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Abstract:
Solar eruptions are nearly always preceded by a slow-rise phase that comprises an ascent of the eventually erupting filament (or prominence) in the corona and a slow increase of the soft X-ray flux. This is a distinct phase characterized by intermediate velocities of typically several to several 10 km/s (in active regions up to ~100 km/s), 1-2 orders of magnitude faster than the quasi-static evolution during energy storage, which scales with the driving photospheric velocities, and 1.5-3 orders of magnitude below the coronal Alfven velocity, V_A, which is the scaling parameter of eruption speeds and their upper limit. Proposed mechanisms of this phase range from distributed small-scale (``tether-cutting'') reconnection events in sheared field to a nonequilibrium and even an ideal magnetohydrodynamic instability of a flux rope. I present simulations of flux cancellation that show the formation of a flux rope, a quasi-static evolution with a rise speed similar to the imposed photospheric driver, then a slightly faster rise, gradually accelerating up to ~ 0.01 V_A, and eventually the eruption of the rope by onset of the torus instability. The flux rope is found to be in a nonequilibrium state during the slow rise.
