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Solar Images

Solar X-Rays

The Yohkoh satellite is an observatory for studying X-rays and gamma rays from the Sun. Launched from Kagoshima, Japan on August 31, 1991; Yohkoh is a project of the Institute for Space and Astronautical Sciences. The spacecraft was built in Japan and the observing instruments have contributions from the U.S. and from the U.K.

Instruments Onboard Yohkoh

The Yohkoh spacecraft has four instruments: a Bragg Crystal Spectrometer (BCS), a Hard X-ray Telescope (HXT), a Soft X-ray Telescope (SXT), and a Wide Band Spectrometer (WBS). This combination of hard and soft x-ray imaging from the telescopes and spectroscopy instruments operating simultaneously, in coordination, makes the Yohkoh satellite unique and powerful tool for studying the Sun. These four instruments cover a wide energy range of the high energy solar spectrum and are extremely useful for studying solar flares which is Yohkoh's primary objective.

The Hard X-ray Telescope

A hard x-ray imaging instrument can not use conventional optics such as a lens or mirrors to form hard x-ray images because the hard x-rays are absorbed by these objects. Instead, the telescope consists of a series of aligned grids which form several collimated x-ray light paths. Each sub-collimator has a non-position-sensitive detector. The signal from each detector is then used to construct an image by computation.

The Soft X-ray Telescope

A soft x-ray telescope can be made with either a special normal incident mirror or glancing incidence mirrors. This telescope uses glancing incidence mirrors to form soft x-ray images of the Sun on a CCD sensor.


This archive contains GIF images available for quick look purpose. GIF images can also be found at the Lockheed Martin Solar and Astrophysics Laboratory. GIF images have 512 x 512 pixels, with only one byte per pixel. The pixel size is about 4.9 arc seconds. Some have only 256 x 256 pixels. FITS images, with same spatial resolution and depth (one byte per pixel), can be found at CRL/Japan. The directory archive is divided in months: for instance, 9511 means November 1995. In each subdirectory, the filenames are in the form:
ysYYMMDD.HHMMSS.gif for 512 x 512 x 1 byte GIF files (quick look).

The field YYMMDD is the date of observation, where YY is the year, MM is the month and DD is the number of the day. The field HHMMSS is the time of observation, where HH is the hour, MM the minute and SS the second of observation in UT.

The suffix .gif means GIF data.


A temporary enhancement of the X-ray emission of the sun. The time-intensity profile of soft X-ray bursts is similar to that of the H-ALPHA profile of an associated FLARE.


Rank of a FLARE based on its X-ray energy output. Flares are classified by the order of magnitude of the peak burst intensity (I) measured at the earth in the 1 to 8 angstrom band as follows:

Class (in Watt/sq. Meter)
B I less than (l.t.) 10.0E-06
C 10.0E-06 l.e.= I l.t.= 10.0E-05
M 10.0E-05 l.e.= I l.t.= 10.0E-04
X I g.e.= 10.0E-04


This ABSORPTION LINE of neutral hydrogen falls in the red part of the visible spectrum and is convenient for solar observations. The H-alpha line is universally used for patrol observations of solar flares. A broad-band image shows the Sun as we see it normally a white or yellow featureless disk, called the photosphere or visible surface. A closer inspection of this telescopic image, however, reveals several dark spots, called sunspots. These appear darker then their surroundings because they are considerably cooler (at 4,000 K that's about 8 times hotter then your oven) then the average temperature of the surrounding photosphere, 6,000 K. They generally occur in pairs or groups. Sunspots are places were the Sun's magnetic field emerges from below the photosphere. If we were to keep a daily record of the sunspot positions, we would discover that the Sun rotates from east to west (from left to right in the image) once every 27 days. in the next couple of years, as the new sunspot cycle begins, spots will occur at latitudes and these spots will rotate at a slightly slower rat

H alpha coronograph :

FITS files: the archive contains FITS compressed images of the H alpha coronograph at Pic du Midi. Images have been compressed using the usual command "compress" of UNIX. Uncompress images with the usual command "uncompress".

