Overview

“Yohkoh” (SOLAR-A) is an X-ray solar observation satellite developed under the leadership of the Institute of Space and Astronautical Science (ISAS) with international cooperation from Japan, the United States, and the United Kingdom. The main mission objectives were to observe solar flares during the solar maximum period and to study the physical phenomena in the solar corona associated with these flares. Yohkoh was launched on August 30, 1991, from the Kagoshima Space Center (now Uchinoura Space Center), and continued to observe solar activity until its operation ended on April 23, 2004.

The satellite weighed approximately 390 kg and had a rectangular shape measuring 1 m x 1 m x 2 m, with two foldable solar panels. It orbited the Earth in a nearly circular orbit at an altitude of 550 km at periapsis and 600 km at apoapsis, with an inclination of 31 degrees, completing one orbit every 98 minutes.

Observation Instruments

Soft X-ray Telescope (SXT)

The SXT, developed through US-Japan collaboration, is a telescope that images the entire Sun in the soft X-ray region (0.5-6 nm). It uses grazing incidence X-ray mirrors to focus the solar image onto an X-ray CCD detector. By imaging the Sun through six different filters, it can determine physical quantities such as the temperature and emission measure of the solar plasma through temperature analysis. The smooth mirrors achieve a high resolution of 2.45 arcseconds. An onboard microprocessor automatically controls filter selection, exposure time, and observation area. SXT was primarily used to capture high-resolution images of the solar corona’s structural dynamics.

Hard X-ray Telescope (HXT)

The HXT consists of 64 slat collimators, making it the world’s first “Fourier synthesis” X-ray telescope. The 64 spatial Fourier components are combined using ground-based computers to synthesize the image. The collimator/detector system’s total effective area reaches about 70 cm², offering sensitivity an order of magnitude higher than conventional telescopes. It captures hard X-ray images of flares in four energy bands (14-23-33-53-93 keV), allowing detailed observation of flare phenomena in the high-energy X-ray region above 30 keV for the first time.

Bragg Crystal Spectrometer (BCS)

The BCS is an instrument that obtains high wavelength resolution energy spectra of soft X-ray emission lines. It uses Bragg reflection to selectively detect specific wavelengths of X-rays (sulfur ions: 0.50385 nm, calcium ions: 0.31769 nm, iron ions: 0.18509 nm, 0.17780 nm), enabling analysis of plasma temperature and composition during solar flares. This instrument was developed through international cooperation among Japan, the United Kingdom, and the United States.

Wide-band Spectrometer (WBS)

The WBS is designed for spectroscopic observation across a broad energy range from soft X-rays (2-30 keV) to gamma rays (0.2-100 MeV). This allows comprehensive observation of the overall energy distribution of solar flares and their behavior across different energy ranges. The gamma-ray spectrometer also served as a monitor for cosmic gamma-ray bursts.

Achievements

Yohkoh was the first scientific satellite to continuously observe nearly an entire solar cycle (approximately 11 years), covering 10 years and 3 months. It provided groundbreaking insights into the dynamic structural changes of the solar corona over various timescales and was the first to discover that explosive phenomena such as flares are actually “magnetic reconnection” events within the corona. These discoveries significantly advanced the understanding of solar physics.