Numerical MHD Simulation of the Coupled Evolution of Collisional Plasma and Magnetic Field in the Solar Chromosphere. I. Gradual and Impulsive Energisation

摘要

The dynamical coupling between the solar chromospheric plasma and magneticfield is investigated by numerically solving a fully self-consistent,two-dimensional initial-value problem for the nonlinear collisional MHDequations including electric resistivity, thermal conduction, and, in somecases, gas-dynamic viscosity. The processes in the contact zone between twohorizontal magnetic fields of opposite polarities are considered. The plasma isassumed to be initially motionless and having a temperature of 50,000 K uniformthroughout the plasma volume; the characteristic magnetic field corresponds toa plasma $\beta\gtrsim 1$. In a physical-time interval of 17~seconds typicallycovered by a computational run, the plasma temperature gradually increases by afactor of two to three. Against this background, an impulsive (in 0.1 secondsor less) increase in the current-aligned plasma velocity occurs at the site ofthe current-layer thinning (sausage-type deformation, or $m=0$ pinchinstability). Such a "velocity burst" can be interpreted physically as an eventof suprathemal-proton generation. Further development of the sausageinstability results in an increase in the kinetic temperature of the protons tohigh values, even to those observed in flares. The form of our system of MHDequations indicates that such increases are a property of the exact solution ofthe system at an appropriate choice of the parameters. Magnetic reconnectiondoes not manifest itself in this solution: it would generate flows forbidden bythe chosen geometry. Therefore, the pinch-sausage effect can act as anenergiser of the upper chromosphere and be an alternative to themagnetic-reconnection process as the producer of flares.