Minimize Solar-A

Solar-A / Yohkoh (X-ray Solar Observatory)

Solar-A is a Japanese solar physics mission of JAXA, formerly ISAS (Institute of Space and Astronautical Science) at the University of Tokyo. The overall objective is the study of high-energy phenomena on the sun's visible disk (solar flares, dynamics of solar corona and active regions with focus on time and spectral resolution) through X-ray observations and energetic neutrons (part of gamma-ray region). Primary science objectives are: a) to further the understanding of the magnetofluid dynamics of the repeated expansion of the solar corona, b) clarification of the changes in the shape of the solar corona, and c) the study of the mechanism underlying the occurrence of plasma disturbance phenomena in interplanetary space.1) 2) 3) 4)

From project initiation in the 1982-1985 timeframe, Solar-A was planned by ISAS as an international collaborative mission with US (NASA) and UK (MSSL, RAL) partners.

Note: On Oct. 1, 2003, ISAS was integrated into JAXA (Japan Aerospace Exploration Agency). JAXA is the new name (merger) of the three former Japanese space organizations into a single national agency, namely: NASDA (National Space Development Agency of Japan), ISAS (Institute of Space and Astronautical Science), and NAL (National Aerospace Laboratory of Japan).


Figure 1: Illustration of the Solar-A spacecraft (image credit: JAXA/ISAS)


The Yohkoh (Japanese for "sunlight" or "sunbeam") minisatellite is three-axis stabilized with arcsec stability, allowing long exposure times with the soft X-ray telescope. The satellite instruments are pointed toward the sun. The spacecraft uses a box-like structure of size: 1 m x 1 m x 2 m (height) and a mass of about 390 kg (100 kg science payload), power supply = 450 W average (560 W of peak power, use of two deployed solar panels), nominal design life of 2 years.

The ACS (Attitude Control Subsystem) uses momentum wheels, magnetic torquers, and control-moment gyros as actuators. Attitude sensing is provided with two sun sensors, a star tracker, and geomagnetic sensors - capable for determining the spacecraft pointing relative to the direction of the sun and to the ecliptic plane. An inertial reference unit (IRU) comprising four gyros detects changes of attitude with time.

Solar-A features also the first regulated EPS (Electrical Power Subsystem) bus on a Japanese scientific spacecraft. 5)


Figure 2: Line drawing of the Solar-A spacecraft (image credit: JAXA/ISAS)


Launch: A launch took place on Aug. 30, 1991 by an ISAS M-3SII-6 launch vehicle from the Kagoshima Space Center, Japan. The Solar-A spacecraft became Yohkoh effective upon launch.

Orbit: slightly elliptical orbit, apogee = 792 km, perigee = 525 km, inclination = 31.3º, period = 97 minutes.

RF communications: On-board recorder (80 Mbit capacity, magnetic bubble data recorder). Real-time data transmission of 32 kbit/s, or 262 kbit/s playback data, both to the Kagoshima ground station of ISAS. Downlink communication in S-band and X-band.

The Yohkoh operations, based at ISAS, involve a multinational team. NASA provided in addition extensive downlink telemetry coverage via Wallops and the DSN (Deep Space Network) tracking stations, but uplink is solely through ISAS's Kagoshima Space Center in Kyushu, Japan.


Status of the Yohkoh mission:

• The Yohkoh spacecraft reentered the Earth's atmosphere on Sept. 12, 2005 over Southeast Asia (India), disintegrating and burning up completely on reentry. 6)


Figure 3: Estimated reentry time: 18:16 hours (JST) on Sept. 12, 2005 (image credit: JAXA)

• The spacecraft experienced a very long operational life of over 10 years up to mid-December 2001. All instruments worked well throughout the mission. However, the aperture entrance filter of SXT (Soft X-ray Telescope) ruptured about 14 months into the mission. This problem made it impossible to derive adequate co-alignment information for SXT images from SXT data. Luckily, sensors on the HXT could be used.

• Yohkoh is the first spacecraft to continuously observe the sun in X-rays over an entire sunspot cycle and carries the longest operating CCD detector in space (SXT): the instrument that has captured over six million images. As such, the Yohkoh mission revolutionized our understanding of the active sun and helped to shape the development of space weather research.7) 8)


Sensor complement: (HXT, SXT, BCS, WBS)

When Solar-A/Yohkoh was launched in 1991, the sun was near the peak of its eleven-year activity cycle, so many active regions (parts of the corona associated with sunspot regions) and flares were imaged and a great deal of data acquired. As the years progressed, solar activity decreased and the numbers and size of active regions declined greatly. The start of activity cycle number 23 in the late 1990s saw another huge increase in the numbers of active regions, with very large flares occurring. 9) 10) 11)


HXT (Hard X-ray Telescope):

