Atmospheric Heating in Emerging Flux Regions

Abstract

Using a two-dimensional MHD code, we study the nonlinear dynamics and associated heating of a magnetic loop emerging from the convection zone into the photosphere, chromosphere, and corona. We find the following: (1) The magnetic flux rises in an approximately self-similar fashion, with the rise velocity of the loop increasing linearly with height up to ≃ 13 km s-1 at a height of 5000 km above the photosphere. Downflows occur along the rising loop, with a maximum velocity (~ 50 km s-1) exceeding that of the local sound and Alfven velocities. (2) Plasmas in the upper chromosphere of the emerging loop are heated to ~ 3 × 104–105 K by strong MHD shock waves produced by the downflows along the loop. The energy flux dissipated at the shocks amounts to ~ 107 erg cm-2 s-1; this may explain in part the chromospheric heating in bright plages in emerging flux regions. (3) Initially weak convection zone magnetic flux (B ≃ 600 G) is amplified up to 1–1.5 kG after emerging into the photosphere by the connective collapse of the flux tube. Shocks are produced in the intense flux tube in the low chromosphere; these shocks may explain the heating of some of the low-chromospheric bright points, such as Moustaches. (4) Reconnection between emerging loops and overlying preexisting magnetic fields or between two different emerging loops lead to substantial heating of the corona.