IMPLOSION IN A CORONAL ERUPTION

We present the observations of the contraction of the extreme-ultraviolet coronal loops overlying the flaring region during the preheating as well as the early impulsive phase of a GOES class C8.9 flare. During the relatively long, 6 minutes, preheating phase, hard X-ray (HXR) count rates at lower energies (below 25 keV) as well as soft X-ray fluxes increase gradually and the flare emission is dominated by a thermal looptop source with the temperature of 20-30 MK. After the onset of impulsive HXR bursts, the flare spectrum is composed of a thermal component of 17-20 MK, corresponding to the looptop emission, and a nonthermal component with the spectral index gamma = 3.5-4.5, corresponding to a pair of conjugate footpoints. The contraction of the overlying coronal loops is associated with the converging motion of the conjugate footpoints and the downward motion of the looptop source. The expansion of the coronal loops following the contraction is associated with the enhancement in Ha emission in the flaring region, and the heating of an eruptive filament whose northern end is located close to the flaring region. The expansion eventually leads to the eruption of the whole magnetic structure and a fast coronal mass ejection. It is the first time that such a large scale contraction of the coronal loops overlying the flaring region has been documented, which is sustained for about 10 minutes at an average speed of similar to 5 kms(-1). Assuming that explosive chromospheric evaporation plays a significant role in compensating for the reduction of the magnetic pressure in the flaring region, we suggest that a prolonged preheating phase dominated by coronal thermal emission is a necessary condition for the observation of coronal implosion. The dense plasma accumulated in the corona during the preheating phase may effectively suppress explosive chromospheric evaporation, which explains the continuation of the observed implosion up to similar to 7 minutes into the impulsive phase.