The size of FITS images is 1024 x 1024 pixels, 2 bytes per pixel (the detector is a CCD camera). The approximate size of each pixel is about 3 arc seconds.

GIF files: the archive contains also GIF images available for quick look purpose. GIF images have 512 x 512 pixels, with only one byte per pixel. The pixel size is about 6 arc seconds.

The directory archive is divided in months: for instance, 9511 means November 1995. In each subdirectory, the filenames are in the form:

prYYMMDD.HHMMSS.fits.Z for 1024 x 1024 x 2 bytes FITS raw files

pcYYMMDD.HHMMSS.fits.Z for 1024 x 1024 x 2 bytes FITS calibrated files

prYYMMDD.HHMMSS.gif for 512 x 512 x 1 byte GIF files (quick look)

The first letter "p" means "Pic du Midi"; the second one means raw data if "r" or calibrated data if "c". Raw data ("r") are only corrected from dark current; calibrated data ("c") include correction of dark current and flat field (the flat field is obtained using a scattering filter). The field YYMMDD is the date of observation, where YY is the year, MM is the month and DD is the number of the day. The field HHMMSS is the time of observation, where HH is the hour, MM the minute and SS the second of observation in UT. The suffix .fits.Z means FITS compressed data. The suffix .gif means GIF data for quick look.

The H alpha coronograph is operated by Dr J.-C. NOENS; complete sets of raw (pr) and calibrated (pc) images, with

good time resolution, are available in FITS format (via network or DAT tapes) by request to the author at

Radio Heliographs

Nobeyama Radioheliograph (NoRH)

is a radio telescope dedicated to observe the Sun. "Helio" means the Sun, "graph" means an imaging telescope. It consists of 84 parabolic antennas with 80 cm diameter, sitting on lines of 490 m long in the east/west and of 220 m long in the north/south. Its construction took 2 years and cost 1.8 billion yen. The first observation was in April, 1992 and the daily 8-hours observation has been done since June, 1992.

Frequency 17GHz (Right and left circular polarization), 34GHz (only intensity)

Field of view Solar full disk

Spatial resolution 10 arcsec (17GHz), 5 arcsec (34GHz)

Temporal resolution 0.1 sec (Event), 1 sec (Steady)

As the NoRH is a radio interferometer, original data are sets of correlation values of all the combination of antennas. They correspond to the spatial Fourier components of the brightness distribution of the solar disk. In most cases, it is necessary to synthesize images from the original raw data.

Observatoire de Paris Nancay Radioheliograph

The Nancay Radioheliograph (NRH; located at 47 N 02 E) provides since the end of 1996 TWO dimensional images of the radio brightness of the Sun with second time resolution. It consists of two linear antenna arrays, on terrestrial east-west and north-south baselines respectively. In its earlier versions it operated in 1D only at 169 MHz with both arrays from 1980 until early 1985, and later at 164 MHz with the east-west array, and in a five-frequency mode with the north-south array from 1986 to 1990. Since late summer 1991both arrays of the NRH are operated in the multifrequency mode. Up to five frequencies are chosen in the range 150 - 450 MHz within the restrictions imposed by terrestrial interference. Since the end of 1996, N/S and E/W arrays can be correlated in the multifrequency mode from 150 to 450 MHz in order to produce 2D radiograms. The characteristics of the instrument are summarized in the following table.

Arrays Number of Minimum Maximum Beamwidth Beamwidth

antennas baseline baseline (150 MHz) (450 MHz)

East-west 19 50.0 m 3200 m 1.3 0.42

North-south 24 54.3 m 1248 m 3.2-8.0 (1) 1.2-2.6 (1)

Time resolution: one 2D image/second at each frequency

Measurement of circular polarization (Stokes par. V)

Dynamic range: > 45 dB

Observing time: 8:30 to 15:30 UT

1) during summer and winter months, respectively

The quicklook data of the daily 2D NRH observations provide information on the occurrence and localization of events at two different frequencies: 164 and 327 MHz. There is about one GIF file per hour.