The HXT was developed by ISAS of the University of Tokyo, and NAOJ (National Astronomical Observatory of Japan). HXT is a Fourier synthesis-type imaging device with 64 collimator elements (Sudsre modulation type) covering an energy range from 10 - 100 keV at 7 arcsec resolution for the whole solar disk. HXT employs multi-pitch bi-grid modulation collimators with NaI scintillators (using a precise grid fabricated by photo-etching and a highly stable X-ray scintillator). There is no other means of X-ray imaging at energies approaching 100 keV. 12) 13)


Figure 4: View of the HXT instrument (image credit: JAXA/ISAS)

The HXT instrument consists of three elements (total instrument mass of 48 kg):

1) HXT-C: The collimator (C) is the X-ray optics part of the instrument. It consists of an assembly of tungsten grid plates 0.5 mm thick, arranged within a metering tube with transverse dimensions 417 mm x 376 mm and 1400 mm long. The HXA (HX aspect system) is installed at the center of the optics system which includes lenses with appropriate filters on the front grid plate and fiducial marks on the rear plate, thus providing white-light images of the sun. The effective sensitive area of the telescope is 70 cm2. The HXT-C has a mass of 13.5 kg.

2) HXT-S denotes the sensor section - a package of 64 detector modules with transverse dimensions 465 mm x 392 mm and 223 mm deep. Each module consists of a NaI(T1) scintillation crystal (25 mm2) and a photomultiplier/ re-amplifier. Eight high-voltage power supply units are attached to the HXT-S. The HXT-S features also two one-dimensional CCDs at the center for detecting HXA visible light signals.

3) HXT-E denotes the electronics units of the system of size 374 mm x 246 mm x 220 mm and a mass of 10.8 kg. It processes the hard X-ray signals from HXT-S. First, it converts pulse-height analog signals into digital signals, then it counts the incident photon number after discriminating the photon energy into four energy bands. The signals from the individual subcollimators are being processed separately and simultaneously. Finally, the HXT-E sends the photon count data to the data processor of the spacecraft.

HXT forms images in four energy bands (15-24, 24-35, 35-57, and 57-100 keV) with a time resolution as fine as 0.5 s and spatial resolution as small as 5-7 arcsec. The HXT instrument responds both to bremsstrahlung from non-thermal electrons and to thermal radiation from "super-hot" sources formed during flares.

The FOV (Field of View) of HXT is about 35 arcmin x 35 arcmin, i.e. covering the entire disk of the sun (the mean diameter of the sun is 32 arcmin). This means that HXT can detect hard X-rays of flares regardless of their position on the sun without re-pointing the spacecraft.


Figure 5: Schematic view of the HXT instrument (image credit: JAXA/ISAS)

Background: The HXT instrument is considered the first solar instrument to obtain imagery of very hard X-ray flares with energies ≥ 30 keV. Yohkoh was also the first satellite in Japan to incorporate true computer telescope control. HXT was the first telescope anywhere to adopt the Fourier synthesis principle in its observation strategy. The sensitivity of HXT was approximately 100 times that of its predecessor instrument of the SMM (Solar Maximum Mission) spacecraft (launch: Feb. 14, 1980), namely HXIS (Hard X-Ray Imaging Spectrometer). HXT collected many examples of hard X-ray flares and helped to answer a number of questions of solar flare characteristics.


SXT (Soft X-ray Telescope):

SXT is a US-Japanese collaboration between NASA/MSFC, LMSAL (Lockheed Martin Solar and Astrophysics Laboratory), of Palo Alto, CA, NIST (National Institute of Standards and Technology), NRL (Naval Research Laboratory), ISAS, NAOJ, and the Tokyo Astronomical Observatory.

SXT employs a grazing incidence reflecting soft X-ray mirror (in the wavelength range of 3-60 Å) and using an X-ray CCD detector (first introduction anywhere of a CCD detector for X-ray imaging). SXT uses a 23 cm diameter Wolter-Nariai optics of 155 cm focal length; it covers the energy range from 0.25 - 3 keV at 2.4 arcsec resolution (1700 km on the sun), far better than any previous spaceborne instrument. Imaging is provided by a 1024 x 1024 pixel CCD behind two six-position filter wheels. SXT forms images at 0.5-2 keV photon energy from plasmas with temperatures in the range of 2-20 million K. SXT has a FOV that covers the whole sun, but on internal command it can obtain a series of small-scale, high-resolution images of flares. Instrument mass of 27 kg.14) 15) 16) 17)


Figure 6: Optics design concept of the SXT (image credit: LMSAL)


Figure 7: Schematic view of the SXT instrument assembly (image credit: LMSAL)

SXT has for the first time provided images of coronal mass ejections - eruptions through the lower corona of the sun of enormous bubbles of superheated plasma. The plasma - estimated to be the equivalent of 17,000 Earths in volume - spews charged particles out into space at velocities of over one million miles an hour. Solar scientists discovered the elusive voids left behind by these colossal events in SXT images, providing another piece in the puzzle of understanding the corona and how the sun throws off wind and transports energy.


