Shock and Discontinuity Formation in the Front of an Expanding Coronal Magnetic Arcade in Two-fluid Simulations

The dynamic evolution of a magnetic arcade associated with footpoint shearing motions is investigated by an ideal two-fluid (electron-ion) code. The two-fluid numerical simulations produce conspicuous differences compared to earlier MHD simulations beyond the inner arcade region. The decoupling motion between electrons and heavier ions during the arcade expansion induces a growing charge separation and strong electric field in the front of the expanding arcade. The presence of this electric field provides an additional force, along with the magnetic and thermal pressures, that drives the growth of an outwardly expanding wave that steepens into a propagating discontinuity in the plasma and magnetic field. The propagation speed of the discontinuity eventually exceeds the local phase velocity of the MHD fast mode and becomes a perpendicular fast-like shock. There is significant heating at the shock due to adiabatic compression, with preferential heating of the ion fluid also being observed. In addition, parameter tests indicate that (1) the propagation speed of the shock before exiting the inner arcade is independent of the maximum shear speed; (2) slower shearing speeds produce weaker shocks with weaker adiabatic heating; (3) the ion-to-electron mass ratio,m(i)/m(e), impacts the strength of the charge separation linearly but has a moderate effect on the propagation speed; and (4) the normalized value of the ion inertial length does not affect the formation and speed of the shock as a whole.