Formation of a Dense Flux Rope by a Siphon Flow

Abstract

The interaction of siphon flow with an initially linear Alfvén wave within an isolated chromospheric loop is investigated. The loop is modeled using 1.5D magnetohydrodynamics (MHD). The siphon flow undergoes a hydrodynamic (HD) shock, which allows the Alfvén instability to amplify the propagating waves as they interact with the shock and loop footpoints. The amplification leads to nonlinear processes strongly altering the loop equilibrium. Azimuthal twists of are generated and the loop becomes globally twisted with an azimuthal magnetic field of . The flow is accelerated to due to the propagating shock waves that form. Near the end of the simulation, where the nonlinear processes are strongest, flow reversal is seen within the descending leg of the loop, generating upflows up to . This flow reversal leads to photospheric material being “pulled” into the loop and spreading along its entirety. Within about 2.5 hr, the density increases by a factor of about 30 its original value.