ABRUPT LONGITUDINAL MAGNETIC FIELD CHANGES AND ULTRAVIOLET EMISSIONS ACCOMPANYING SOLAR FLARES

Abstract

We have used Transition Region and Coronal Explorer (TRACE) 1600 \\AA images\nand Global Oscillation Network Group (GONG) magnetograms to compare ultraviolet\n(UV) emissions from the chromosphere to longitudinal magnetic field changes in\nthe photosphere during four X-class solar flares. An abrupt, significant, and\npersistent change in the magnetic field occurred across more than ten pixels in\nthe GONG magnetograms for each flare. These magnetic changes lagged the GOES\nflare start times in all cases, showing that they were consequences and not\ncauses of the flares. Ultraviolet emissions were spatially coincident with the\nfield changes. The UV emissions tended to lag the GOES start times for the\nflares, and led the changes in the magnetic field in all pixels except one. The\nUV emissions led the photospheric field changes by 4 minutes on average with\nthe longest lead being 9 minutes, however, the UV emissions continued for tens\nof minutes, and more than an hour in some cases, after the field changes were\ncomplete. The observations are consistent with the picture in which an\nAlfv\\'{e}n wave from the field reconnection site in the corona propagates field\nchanges outward in all directions near the onset of the impulsive phase,\nincluding downwards through the chromosphere and into the photosphere, causing\nthe photospheric field changes, whereas the chromosphere emits in the UV in the\nform of flare kernels, ribbons and sequential chromospheric brightenings during\nall phases of the flare.\n