NONPOTENTIAL MAGNETIC-FIELDS AT SITES OF GAMMA-RAY FLARES

In this paper the relation between the degree of nonpotentiality of photospheric magnetic fields and the occurrence of gamma-ray flares is examined. We use the parameter ΔΦ (termed magnetic "shear") and the strength of the magnetic field intensity as measures of the degree of nonpotentiality, where ΔΦ is defined as the angular difference between the observed direction of the transverse component of the photospheric field and the direction of the potential field prescribed by the distribution of measured photospheric flux. An analysis of the great flare of 1984 April 24-25 is presented as an example of this technique to quantify the nonpotential characteristics of the preflare magnetic field. For this flare, which produced a large gamma-ray event, we found that strong shear and high field strengths prevailed over an extended length of the magnetic neutral line where the flare occurred. Moreover, the flare began near the area of strongest measured shear (89°-90°). Four other flaring regions were analyzed; one of these produced a moderate gamma-ray event while the other three did not produce detectable gamma rays. For all four regions the flares were located in the area where the local field was most nonpotential, regardless of the class of flare. The fields of the gamma-ray flares were compared with those associated with the flares without gamma rays, and we found little distinction in the degree of magnetic shear. The major difference is seen in the extent of the sheared field: for gamma-ray events, the field is sheared over a longer length of the neutral line.