Evolution of emerging force-free flux tube to be a flare loop

Evolution of a filamentary magnetic flux tube emerging from the photosphere is investigated in the assumption that the magnetic field is force-free and unchanged during the evolution.If a characteristic radius of the flux tube is 3 km or less setting the field to 1000G, the temperature increases at first due to Joule heating up to about one million degree keeping the plasma density almost constant, and then the density decreases down to a critical value at which a current instability may occur. Thus, a.strong field-aligned electric field of 200 million volts or more is expected to be produced during the following anomalous Ohmic decay of the magnetic field as already shown by a numerical simulation.