The dynamics of filament formation - The thermal instability in a sheared magnetic field

Abstract

There exists no good model which explains the initial temporal and spatial development of filaments and prominences. It is believed that they form in conditions of sheared coronal magnetic fields, as evidenced by their location above photospheric polarity-inversion lines and their occurrence after a period of increasing fibrille inclination. The general mechanism giving rise to thermal instability in a uniform medium such as the solar corona, which exhibits increasing radiative output as it cools, is well known. In this brief Letter we describe a self-consistent calculation of this instability in a nonuniform magnetic field, which shows the dynamic response of the pressure, density, and temperature to the competing effects of optically thin radiation and field-collimated thermal conduction. The condensation of characteristic ''knife-blade'' filaments is then demonstrated and their observable temporal and spatial scales detailed.