Observations of Preflare Activity with TRACE and Yohkoh [Invited]

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Despite several decades of observational and theoretical effort, a complete understanding of solar flares remains elusive. It has been especially difficult to understand how the evolution of the magnetic field triggers a flare and drives the release of energy. In this talk I will review TRACE and Yohkoh observations of pre-flare and impulsive phase dynamics related to nonthermal broadening, flare ribbon evolution, and breakout reconnection. Studies of these phenomena suggest that pre-flare activity is a potentially rich source of information on the mechanisms that power a flare. For example, Yohkoh BCS measurements of nonthermal broadening have shown that the largest nonthermal velocities can occur before the onset of significant hard X-ray emission. This suggests that nonthermal broadening is a signature of a turbulent phase of the flare, which can begin several minutes before the onset of the hard x-ray emission. TRACE observations have also yielded evidence for ribbon brightenings that precede the onset of the hard X-ray emission. The analysis of very high cadence TRACE data, however, indicates that energy release during the pre-flare and impulsive phases of the flare is occurring on different loops. Finally, comparisons of pre-flare TRACE images with potential field extrapolations have shown that pre-flare activity associated with a null point in the field is an essential component of eruptive flares. Understanding the relationships between these phenomena will require coordinated observations between many instruments. I will discuss how future observations from Yohkoh, TRACE, SoHO, HESSI, Stereo, Solar-B, and ground-based observatories will be used to advance our understanding of flare physics.

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