Flare research with the NASA/MSFC vector magnetograph: Observed characteristics of sheared magnetic fields that produce flares

The present MSFC Vector Magnetograph has sufficient spatial resolution (2.7 arcsec pixels) and sensitivity to the transverse field (the noise level is about 100 gauss) to map the transverse field in active regions accurately enough to reveal key aspects of the sheared magnetic fields commonly found at flare sites. From the measured shear angle along the polarity inversion line in sites that flared and in other shear sites that didn't flare, we find evidence that a sufficient condition for a flare to occur in 1000 gauss fields in and near sunspots is that both (1) the maximum shear angle exceed 85 degrees and (2) the extent of strong shear (shear angle > 80 degrees) exceed 10,000 km.