Flare precursors and triggering

Abstract

Large solar flares are unpredictable. Some degree of successful forecasting is possible for the time of occurrence, but even with this other properties of the flare will still be a mystery. One aspect of this problem is the identification and interpretation of precursors. We review the various coronal and photospheric/chromospheric events which precede flares, which include the radio, Hα, UV and X-ray regions of the spectrum, together with the magnetograph data. We show how these observations constrain the various models for the energy release in flares. There is evidence that the triggering of the energy release is not coincident with the impulsive hard X-ray burst. This provides constraints both on the nature of the primary energy transfer from the magnetic field and on its location in the solar atmosphere.