On August 31, 1991, a satellite was launched into space from the Kagoshima Space Center (KSC) in Southern Japan. This satellite, known as Yohkoh ("Sunbeam"), was a project of the Japanese Institute of Space and Astronautical Science (ISAS). The scientific objective was to observe the energetic phenomena taking place on the Sun, specifically solar flares in x-ray and gamma-ray emissions.
There were four instruments on the satellite that detected energetic emissions from the Sun:
A team from the United States collaborated on SXT, and teams from the United States and Great Britain collaborated on BCS.
The BCS consisted of four bent crystal spectrometers. Each was 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 provided information about the temperature and density of the hot plasma, and about motions of the plasma along the line of sight. Images were not obtained, but this was offset by enhanced sensitivity to the line emission, high spectral resolution, and time resolution on the order of one second.
The WBS consisted 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 were not obtained.
The SXT imaged x-rays in the 0.25 - 4.0 keV range. It used thin metallic filters to acquire images in restricted portions of this energy range. SXT could resolve features down to 2.5 arc seconds in size. Information about the temperature and density of the plasma emitting the observed x-rays was obtained by comparing images acquired with the different filters. Flare images could be obtained every 2 seconds. Smaller images with a single filter could be obtained as frequently as once every 0.5 seconds.
The HXT observed hard x-rays in four energy bands through sixty-four pairs of grids. These grid pairs provided information about 32 spatial scales of the x-ray emission. This information was 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 could be resolved. These images could be obtained as frequently as once every 0.5 seconds.
The Yohkoh Spacecraft was in a slightly elliptical low-earth orbit, with an altitude ranging from approximately 570 km to 730 km. The orbital period was 90 minutes. Sixty-five to seventy-five minutes of this time was spent in sunlight. During five to six of it's orbits per day, Yohkoh passed through the radiation belts of the South Atlantic Anomaly where the instruments using high voltages had to be turned off (the BCS, HXT, and most WBS channels). Otherwise the radiation could destroy the instruments or the satellite.
Observations from the instruments were stored in the Spacecraft Bubble Data Recorder (BDR). The capacity of the BDR was 10 Mbytes. In order to optimize the recorder, it could operate at several bit-rates; high, medium, and low. Switching between the bit-rates was controlled two different ways, by the on-board deferred commands and automatically. This switching was necessary since the high-bit rate only held 42 minutes worth of data. Some overwriting of the data was permitted.
The satellite could operate in a large number of spacecraft modes and several different subsystem modes. The two modes of principal interest were the Quiet Mode and Flare Mode. Switching between these two particular modes was controlled by a flare flag generated by the WBS instruments. Allocation of which instruments could collect what data and how much of it depended on which mode Yohkoh was operating in. Generally, more HXT data was taken during the Flare Mode as opposed to the Quiet Mode.
During each orbit, about five or six times a day, Yohkoh passed over the Kagoshima Space Center. Commanding of the satellite could be performed at this time. (The rest of the time the satellite was controlled by on-board deferred command storage.) In addition, Kennedy Space Center also received the current data from the Data Recorder. At other locations in the orbit, the data were sent to ground stations in the NASA Deep Space Network.
Yohkoh ceased operations on December 14, 2001.
Solar flares are now observed in X-rays and gamma-rays by the Ramaty High Energy Solar Spectroscopic Imager.
For more information about the Yohkoh satellite and mission, see the Yohkoh Solar Observatory Web Site.
Institute of Space and Astronautical Science (ISAS)
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