Magnetic field-related heating instabilities in the surface layers of the Sun and Stars

We investigate the stability of a magnetized low-density plasma to current-driven filamentation instabilities and apply our results to the surface layers of stars. Unlike previous studies, the initial (i.e., ''precoronal'') state of the stellar surface atmosphere is taken to be a low-density, optically thin magnetized plasma in radiative equilibrium. Our (linear) analysis shows that the surface layers of main-sequence stars (including the Sun) which are ''threaded'' by magnetic fields are unstable; the instabilities considered here lead to structuring perpendicular to the ambient magnetic fields. These results suggest that relatively modest surface motions, in conjunction with the presence of magnetic fields, suffice to account for the presence of inhomogeneous chromospheric and coronal plasma overlying a star's surface