Yohkoh
Encyclopedia
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
observatory spacecraft of the Institute of Space and Astronautical Science
Japan Aerospace Exploration Agency
The , or JAXA, is Japan's national aerospace agency. Through the merger of three previously independent organizations, JAXA was formed on October 1, 2003, as an Independent Administrative Institution administered by the Ministry of Education, Culture, Sports, Science and Technology and the...
(Japan
Japan
Japan is an island nation in East Asia. Located in the Pacific Ocean, it lies to the east of the Sea of Japan, China, North Korea, South Korea and Russia, stretching from the Sea of Okhotsk in the north to the East China Sea and Taiwan in the south...
) with United States
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
and United Kingdom collaboration. It was launched into Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
August 30, 1991 by the M-3S-5
Mu (rocket)
The Mu, also known as M, was a series of Japanese solid-fuelled carrier rockets, which were launched from Uchinoura between 1966 and 2006...
rocket from Kagoshima Space Center.
The satellite was three-axis stabilized and in a near-circular orbit. It 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). About 50 MB
Megabyte
The megabyte is a multiple of the unit byte for digital information storage or transmission with two different values depending on context: bytes generally for computer memory; and one million bytes generally for computer storage. The IEEE Standards Board has decided that "Mega will mean 1 000...
were generated each day and this was stored on board by a 10.5 MB bubble memory
Bubble memory
Bubble memory is a type of non-volatile computer memory that uses a thin film of a magnetic material to hold small magnetized areas, known as bubbles or domains, each storing one bit of data...
recorder.
Because SXT utilized a charge-coupled device
Charge-coupled device
A charge-coupled device is a device for the movement of electrical charge, usually from within the device to an area where the charge can be manipulated, for example conversion into a digital value. This is achieved by "shifting" the signals between stages within the device one at a time...
(CCD) as its readout device, perhaps being the first X-ray astronomical telescope to do so, its "data cube" of images was both extensive and convenient, and it revealed much interesting detail about the behavior of the solar corona. Previous solar soft X-ray observations, such as those of Skylab
Skylab
Skylab was a space station launched and operated by NASA, the space agency of the United States. Skylab orbited the Earth from 1973 to 1979, and included a workshop, a solar observatory, and other systems. It was launched unmanned by a modified Saturn V rocket, with a mass of...
, had been restricted to film as a readout device. Many interesting new discoveries were also made; for glimpses of these see the Yohkoh Science Nuggets, for example.
The mission ended after more than ten years of successful observation when it went into its "safehold" mode during an annular eclipse on December 14, 2001 after the spacecraft lost lock on the sun. Operational mistakes and other flaws conspired in such a way that its solar panels could no longer charge the batteries, which drained irreversibly; several other solar eclipses had successfully been observed.
On September 12, 2005 the spacecraft burned up during reentry over South Asia. The time of reentry, as provided by the U.S. Space Surveillance Network, was 6:16 pm Japan Standard Time
Japan Standard Time
Japan Standard Time or JST is the standard timezone of Japan, and is 9 hours ahead of UTC. For example, when it is midnight in UTC, it is 09:00 in Japan Standard Time. There is no daylight saving time, though its introduction has been debated several times. Japan Standard Time is the same as...
(JST).
Instruments
Yohkoh carries four instruments :- Soft X-ray Telescope (SXT) is an X-ray telescope with glancing incidence X-ray mirror and a CCD sensor. There is also a co-aligned optical telescope using the same CCD, but after the failure of the entrance filter in November 1992 it became unusable.
The CCD is 1024×1024 pixels with pixel angular size of 2.45″×2.45″, point spread function core width (FWHM) is about 1.5 pixels (i.e. 3.7″), field of view is 42′×42′, which is a little larger than the whole solar disk. Typical time resolution is 2 s in flare mode and 8 s in quiet (no flare) mode, the maximum time resolution in 0.5 s.
For spectral discrimination, STX employs wide-band filters installed on a filter wheel. There are five usable filter positions: 1265 Å-thick Al filter (2.5 Å–36 Å pass band), Al/Mg/Mn filter (2.4 Å–32 Å), 2.52 μm Mg filter (2.4 Å–23 Å), 11.6 μm Al filter (2.4 Å–13 Å), 119 μm Be filter (2.3 Å–10 Å). Before the entrance filter failure in November 1992 three more filter positions were available: no analysis filter (2.5 Å–46 Å), Wide band optical filter (4600 Å–4800 Å), Narrow band optical filter (4290 Å–4320 Å).
- Hard X-ray Telescope (HXT) is a Fourier synthesis type X-ray imager. HXT is sensitive to photons with energies from 14 keV to 93 keV, this range is divided into four energy bands (called L, M1, M2, H). The angular resolution is about 5″, image synthesis field of view is 2′×2′, maximum time resolution is 0.5 s.
- Bragg Crystal Spectrometer (BCS) is two bent crystal spectrometerSpectrometerA spectrometer is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify materials. The variable measured is most often the light's intensity but could also, for instance, be the polarization...
s sensitive in four spectral lines: the line of ion Fe XXVI (1.76 Å–1.81 Å), ion Fe XXV (1.83 Å–1.90 Å), ion Ca XIX (3.16 Å–3.19 Å), and ion S XV (5.02 Å–5.11 Å). Spectral resolution varies in the range of λ/Δλ=3000–8000, typical time resolution in flare mode is 8 s, maximum is 0.125 s. BCS integrates radiation over the whole solar disc. - Wide Band Spectrometer (WBS) has spectroscopic capabilities in a wide energy band from 3 keV to 100 MeV. WBS is a set of four subinstruments, eаch of them outputs Pulse Count (PC) corresponding to intensity integrated over a band, and Pulse Height (PH) profile which corresponds to spectrum. Time resolution for PC (0.125 s–4 s for different subinstruments and modes) is 8–16 times better than for PH (1 s–32 s). WBS integrates radiation over the whole Sun and does not resolve source position.
- Soft X-ray Spectrometer (SXS) consists of two proportional gas countersProportional counterA proportional counter is a measurement device to count particles of ionizing radiation and measure their energy.A proportional counter is a type of gaseous ionization detector. Its operation is similar to that of a Geiger-Müller counter, but uses a lower operating voltage. An inert gas is used to...
with nominal energy band 5 keV–40 keV, which is divided into two PC channels and 128 PH channels. It was found after the launch that PH to energy relationship was distorted. No energy calibration for WBS PH data was available in 1999. - Hard X-ray Spectrometer (HXS) is a NaI(Tl) scintillatorScintillation counterA scintillation counter measures ionizing radiation. The sensor, called a scintillator, consists of a transparent crystal, usually phosphor, plastic , or organic liquid that fluoresces when struck by ionizing radiation. A sensitive photomultiplier tube measures the light from the crystal...
. The energy band after June 1992 is 24 keV–830 keV. It is divided into 2 PC channels and 32 PH channels. - Gamma-ray Spectrometer (GRS) consists of two identical bismuth germanate oxide scintillators. It covers energy range 0.3 MeV–100 MeV, which is divided into 6 PC channels and 128+16 PH channels.
- Radiation Belt Monitor (RBM) unlike the other three isn't aimed at solar flare observations and serves to sound the alarm for radiation belt passage.
- Soft X-ray Spectrometer (SXS) consists of two proportional gas counters