According to recent computer simulations, wave-particle interactions give thin plasmas a useful viscosity that controls their turbulence and heating.
Plasma, a chaotic state characterized by charged particles collectively interacting with electromagnetic fields, makes up most of the normal matter in the Universe. A three-dimensional fluid theory called magnetohydrodynamics can accurately describe bulk plasma motions when individual particle collisions occur at scales significantly smaller than bulk plasma motions. This is present in protoplanetary accretion disks as well as in the interiors of stars and planets. However, slight collisions are present in many streams of hot, low-density astrophysical plasma.
Behavior of Plasmas
A statistical kinetic description of particle positions and velocities in a 6-dimensional universe is required to account for stellar winds, agglomeration around black holes, and movements of plasma penetrating intergalactic space. Numerical simulations by Lev Arzamasskiy of the Institute for Advanced Studies in Princeton, New Jersey, and colleagues have revealed new information about the magnetized kinetic turbulence in these plasmas. A coarse-grained description of plasma dynamics at the enormous scales that astronomers see may also be possible thanks to these simulations.
Powerful processes such as accretion into a neutron star or nuclear fusion inside a main sequence star affect cosmic fluid dynamics on large scales. Viscosity, proportional to the mean free path of particles in a gas, determines whether a flow is laminar or turbulent. The viscosity is typically too low for viscous friction to dissipate energy on large astronomical scales in regions of the universe where particle collisions are common. Instead, it descends to much smaller scales where viscous stresses eventually cause it to turn into heat.
Entropy in Stars
The primary mechanism for the macroscopic transport and mixing of important physical quantities, such as entropy in stars or angular momentum in accretion processes, is this turbulent, nonlinear dynamic response of a fluid to a lack of equilibrium at large scales. As a result, it controls the energy and development of numerous cosmic systems.
However, individual particles travel great distances with momentum in collision-free cosmic plasmas. These plasmas may or may not have low viscosity. Is it possible that they are turbulent? The presence of wave-particle resonances and kinetic instabilities in non-collisional plasmas scatter and trap particles in electromagnetic fluctuations, unlike collisional fluids. All of these impede particle motion, giving the plasma an effective collision property.
How kinetic dynamics affect thermalization, microscopic (conflicting) and large-scale (turbulent) transport processes is a key issue of enormous complexity.
Long-term efforts have been made by scientists working on magnetically controlled fusion to identify the many transport mechanisms that prevent the efficient confinement of hot plasma. However, a phenomenological representation of turbulence in cosmic plasmas has only recently emerged. The radius of rotation of the magnetic field is typically significantly smaller in cosmic plasmas than their fusion equivalents, although they typically have much less magnetic energy. However, due to poor collision characteristics. Large-scale plasma mixing in cosmic plasmas causes pressure anisotropies associated with the dynamic evolution of the magnetic field, which feeds rapid kinetic instabilities.
At microscopic scales, the latter, well known in heliospheric plasmas, scatter particles creating significant electromagnetic fluctuations that present a certain amount of effective collision properties at larger scales.
High-performance computation is required to simulate a system that exhibits extreme multi-scale nonlinearity. However, 6D cosmic plasmas are in a separate class of computational complexity. In this context, the work of Arzamasskiy et al. is a show of strength. Authors, 1011 performed high-resolution intracellular particle kinetic simulations of the magnetohydrodynamic turbulence problem without collision with the macroparticle.
According to the study of the dynamics of particles, two important effects change the fluid picture. The first is collisionless damping, a wave-particle resonance process in which the wave energy present in large-scale magnetic Alfvén waves is transferred to particles. Lev Landau first proposed this process in 1946. The second effect is the dispersion of particles by nonlinear pressure-anisotropy-driven microscale instabilities that form above the growing Alfvén waves, dominated by an instability resembling the writhing of a pressurized fire hose. Although some plasma collisions occur with this scattering, the estimated effective viscosity is still very high, limiting turbulent cascading to some extent. Therefore, it appears that it is more difficult for weakly magnetized collisionless plasmas to become turbulent.
The findings of Arzamasskiy et al relate to a number of open astrophysical issues. For example, the thin, hot plasma of a galaxy cluster radiates energy so effectively that it should cool and sink into the gravitational well at the center of the cluster. However, that doesn't happen. The dominant galaxy at the center of the cluster can hurl particles and photons into the plasma, creating just the right amount of turbulence to promote dynamic thermalization and prevent it from cooling and collapsing.
Despite recent advances, researchers are only just beginning to unravel the secrets of magnetized plasma turbulence. Arzamasskiy, his associates, and others focused on the collision-free dynamics of ions, while treating electrons as a fluid for computational purposes. In fact, the same kinetic processes apply to lighter, collisionless electrons in hot, dilute cosmic plasmas, but on smaller scales. These electron-scale dynamics, which have important implications for magnetic production, reconnection, radiation, heat and charge transport, are just beginning to attract the attention of researchers.
Bridging kinetic simulations such as those of Arzamasskiy and colleagues with magnetohydrodynamic simulations of the evolution of cosmic structures or black hole deposition is another major challenge.
Because the scales in the intra-cluster environment span 14 orders of magnitude, it is difficult to model astrophysical systems from their largest scales to electron scales. However, kinetic simulations such as Arzamasskiy et al.'s and accompanying analyzes are quite useful. They enable the development of physically driven models that capture the net transport effects of microscale physics on fluid dimensions, without having to copy all the kinetics. It is now the community's task to use these findings to increase our shared understanding of astrophysics.
📩 27/04/2023 13:00