Soft X-ray Image of the Sun taken May 8, 1992 by the Soft X-ray Telescope (SXT) on board the Yohkoh Satellite. This is a re-processing of the same data set that was used to create educational outreach poster of the Sun in 1992. The posters were created in collaboration with Keith Strong and myself and included an explanation of the types of light emitted by the sun otherwise known as the solar spectrum as well as instructions on how to build a solar eclipse project box and a sundial on the backside of the poster. Copies of the poster were distributed to several schools throughout the USA. I wanted to re-process this data set with more modern image processing tools from my astrophotography hobby to see if I could improve the image and bring out more corona structure on the disk and close to the limb of the sun. This image is a blend of a luminance image and a synthetic color image that I made from assigning three different colors to fundamentally different brightness regions: the bright cores of Active Regions, the bright active areas, and the dim corona. These false colors were added to aid in enhancing the detail structures of the corona.  The image with north at the top and east on left is a composite image from East and West off-points. Each off-point image is a combination of 3 exposures (long, medium, and short) to improve dynamic range. The original purpose of the off-point observations was to better show the extended corona about the east and west limbs in soft X-rays, using the thin Al filter with a peak bandpass at 10 Angstroms.

The Yohkoh satellite was launched in August 1991 during the peak of Solar Cycle 22, with the main goal of observing energetic phenomena from emitting structures such as solar flares in X-rays and gamma-rays with a coordinated set of instruments.There are several interesting features displayed in this image. First and foremost is that the surface of the Sun at 5,500 C is too cool to emit X-rays and is therefore black. Only the atmosphere of the Sun, also called the corona, which has temperatures measured in the millions degrees C is hot enough to emit X-rays. It is believed that solar activity with the ionized gases known as plasma, interacting with the Sun’s magnetic field leads to the high coronal temperatures. The structure of the corona displayed in this image traces the magnetic fields, like the pattern of iron filings around a bar magnet, as the magnetic fields channel the hot plasma within the atmosphere.Some of the more interesting coronal features revealed in this image are:
1. The Coronal Hole - where the corona is dark and is associated with open magnetic field lines and are often found at the Sun’s poles, such as the fairly large corona hole displayed here at the top of the Sun.
2. Polar Plumes - which are non thin streamers that project outward from the Sun’s north and south poles, also shown at the top of the Sun.
3. X-ray bright points - which are small point-like emission regions such as shown on the western edge of the coronal hole and scatted about over the disk of the sun.
4. Coronal Loops - which are displayed in this image over the dark disk of the Sun, sometimes looking like pinched elbow tube-shaped pastas, as well as the loops on the limbs that show the extension high into the corona. Sometimes a series of loops form larger structures such as arcades, such as those displayed at about 8 o’clock on an hour-hand from the disk center and on the west limb at about 3 o’clock where our view is down the axis of an arcade.
5. Helmet Streamers are large cap-like coronal structures with long pointed peaks. While this image doesn’t have a large enough field of view or brightness to show the pointed peaks of helmet streamers, the area at about 10 o’clock on the eastern limb is a good candidate for such an event. Helmet streamers often have a prominence or filament lying at the base of the structure. There is also a filament channel at about 7 o’clock just inside the edge of the limb. The ejection of a prominence or filament into space can lead to solar auroras when the ejected plasma collides with the earth’s magnetic field.

Image Credit: Yohkoh data courtesy of the NASA supported Yohkoh Legacy Archive, MSU; SXT Instrument: LPARL, USA; NAOJ, Japan
I also want to thank Loren Acton and Aki Takeda for their guidance and background information related to processing the Yohkoh-SXT data.

Yohkoh was launched in August 1991 from the Kagoshima Space Center. Due to data rates from the Satellite to ground stations in 1992 the 1024x1024 CCD detector was not used in full resolution for this data set. This data set was done with a 2x2 on-chip summing which produced image resolution of 512x512. Original Image process was done using Interactive Data Language (IDL). The SXT instrument is a glancing incident X-ray mirror with a geometrical area of 262 mm² and a focal length of 1535 mm. The spectral range using various thin metal filters is 3--45Å. The back illuminated CCD detector has a pixel size of 18.3 µm.

The image data comes from the Yohkoh Legacy Archive at MSU and I used the Yohkoh SXT level-1 FITS data.
The image processing for this image was done primarily in PixInsight. GIMP was used to stitch the east and west limb images and Photoshop was used for removal of the seam between the two off-points and small artifact clean-up, and a high-pass filter was applied to further sharpen details.

Image workflow summary:
1. Stacked 3-short and 3-Med exposure images for the East and West limbs. Used just a single long exposure for the East and West.
2. Created a mask for the bloom areas on the long exposure images. Then used PixelMath to effectively replace the bloomed areas in the long exposure images with the exact same regions from the med exposures but scaled to the long exposure length.
3. Created two HDR images using the short, med, and long exposures: one for the East and West limbs. This is 32-bit float operation.
4. Ran a DeNoise process to reduce the image noise in both images.
5. Linear to Non-Linear stretch. Applied the same mild stretch to both images.
6. Used HDR Multiscale Transform on each image. I ended up using 6-layers which corresponds to 6 different spacial scale transforms. The results are then blended together with a PixelMath using a weighting factor to selectively enhance different spacial scales in the coronal structures. This is 64-bit float operation.
7. Used a generalized hyperbolic stretch to further enhance specific regions of the images. Used the same stretch for both images.
8. Here I had to exported the two images as 32-bit float TIFF format images. Used GIMP to merge the east and west limb images into a single image. (PIxInsight would normally be the place to do this but it requires that I plate solve each the two images in order to make single mosaic image and I didn’t have that information.)
9. Imported the TIFF from GIMP back into PixInsight. Then did a slight crop of the image as there was a slight shift of sun center in the fames between east and west. Then used a dynamic background extraction to remove any background gradient between the two images.
10. Used a local histogram equalization process to further stretch the image.
11. Had to export the image as 16-bit int to Photoshop for a final clean-up. This is where I was able to remove the artifacts from the seam between the two images and removed any small artifacts in the bloom areas. Applied a high-pass filter to effectively sharpen the structures.
This finished the processing for the mono image or what I call the luminance image. 
12. I took things a step further by generating a synthetic color from the luminance image by masking three principally different regions in brightness (AR cores, general active areas, dim corona). Applied a different color to each region and blended the three into a color rendering of the luminance image.
13. Finally, a process often done in astrophotography is to combine the luminance with the RGB. So, I blended the luminance image with the synthetic color image to further bring out structural details.
The displayed image is this blended luminance image with the synthetic color image.