Preflare current sheets in the solar atmosphere

Neutral current sheets are expected to form in the solar atmosphere when photospheric motions or the emergence of new magnetic flux causes oppositely directed magnetic fields to be pressed together. Magnetic energy may thus be stored slowly in excess of the minimum energy associated with a purely potential field and released suddenly during a solar flare. For simplicity, we investigate the neutral sheet which forms between two parallel line dipoles when either the distance between them decreases or their dipole moments increase. It is found that, when the dipoles have approached by an amount equal to a tenth of their original separation distance, the stored energy is comparable with that released in a major flare. In addition, a similarity solution for one-dimensional magnetohydro-dynamic flow within such a neutral sheet is presented; it demonstrates that rapid conversion of magnetic energy into heat is possible provided conditions at the edge of the neutral sheet are changing sufficiently quickly.