic ranges. Large scale magnetic fluctuations are dominated by non-linear MHD wave-wave interactions following a −5/3 or −2 slope power-law spectrum. After the break, at scales in which kinetic effects take place, the magnetic spectrum follows a steeper power-law k−α shape given by a spectral index α > 5/3. Despite its ubiquitousness, the possible effects of a turbulent background spectrum in the quasilinear relaxation of solar wind temperatures are usually not considered. In this work, a quasilinear kinetic theory is used to study the evolution of the proton temperatures in an initially turbulent collisionless plasma composed by cold electrons and bi-Maxwellian protons, in which electromagnetic waves propagate along a background magnetic field. Four wave spectrum shapes are compared with different levels of wave intensity. We show that a sufficient turbulent magnetic power can drive stable protons to transverse heating, resulting in an increase in the temperature anisotropy and the reduction of the parallel proton beta. Thus, stable proton velocity distribution can evolve in such a way as to develop kinetic instabilities. This may explain why the constituents of the solar wind can be observed far from thermodynamic equilibrium and near the instability thresholds.
Highlights
In many space environments the media is filled by a weakly collisional plasma
From the theoretical kinetic plasma physics point of view, on the basis of the linear and quasilinear theory approximations of the dynamics of the plasma, it is possible to predict the thresholds in the temperature anisotropy and plasma beta parameter space that separate the stable and unstable regimes, and how the plasma evolves toward such states
Turbulence is ubiquitous in space environments and any relaxation process should occur in the presence of a background turbulent magnetic spectrum, e.g., relaxation to quasi-stationary states out of thermodynamic equilibrium, non-Maxwellian characteristics in poorly collisional plasmas, and temperature anisotropy regulation by micro-instabilities or other processes, among others
Summary
In many space environments the media is filled by a weakly collisional plasma. Coulomb collisions represent an efficient mechanism for relaxing plasma parcels toward a thermodynamic equilibrium state in which the particle Velocity Distribution Functions (VDFs) achieve a Maxwellian profile [1, 2], when collisions are scarce Coulomb scattering becomes ineffective in establishing equilibrium. From the theoretical kinetic plasma physics point of view, on the basis of the linear and quasilinear theory approximations of the dynamics of the plasma, it is possible to predict the thresholds in the temperature anisotropy and plasma beta parameter space that separate the stable and unstable regimes, and how the plasma evolves toward such states These models are useful to study the generation and first saturation of the electromagnetic energy at the expense of the free energy carried by the plasma. We perform such systematic study by computing the quasilinear relaxation of the ion-cyclotron temperature anisotropy instability, considering different choices of the initial level of the magnetic field fluctuations, and the shape of the spectrum We analyze their effect on the relaxation of the instability and the time evolution of the macroscopic properties of the plasma that are involved.
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