SHEAR ANGLE OF MAGNETIC-FIELDS

In this paper we introduce a new parameter, the shear angle of vector magnetic fields, Delta psi, to describe the non-potentiality of magnetic fields in active regions, which is defined as the angle between the observed vector magnetic field and its corresponding current-free field. In the case of highly inclined field configurations, this angle is approximately equal to the 'angular shear', Delta phi, defined by Hagyard et al. (1984). The angular shear, Delta phi, can be considered as the projection of the shear angle, Delta psi, on the photosphere. For the active region studied, the shear angle, Delta psi, seems to have a better and neater correspondence with flare activity than does Delta phi. The shear angle, Delta psi, gives a clearer explanation of the non-potentiality of magnetic fields. It is a better measure of the deviation of the observed magnetic field from a potential field, and is directly related to the magnetic free energy stored in non-potential fields.