Theoretical astrophysics and plasma physics at RPC

Abstract:

Abstract Magnetic reconnection is a fundamental plasma process responsible for the sometimes explosive release of magnetic energy in space and laboratory plasmas. Inside the diffusion regions of magnetic reconnection, the plasma becomes demagnetized and decouples from the magnetic field, enabling the change in magnetic topology necessary to power the energy release over larger scales. Since it was launched in 2015, the Magnetospheric MultiScale (MMS) mission has significantly advanced the understanding of the particle dynamics key to magnetic reconnection by providing high-resolution, in-situ measurements able to resolve ion and electron kinetic scales, i.e. a fraction of a gyroradius, that have confirmed theoretical predictions, revealed new phenomena, and refined existing models. These breakthroughs are critical for understanding not only space plasmas but also laboratory and astrophysical plasmas where magnetic reconnection occurs. In this work, we review the ion and electron dynamics occurring within the diffusion regions, in the inflow, along the separatrices, and downstream of the diffusion regions, in different reconnection configurations: symmetric, asymmetric, antiparallel, and guide field reconnection.

Abstract:

A set of analytic solutions for the plunging region thermodynamics has been developed recently under the assumption that the fluid undergoes a gravity-dominated geodesic plunge into the black hole. We test this model against a dedicated 3D global general relativistic magnetohydrodynamics simulation of a thin accretion disc around a Schwarzschild black hole using the code athenak . Provided that we include the effects of non-adiabatic heating (plausibly from grid-scale magnetic dissipation), we find excellent agreement between the analytic model and the simulated quantities. These results are particularly important for existing and future electromagnetic black hole spin measurements, many of which do not include the plunging fluid in their emission modelling. This exclusion typically stems from the assumption of a zero-stress boundary condition at the innermost stable circular orbit (ISCO), forcing all thermodynamic quantities to vanish. Instead, we find a non-zero drop in the angular momentum over the plunging region, which is consistent with both prior simulations and observations. We demonstrate that this stress is small enough for the dynamics of the fluid in the plunging region to be well-described by geodesic trajectories, yet large enough to cause measurable dissipation near to the ISCO – keeping thermodynamic quantities from vanishing. In the plunging region, constant -disc models are a physically inappropriate framework.