Torsional Alfvén Oscillation in the Firehose-Instability Regime as a Mechanism of Plasma Stratification in a Laboratory Experiment Modeling a Coronal Arch

Abstract

The compact laboratory setup “Solar Wind” (IAP RAS) forms an arch-shaped structure of the coronal-loop type, in which the plasma pressure varies from zero to values of the order of or above the magnetic pressure. The arc discharge at each footpoint of the magnetic tube creates a plasma characterized by a significantly higher ion temperature along the magnetic field line than across it. In the stationary state (when the ion pressure remains below the threshold for rupture of the system at the loop apex), the plasma is found to be stratified either as a cylindrical layer along the outer wall of the tube or, possibly, as two belts along the upper and lower vaults of the arch. The paper discusses the excitation of a torsional Alfvén oscillation in the loop under firehose-instability conditions. In the case of rapid growth (with an increment on the order of the ion cyclotron frequency), the unstable Alfvén oscillation substantially redistributes particles between the central axis and the tube wall, manifesting itself as the observed cylindrical layer.