Convection-driven Emergence of Small-Scale Magnetic Fields and their Role in Coronal Heating and Solar Wind Acceleration

Abstract

Recent observations by the Solar Optical Telescope (SOT) on board Hinode have revealed that the surface of the Sun is, on average, covered with small-scale horizontal magnetic fields. Frequent emergence of horizontal magnetic flux on a granular scale is found in the quiet Sun and in plage regions. In this Letter we present the results of magnetohydrodynamic simulations that cover the upper convection zone and the corona. It is found that, even when the initial magnetic field is uniform and vertical, a disordered magnetic field is produced in the convection zone. The photospheric magnetic field is then characterized by strong vertical fields concentrated in the intergranular lanes and relatively weak, horizontal fields both in the granules and in the intergranular lanes. Occasionally, fragments with large magnetic fluxes are driven above the photosphere by the upward convective flows. These characteristics are consistent with the SOT observations. Moreover, the simulated flux emerging on a granular scale undergoes magnetic reconnection with the expanding vertical magnetic concentrations in the chromosphere. These reconnection events heat the local plasma and emit high-frequency waves that propagate into the corona. Such an interplay between the small-scale horizontal fields and the vertical flux may play an important role in coronal heating and wind acceleration in the Sun and stars.