Filenames Image filenames are in the form:

naYYMMDD.HHMMSS.gif for 164 MHz observations (gif files)

nbYYMMDD.HHMMSS.gif for 327 MHz observations (gif files)

naYYMMDD.HHMMSS.fits for 164 MHz observations (fits file)

nbYYMMDD.HHMMSS.fits for 327 MHz observations (fits file)

where YY, MM and DD identify respectively the year, month and day of observation, and HH, MM, SS the hour, minute and second of observation. The size of images is about 256 x 256 pixels, with 8 bits per pixel for gif files, and 32 bits per pixel (REAL*4) for fits files. The direction of the North pole of the Sun is up. The field of view is -2 Rs to +2 Rs in both directions N/S and E/W, where Rs is the solar radius. Hence, the pixel size is 0.25 arc min or 15".

The database contains about one image per hour for each frequency.


The 150-Foot Solar Tower at Mt. Wilson Observatory

The solar magnetograph was first developed by Horace W. Babcock in 1953. Four years later, a magnetograph was installed for full time use at the Mt. Wilson 150-foot solar tower. The magnetograph currently in use at the tower was constructed in 1982, and upgraded in 1994 and 1996. From these numerous modifications, alterations, and advancements this magnetograph has remained a state of the art instrument.

Entrance Slit:

The image of the solar disk is projected onto a small "table" at ground level of the telescope. Light from the solar disk is allowed to pass through a 4-position filter wheel, which is generally used in the "no filter" position. Next, a combination of a KD*P crystal (Pockels Cell) and a Glan-Thompson prism is used to act as a circular polarization analyzer. A discrete portion of the solar disk is selected with a 20 or 12.5 arc second square aperture, which, in turn, is laid out by way of a Walraven image slicer onto the 250 micron entrance slit of the instrument. The light is then allowed into the spectrograph pit.

Spectrograph Pit:

The spectrograph pit employs the use of a diffraction grating in a Littrow mounting. The Littrow lens is a 75-foot focal length air-spaced Brashear doublet with an aperture of 9 inches. The diffraction grating, produced by Milton-Roy in 1994, is 250 by 408 millimeters in size, ruled at 367.5 lines per millimeter, and is blazed at the ninth order green (60 degrees). This design allows for the simultaneous examination of the Fe I line at 5250.2? (9th order), Na I D1 at 5895.9?, Na D2 at 5890.0? (8th order), Ni I at 6767.8? (7th order), and Ca II K line at 3933.7? (12th order), all within the three foot exit slit box. The different orders are separated by the use of narrow band-pass filters, located in front of the exit slits which follow.

Exit Slit Assembly:

In the current exit slit assembly, two separate stages maintain positions by servoing on absorption features in the solar spectrum. Generally, for the observations that are posted on the Web, the selected "servo" lines are Fe I at 5250.216? and Na I at 5895.940?. Each stage is equipped with a pair of fiber optic bundles that transfer light from the solar spectrum to 24 photomultiplier tubes (Hamamatsu R1477). The spectrum side of the fiber optic bundle is rectangular in shape, but through randomly rearranging the individual fiber optics, the PMT side of the bundle is circular. Each fiber optic bundle samples the light intensity at selected distances into the wing of the absorption line, and up to 10 points may be observed in the line profile.

The stage maintains its position by incrementally serving blueward or redward in the spectrum such that PMT intensities from both fiber optic bundles remain of approximately equal value. By this method, doppler shifts may be calculated. Zeeman line splitting is also observed and, by means of a 400Hz modulation of the KD*P crystal, magnetic field strengths and polarities are determined. Thus, both motion and magnetic fields on the sun may be measured with the use of this magnetograph. Two other absorption lines in the solar spectrum, Cr II at 5237.325? and Ni I at 6767.782?, are also observed. However, data from these lines are not posted on the 150-Foot Solar Tower Home Page, but are indeed archived for research purposes.