The objective of Yohkoh (Japanese for sunbeam) is to study the high-energy radiations from solar flares (hard and soft X-rays and energetic neutrons) as well as quiet structures and pre-flare conditions. The mission is a successor to Hinotori, a previous Japanese spacecraft flown at the previous solar activity maximum in 1981. Yohkoh was launched on August 30, 1991.
Yohkoh is a three-axis stabilized, observatory-type satellite in a nearly-circular Earth orbit. It carries four instruments: two imagers and two spectrometers. The imaging instruments have almost full-Sun fields of view, to avoid missing any flares on the visible disk of the Sun. American solar physicists at Lockheed Palo Alto Research Laboratory are collaborating in the soft X-ray telescope production and data analysis.
Approximately 50 MB of data are accumulated per day, and stored on an on-board tape recorder with 10.5 Mbyte capacity. The Yohkoh mission is a cooperative mission of Japan, the US, and England.