Solar Magnetic Field Studies Using the 12 Micron Emission Lines. IV. Observations of a Delta Region Solar Flare

We have recently developed the capability to make solar vector (Stokes IQUV) magnetograms using the infrared line of Mg I at 12.32 μm. On 2001 April 24, we obtained a vector magnetic map of solar active region NOAA 9433, fortuitously just prior to the occurrence of an M2 flare. Examination of a sequence of SOHO/Michaelson Doppler Imager magnetograms and comparison with ground-based Hα images shows that the flare was produced by the cancellation of newly emergent magnetic flux outside of the main sunspot. The very high Zeeman sensitivity of the 12 μm data allowed us to measure field strengths on a spatial scale which was not directly resolvable. At the flare trigger site, opposite polarity fields of 2700 and 1000 G occurred within a single 2" resolution element, as revealed by two resolved Zeeman splittings in a single spectrum. Our results imply an extremely high horizontal field strength gradient (5 G km-1) prior to the flare, significantly greater than seen in previous studies. We also find that the magnetic energy of the cancelling fields was more than sufficient to account for the flare's X-ray luminosity.