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We report the development of a millimeter-wave electron-spin-resonance (ESR) measurement system at the University of Fukui using a 3He/4He dilution refrigerator to reach temperatures below 1 K. The system operates in the frequency range of 125–130 GHz, with a homodyne detection. A nuclear-magnetic-resonance (NMR) measurement system was also developed in this system as the extension for millimeter-wave ESR/NMR double magnetic-resonance (DoMR) experiments. Several types of Fabry–Perot-type resonators (FPR) have been developed: A piezo actuator attached to an FPR enables an electric tuning of cavity frequency. A flat mirror of an FPR has been fabricated using a gold thin film aiming for DoMR. ESR signal was measured down to 0.09 K. Results of ESR measurements of an organic radical crystal and phosphorous-doped silicon are presented. The NMR signal from 1H contained in the resonator is also detected successfully as a test for DoMR.

We have developed millimeter-wave Fabry-Perot type resonator (FPR) aiming for simultaneous electron-spin and nuclear-spin resonance (ESR/NMR) measurements at very low temperatures below 1 K. The flat mirror of FPR is fabricated using a gold thin layer in order to minimize disturbance to radio frequency wave for NMR. An optimum thickness of the gold layer by measuring $Q$ value is found. We have actually succeeded to detect both ESR and NMR at very low temperatures.

To clarify the relationship between the regional variability of water temperatures and the spatially non-uniform appearances of coral mass bleaching and consequent mortality, we have investigated the near-shore temperatures continuously monitored around the Ryukyu Islands in 1999 and NOAA–SST image data in 1998 and 1999. They indicate appreciable regional differences in temperature (e.g., at the Kerama Islands located southwest of Okinawa Island the temperature in summer was generally lower than that at the Okinawa Island coasts). Comparison of the solar radiation distribution and NOAA-SST images with bottom bathymetry and a numerical experiment suggest that the presence of the shelf around the Kerama Islands caused the relatively lower temperature (i.e., the warm water mass from the Kuroshio currents may be propagated toward Okinawa Island but may be blocked at the shelf edge, resulting in relatively lower temperature at the Kerama Islands). Further comparison with the reported changes in the coral coverage in the Ryukyu Islands after the mass coral bleaching event in summer 1998 suggests that one of the primary causes of the regional variability of the coral bleaching and consequent mortality is the regional variation of water temperature.