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

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The sun observing satellite YOHKOH has been successfully operated for five years from the previous activity maximum and throughout the declining phase.All the scientific instruments onboard has been in good condition and the soft X-ray instruments can observe the quiet sun even at its minimum.Investigating the whole set of data, the keyword of "magnetic reconnection" has been emerged as a fundamental process for producing high temperature plasmas in the corona.Loop-top hard X-ray sources newly found by Yohkoh show consistency with the "magnetic reconnection scenario." Characteristic of nonthermal line broadenings seen in helium-like sulphur lines were systematically investigated.The locations of hot plasmas at the time of the line broadening maximum were either footpoints or loop top of flaring loops.Statistical studies of X-ray jets were also completed and they were mostly found in active regions with mixed magnetic polarity and strongly associated with emerging-flux regions.The interaction of emerged magnetic fields with pre-existing coronal field causes X-ray jets.Correspondence with micro-flares also suggests that the both phenomena come from the same mechanism.On the other hand, some evidence also appeared for the mechanism of basic coronal heating was something other than "magnetic reconnection."The Yohkoh dataset was compiled for nearly half of the solar cycle, with which studies of solar cycle and solar dynamo mechanisms will be done in future.The SoHO satellite launched in December 1995 will now station at the Lagrangean point and start its observations.Joint observations and collaborative studies with SoHO will be highly encouraged for the first space observations in the rising phase of the solar activity.