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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 is a three-axis stabilized observatory-type satellite in a nearly-circular Earth orbit, carrying four instruments: two imagers and two spectrometers. The spacecraft is a rectangular solid about 2 m square and 4 m long. The imaging instruments have almost full-Sun fields of view, to avoid missing any flares on the visible disk of the Sun. The hard X-ray telescope is a multi-grid synthesis type with a spatial resolution of 7 arcsec, operating in the 20 - 80 keV range. The soft X-ray telescope uses grazing-incidence optics and achieves 4 arcsec spatial resolution, operating in the 0.1 - 4 keV range and using 1024 x 1024-pixel CCDs. US solar physicists at Lockheed Palo Alto Research Laboratory are collaborating in the soft X-ray telescope production and data analysis. There is also a continuum spectrometer for X-rays and gamma-rays from 3 keV to 20 MeV (also sensitive to neutrons) and a Bragg crystal spectrometer for the X-ray lines Fe XXV, Fe XXVI, Ca XIX, and S XV.

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 the United Kingdom.

The Solar Data Analysis Center (SDAC) at NASA/GSFC can provide small quantities of data, by request to Sample data from the SXT images may be obtained via the World Wide Web from the SDAC at:

The Yohkoh analysis guide is available in PostScript form in three parts from:

Alternate Names

  • 21694
  • Solar-A
  • solara

Facts in Brief

Launch Date: 1991-08-30
Launch Vehicle: M-3SII
Launch Site: Uchinoura Space Center, Japan
Mass: 390 kg
Nominal Power: 200 W

Funding Agencies

  • Institute of Space and Aeronautical Science, U of Tokyo (Japan)
  • NASA-Office of Space Science Applications (United States)


  • Solar Physics

Additional Information

Questions and comments about this spacecraft can be directed to: Dr. H. Kent Hills



NameRoleOriginal AffiliationE-mail
Prof. Keizo KaiMission Principal InvestigatorNational Astronomical Observatory of Japan
Dr. Yutaka UchidaProject ScientistUniversity of Tokyo
Dr. Yoshiaki OgawaraProject ManagerInstitute of Space and Aeronautical Science

Selected References

  • The launching of M-3SII-6 rocket and ``Yohkoh'' satellite, Institute of Space and Astronautical Science, Dec. 1991.
  • Kai, K., Rapid fluctuations in solar flares, NASA, Conference Pub. 2449, p. 647, 1986.
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