(1) Big Bear Solar Observatory, California Institute of Technology, Pasadena, CA 91125, U.S.A.
(2) National Astronomical Observatory, Mitaka, Tokyo 181, Japan
Our colleague and prominent solar physicist, Katsuo Tanaka died in Tokyo on January 2, 1990, by leukemia which he had been fighting against for many years. He was only at the age of 46, in the midst of his scientific career.
He was born in 1943 in Tokyo. After finishing the Astronomy Course of the Department of Physics, the University of Tokyo in 1965, he proceeded to the graduate course there and majored astronomy. In 1967 he was appointed as a research associate at the Department of Astronomy, the University of Tokyo. His main topic in research was radiative transfer and line formation. He obtained his Ph.D degree in 1971 from the University of Tokyo for the study of the solar chromosphere based on the data taken at the eclipse of 1966 in Peru.
In 1970 he moved to Tokyo Astronomical Observatory of the University of Tokyo. His interest was then shifted to the observation of solar activity, particularly solar flares, in high resolution. His thesis advisor Prof. Z. Suemoto advised him to visit Caltech, and contacted with H. Zirin of Big Bear Solar Observatory. For two years from 1971, he stayed at Big Bear and made cooperative research with H. Zirin. One of the outcome was the study on the large flares of August 1972, which still stands as a landmark achievement in solar flare study.
When he came back to Tokyo, the Astro-A satellite project was about to start. This project aimed at studying high energy phenomena in solar flares. The major instrument was a hard X-ray telescope by using the modulation collimator (Oda collimator). The image synthesis was supposed to be made by utilizing the spin of the satellite. Tanaka on the other hand recognized the importance of X-ray spectroscopy and participated in this project with his Bragg Crystal spectrometer. This ingenious instrument scans the spectra by using the spin of the satellite. As a test, the same instrument was flown on Tansei-IV satellite in 1980, and recorded X-ray spectra from flares which suggested the temperature approaching 2X107 K. The real significance of this observation was recognized when the Astro-A, to which Tanaka gave the name `Hinotori' (meaning `Phoenix' in Japanese), started its operation.
The Hinotori (Astro-A) satellite was launched by the Institute of Space and Aeronautical Science, the University of Tokyo (currently the Institute of Space and Astronautical Science, the Ministry of Education of Japan) in 1981. The satellite recorded more than 700 flares during its mission (February 1981 to June 1982). His X-ray spectrometer detected a superhot component (>3X107 K) in solar flares, and the blue-shifted spectral lines in the initial phase of flares which indicate the chromospheric evaporation. He also studied the K-alpha emission from flares and concluded that it is due to the fluorescence by the X-rays from hot flare plasma in the corona. Combining these results with hard X-ray images taken by the X-ray telescope on board Hinotori, he classified X-ray flares into three types: type A or hot thermal flares, type B or impulsive flares, and type C or gradual hard flares. These differences would originate, as he believed, from the environmental variety of flare sites, namely the density in flaring loops, geometry of magnetic fields, and so on. He summarized his view in an article in the Publications of Astronomical Society of Japan in 1987. This was the first invited review paper in this journal. He was awarded the Inoue Science Foundation Prize in 1985 for his achievement in flare research.
Although he was fully occupied with the Hinotori project from late 70's to early 80's, he had never lost his interest in ground-based observations. Seeing the development of helioseismology and studies on solar-stellar connections, he reached the idea that a basic study to be made is to understand the solar activity cycle from comprehensive data sets. Namely observations of surface magnetic field, velocity field, chromospheric and photospheric structures, and diagnostics by means of helioseismology, if all combined, would make a solid observational basis for the study of solar activity cycle. This he named the Solar Cycle Telescope Project. The idea was partly stimulated when he had a chance to discuss with O.C. Wilson during Tanaka's stay at Caltech. As the first step he started to develop magneto-optical filters, which he thought are the best instrument in measuring the velocity field because of the stability and narrowness in pass band and high transmission.
Now the 22-nd solar activity cycle is nearly at maximum, and the project following the Hinotori satellite is in progress. The satellite Solar-A will be launched in August 1991 by the Institute of Space and Astronautical Science, Japan. The initiation of this project owed very much to Tanaka's enthusiasm as well as his success in the Hinotori project. In 1983, however, he started suffering from constant fever. This was an initial manifestation of leukemia which finally deprived him of his life.
Even after he knew his disease, his scientific activity never declined, although he tried to concentrate only on the most important matters he had to finish. In 1987 he obtained the professorship at the University of Tokyo, and had been the key person in both research and education at Tokyo Astronomical Observatory. In 1988 he obtained a grant from the Ministry of Education and started to construct a telescope which observes the magnetic field, velocity field, H-alpha and white light images of solar active regions simultaneously. He believed that the energy source of flares is the emergence of magnetic flux tubes which are twisted (stressed) underneath the solar surface. With this instrument which he called the `Solar Flare Telescope', he planned to verify his hypothesis, and to back up the X-ray observations of flares by the Solar-A satellite. The telescope will be in operation from 1991 summer, but sadly he is not able to see whether his theory is proved with this instrument.
In the last one or two years he had to stay mostly at hospital. There he studied literatures on leukemia and haematology, and discussed with his doctor on how they can attack his disease. At the hospital he plotted the number of white blood corpuscles in response to the injection of interferon. He tried to save his life with his own effort.
In spite of his keen attitude in science, he was a mild and enjoyable person in daily life. He loved pop music, sci-fi movies, swimming, yoga, and so on. When planning the eclipse expedition to Australia in 1976, opinions at the observatory did not converge on where the observing site should be. Tanaka went to a prophet and selected one site. His recommendation was not taken because it lacked scientific justification, but the result showed that he was right!
Till the last moment he had been working on his theory on the mechanism of solar flares. The paper will appear in Solar Physics in the near future. The paper was supposed to be part one of four papers in series, but the remaining three papers are not yet completed. His `Solar Cycle Telescope Project' is not yet funded either. His name Katsuo means `victory' and `quickness' in Japanese, which, in retrospect, might indicate his great achievements in such a short life. His too early death left many things behind. Our hope is to dedicate to him successful outcome from Solar-A, and to bring his idea of Solar Cycle Telescopes to reality. Then we will recognize again how he was a talented scientist, with adequate insight into the future direction of solar physics research. As we remember his warm smile, and as we see his vacant office with full of unfinished material, we cannot help thinking how great his loss is to us all.