UK-Japan Cooperative Study of Solar Flares with the Yohkoh Mission

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Yohkoh has been working very well for more than five years and a half with no serious degradation and its operation entered into a phase in which we can do systematic activity-cycle related analyzes. Collaboration with the SoHO satellite had an important role. SoHO was successfully launched in December, 1995. Database of the half solar cycle and softwares for analysis were completed. Yohkoh Data Analysis Center (YDAC) set up in MSSL has playd a key role in organizing active international collaboration in Europe and became the European center of Yohkoh data flow. Compilation of atmic data helpful for analyzing the Yohkoh and SoHO data also made a good progress by activating discussions among UK and Japanese scientists.The followings are brief summary of the new results : (1)BCS without collimators could diagnose SXV emission lines from single active region on the solar disk that appeared in July, 1996, and collaboration with a rocket experiment, SERTS,made it possible to reveal the temperature structure of an active region in May 1996. (2)SoHO EIT/CDS and Yohkoh SXT simultaneously observed jetlets (tiny jet-like structure) around the polar region, and found that these phenomena were also triggered by magnetic reconnection. (3)Analysis of a large number of impulsive flares seen in HXT suggested a new interpretation for nonthermal particle acceleration, especially for low energy cutoffs.