On the Origin of Solar Flares

Two major problems in the interpretation of solar flares are discussed: firstly, by what means high concentrations of energy can be stored in the chromosphere; and secondly, what process can account for the suddenness with which this energy is released during a flare? It is shown that the energy storage can be accounted for only by a particular class of magnetic fields whose lines of force have the general shape of twisted loops protruding above the photosphere. In active regions with strong magnetic fields it is likely that chromospheric motions are entirely dictated by the flow of the much more massive photospheric material. Forces arising from pressure, weight or acceleration of chromospheric gas are most of the time small, so that the motions there are closely those that retain at each moment a force-free field compatible with the magnetic boundary conditions at the photosphere and, as the chromosphere is a good conductor, with the past history of the motion. A sudden release of energy in such a system can result only in a case where the magnetic forces act so as to drive the system away from the force-free configuration, and the energy associated with chromospheric currents can then be dissipated into motion or heat. This will occur when twisted magnetic loops of opposite sense and opposite twist meet. Such loops attract each other, and the annihilation of the longitudinal component of the field where they meet leads to a sudden constriction of the current and through this to a dissipation of the energy associated with that current.