Formation of a Dense Flux Rope by a Siphon Flow

investigated. The loop is modeled 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 nonlinear processes strongly altering the loop equilibrium. Azimuthal twists of 50 km s(-1) are generated and the loop becomes globally twisted with an azimuthal magnetic field of B-theta approximate to 5 x B-z. The flow is accelerated to approximate to 70 km s(-1) 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 28 km s(-1). 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.