Formation of a Dense Flux Rope by a Siphon Flow

The interaction of siphon ow with an initially linear Alfven wave within an isolated chromospheric loop is investigated. The loop is modelled using 1.5D magnetohydrodynamics (MHD). The siphon flow undergoes a hydrodynamic (HD) shock, which allows the Alfven instability to amplify the propagating waves as they interact with the shock and loop footpoints. The amplification leads to non-linear processes strongly altering the loop equilibrium. Azimuthal twists of 50 km/s are generated and the loop becomes globally twisted with an azimuthal magnetic field 5 times stronger than the longitudinal field. The flow is accelerated to 70 km/s due to the propagating shock waves that form. Near the end of the simulation, where the non-linear processes are strongest, flow reversal is seen within the descending leg of the loop, generating up flows up to 28 km/s. This flow reversal leads to photospheric material being `pulled' into the loop and spreading along its entirety. Within about 2.5 h the density increases by a factor of about 30 its original value.