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Yohkoh mission graphic

Phase: Past

Launch Date: August 30, 1991

Mission Project Home Page -

Program(s):Heliophysics Research

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The Yohkoh Mission is a Japanese Solar mission with US and UK collaborators. It was launched into Earth orbit in August of 1991 and provided valuable data about the Sun's corona and solar flares. The satellite carried four instruments - a Soft X-ray Telescope (SXT), a Hard X-ray Telescope (HXT), a Bragg Crystal Spectrometer (BCS), and a Wide Band Spectrometer (WBS).

YohkohYohkoh suffered a spacecraft failure in December 2001 that has put an end to this mission. During the solar eclipse of December 14th the spacecraft lost pointing and the batteries discharged. The spacecraft operators were unable to command the satellite to point toward the Sun.

There were four instruments on the satellite that detect energetic emissions from the Sun:

The Bragg Crystal Spectrometer (BCS) consists of four bent crystal spectrometers. Each is designed to observe a limited range of soft x-ray wavelengths containing spectral lines that are particularly sensitive to the hot plasma produced during a flare. The observations of these spectral lines provide information about the temperature and density of the hot plasma, and about motions of the plasma along the line of sight. Images are not obtained, but this is offset by enhanced sensitivity to the line emission, high spectral resolution, and time resolution on the order of one second.

The Wide Band Spectrometer (WBS) consists of three detectors: a soft x-ray, a hard x-ray, and a gamma-ray spectrometer. They were designed to provide spectra across the full range of wavelengths from soft x-rays to gamma rays with a time resolution on the order of one second or better. Like the BCS, images are not obtained.

The Soft X-Ray Telescope (SXT) images x-rays in the 0.25 - 4.0 keV range. It uses thin metallic filters to acquire images in restricted portions of this energy range. SXT can resolve features down to 2.5 arc seconds in size. Information about the temperature and density of the plasma emitting the observed x-rays is obtained by comparing images acquired with the different filters. Flare images can be obtained every 2 seconds. Smaller images with a single filter can be obtained as frequently as once every 0.5 seconds.

The  Hard X-Ray Telescope (HXT) observes hard x-rays in four energy bands through sixty-four pairs of grids. These grid pairs provide information about 32 spatial scales of the x-ray emission. This information is combined on the ground to construct an image of the source in each of the four energy bands. Structures with angular sizes down to about 5 arc seconds can be resolved. These images can be obtained as frequently as once every 0.5 seconds.