Astronomy and Astrophysics – Astronomy
Scientific paper
May 2009
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2009spd....40.1918l&link_type=abstract
American Astronomical Society, SPD meeting #40, #19.18; Bulletin of the American Astronomical Society, Vol. 41, p.850
Astronomy and Astrophysics
Astronomy
Scientific paper
We present the observations of the contraction of the EUV coronal loops overlying the flaring region during the pre-heating as well as the early impulsive phase of a C8.9 flare. During the relatively long, 6 minutes, pre-heating phase, the flare emission is dominated by a thermal looptop source with the temperature of 20 - 30 MK. After the onset of impulsive hard X-ray 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 (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 H-alpha 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 min at an average speed of 5 km/s. 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 pre-heating 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 pre-heating phase may effectively suppress explosive chromospheric evaporation, which explains the continuation of the observed implosion up to 7 minutes into the impulsive phase.
Alexander Dave
Liu Rui
Wang Hai-Hong
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