BCS (Bragg Crystal Spectrometer):

BCS is being used for high-resolution soft X-ray emission-line spectroscopy. The main objective is the precise observation of the high-temperature plasma that is generated in solar flares. Solar flares create plasma temperatures of 10-50 x 106 K; special emphasis is the heating and dynamics of the plasma during the impulsive phase. The line ratios give flare temperatures, and Doppler-shifted components give plasma flow velocities. BCS is a collaboration between Japan, UK and the US involving MSSL (Mullard Space Science Laboratory) of the University College London and RAL (Rutherford Appleton Laboratory), and Tokyo Astronomical Observatory. Instrument mass of 13 kg. 18)

BCS is a polarimeter/spectrometer operating in the 2-8 keV range. It uses two large plane crystals, lithium hydride and graphite, in a Bragg spectrometer with a honeycomb collimator. The instrument is mounted to view along the satellite spin axis and to examine the radiation of individual X-ray sources for possible polarization and/or the existence of line emissions. The spectrometer employs four bent germanium crystals, views the whole Sun , and observes the resonance line complexes of H-like FeXXVI, CaXIX, and SXV in four narrow wavelength ranges with a resolving power (λ/Δλ) of between 3000 and 6000.


Figure 8: Observation scheme of BCS (image credit: University of Montana)


WBS (Wide-band Spectrometer):

WBS was designed and developed at ISAS and NAOJ. WBS instrument mass of 16 kg. WBS is a non-imaging instrument consisting of three separate spectrometers/detectors viewing the whole-sun radiation in soft X-rays [SXS (Soft X-ray Spectrometer)], hard X-rays (HXS), and in gamma-rays (GRS), with the fourth detector monitoring the terrestrial radiation belts, referred to as RBM (Radiation Belt Monitor). The first three of the spectrometers aim at solar flare observations while the RBM serves to sound the alarm for radiation belt passage. WBS uses a set of proportional counters, a NaI scintillator and two BGO (Bismuth Germanate Oxide) scintillators. Measurement ranges: 19)

SXS (Soft X-ray Spectrometer): 2 - 30 keV

HXS (Hard X-ray Spectrometer): 20 - 400 keV

GRS (Gamma Ray Spectrometer): 0.2 - 100 MeV.


Figure 9: Illustration of the WBS spectrometers (image credit: University of Montana)

The WBS output is a set of simultaneous time-resolved spectra.


Figure 10: Last image of the SXT instrument observed on December 14, 2001 (image credit: LMSAL)

1) "The SOLAR-A Mission," The Solar-Terrestrial Science Project of the Inter-Agency Consultative Group for Space Science, ESA SP-1107, November 1990, pp. 74-76


3) "Yohkoh's Prodigious Output helps Scientists Study Sun," Space News, June 7-13, 1993, p. 12


5) M. Igarashi, M. Homma, E. Hayashi, Y. Takizawa, Y. Nozaki, Y. Tanaka, Y. Ogawara, T. Takano, H. Inoue, M. Tajima, "Development of electrical power subsystem for SOLAR-A and ASTRO-D," 18th International Symposium on Space Technology and Science, Kagoshima, Japan, May 17-22, 1992. Vol. 1 & 2, . A95-82299, pp. 2589-2594

6) "Result of Re-entry of the Solar X-ray Observatory "Yohkoh" (SOLAR-A) to the Earth's Atmosphere", JAXA, Sept. 13, 2005, URL:

7) L. Harra, J. L. Culhane, "A solar cycle in X-rays," URL:

8) Y. Ogawara, “Yohkoh (Solar-A) observations of solar activity,” Journal of Atmospheric and Terrestrial Physics, Volume 57, Issue 12, October 1995, pp. 1361-1368


10) “Yohkoh Analysis Guide - Instrument Manual,” 1999, URL:

11) “Instruments Onboard Yohkoh,” URL:


13) T. Kosugi, K. Makishima, T. Murakami, T. Sakao, T. Dotani, M. Inda, K. Kai, S. Masuda, H. Nakajima, Y. Ogawara, M. Sawa, K. Shibasaki, "The Hard X-Ray Telescope (HXT) for the Solar-A Mission," Solar Physics Vol., 136, 1991, pp. 17-36

14) “Yohkoh Solar Observatory,” URL:

15) “The Yohkoh Soft X-ray Telescope,” URL:

16) S. Tsuneta, L. Acton, M. Bruner, J. Lemen, W. Brown, R. Caravalho, R. Catura, S. Freeland, B. Jurcevich, J. Owens, "The soft X-ray telescope for the Solar-A mission," Solar Physics, Vol. 136, 1991, pp. 37-67


18) “Bragg Crystal Spectrometer (BCS),” URL:


The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.