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Astron. Astrophys. 361, 759-765 (2000)

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4. Conclusions and discussions

In this paper we have presented an example of a hot plasma jet, seen in YOHKOH SXT and SOHO EIT images, that is co-spatial with cool plasma ejection, observed as an H[FORMULA] surge. However, the X-ray jet took place only in the late stage of the H[FORMULA] surge. As shown by previous studies, the surge appeared at the boundary between newly emerging magnetic flux, e.g. parasite polarity, and the pre-existing main flux of an active region. Evidence is first shown by this study that with the interaction of parasite and main magnetic flux, threads of the surge become twisted, and present complicated Doppler patterns. The X-ray jet appears right after the H[FORMULA] surge, became untwisted, and downstreaming appeared at the surge base.

For the event studied we would suggest that the H[FORMULA] surge was caused by the slow reconnection in the lower atmosphere at the very early stage of a vigorous EFR which was fully developed on November 4. It was suggested that surges are due to magnetic reconnection between a twisted cool loop and open field lines (Schmieder et al. 1995). The twisting of surge threads is indicative that the large-scale loops in the corona had been charged, i.e. became current-carrying. In the other words, free magnetic energy and magnetic complexity had been accumulated in the surge threads. The fast reconnection then took place and manifested as an X-ray jet. Both the transient X-ray brightening and the downstreaming which was observed in the later phase of the H[FORMULA] surge might signal the fast reconnection in the higher atmosphere. This two-step reconnection process seems to be common in transient solar activities. Chae et al. (1999) found indications of the two-step process in transition region explosive events. Further observational studies and theoretical approaches are crucially required to confirm, improve, or disprove the two-step reconnection scenario of solar transient events.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001