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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ASTROMULTISCALE (Multiscale dynamics of astrophysical plasmas: pressure-anisotropy-driven instabilities and large-scale dynamical processes)

Teaser

Summary

The project focuses on dynamical processes in plasmas, consisting of ionized atoms and electrons, which are dynamically sensitive to magnetic fields and may change the magnetic fields by their motion. The complexity of the interaction of plasma particles and the electromagnetic field leads to difficulties in effective mathematical description and physical understanding of plasma dynamics. The primary purpose of the project is deeper understanding of processes related to development of the pressure anisotropy in collisionless or weakly collisional plasmas. The pressure anisotropy can be spontaneously generated by velocity shear, plasma expansion or compression and temperature gradients when characteristic time scale for these processes is smaller than the time scale for Coulomb collisions.

Results of the project are expected to be important for general plasma physics as related to the problem of multiscale phenomena in plasmas and coupling between microphysics and macrophysical processes. The study falls into the field of fundamental research aiming at general understanding of pressure-anisotropy-related phenomena in plasmas with possible direct applications to other fields: dynamics of galaxy-cluster plasmas, dynamical processes in stellar winds and planetary magnetospheres. Due to numerous practical applications of plasmas, results of the project potentially may have some practical applications in future.

In terms of objectives, the project focuses on the investigation of (i) the transfer of the energy between the components: kinetic (mechanical), thermal and magnetic, in the presence of pressure anisotropy and related instabilities, (ii) constraints on the pressure anisotropy and other variables describing the state of the system, (iii) development of computational tools for numerical simulations of pressure-anisotropic plasmas. The study includes links between the microphysics and large-scale dynamics and the role of instabilities and effective viscosity of pressure-anisotropic plasmas. Theoretical results are contrasted with spacecraft measurements in turbulent solar wind.

Work performed

During the project two numerical codes were developed to study the problem of influence of the pressure anisotropy on plasma dynamics: a fluid Hall-MHD/MHD code with the anisotropic pressure and hybrid particle-in-cell code (PIC) with kinetic model for ions and fluid model for electrons. The two codes allow one to perform massively parallel computations using large number of processors on supercomputer systems with MPI libraries. The entire numerical setup allows to arrange easily initial setups for one, two, and three dimensional simulations.

The influence of the pressure anisotropy on plasma dynamics was investigated in an extensive series of numerical simulations of plasmas with different ratio of the thermal and magnetic energy density. A new mechanism of the transfer of the kinetic energy of plasma flows to the thermal energy has been identified as specifically related to the presence of the pressure anisotropy. The mechanism can be interpreted in terms of effective viscosity leading to constraints on the pressure anisotropy.

Constraints on the amplification of the magnetic field (so-called dynamo problem) were studied as related to the pressure-anisotropy generation in collisionless or weakly-collisional plasmas. A hierarchy of plasma models of increasing complexity was considered in the context of constraints on the small-scale dynamo action. It was shown that the conservation of the invariants of so-called double-adiabatic or CGL theory makes the maximum available magnetic energy in the dynamo process to scale inversely with the ratio of the thermal and magnetic energy density.

Results obtained in the project has been published in two peer-reviewed papers and presented in eight conference/seminar talks.

Final results

The project contributes to better understanding of differences between the energy-transfer channels in plasmas with isotropic and anisotropic pressure and provides insight into specific constraints on dynamics that appear in case of collisionless or weakly-collisional plasmas. Results on limitations on the amplification of the magnetic field in pressure-anisotropic plasmas extend knowledge of the small-scale dynamo processes in collisionless plasmas. An important aspect of the project is related to development of high-performance computational techniques for numerical simulations of plasmas.

Results of the project have possible direct applications to other fields of fundamental research: dynamics of collisionless turbulent plasma flows, dynamics of galaxy-cluster plasmas, dynamical processes in stellar winds and planetary magnetospheres. Due to numerous practical applications of plasmas, results of the project potentially may have some practical applications in future.