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On the latest solar images from NASA, there is one picture lablled "continuum 4500 Å". There are bands diagonally in the picture as well.

enter image description here

What is the significance and meaning of this image?
(There are several strange features show there.)


Related but unhelpful question:

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    $\begingroup$ The top left quadrant shows the faint outline of a sun dragon. You don't want to annoy him. $\endgroup$ Commented Jan 17 at 20:02

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The 4500Å images (in the blue part of the visible light spectrum) are designed to show the Sun's visible "surface" or photosphere. I'm not 100% sure, but by the looks of it, the banding and fine filamentary structure in the image look like they are CCD artifacts, i.e., products of the imaging system rather than physical structures on the Sun itself. It looks like the image has not had a flat-field correction applied to try and compensate for the CCD irregularities. The real physical features that can be seen are the dark spots, which are sunspots. The fact that the Sun's disk gets darker towards the edges is also a real feature known as limb darkening.

Update

If the image is a raw "Level 0" image, rather than a processed "Level 1" image, then the following will not have been applied:

  • Removal of "over scan" rows and columns.
  • Removal of a dark image to account for the digital offset of the camera, CCD read noise and dark current.
  • Flat-field correction to account for detector non-uniformities, gain differences between CCD quadrants, vignetting and shadowing caused by focal plane filter nickel mesh.
  • Correction to individual pixels using two algorithms is done. First to permanently correct "bad" pixels that do not respond correctly to light and second, to remove the "spikes" that appear as a result of the interaction of energetic particles that either deposit energy directly in the CCD, or interact with the instrument structure and give rise to high-energy photons that are detected in the CCD.
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    $\begingroup$ +1 The diagonal dark-light pattern looks like some kind of fringe or diffraction patter superimposed - thus likely some artefact from the image acquisition system. $\endgroup$ Commented Jan 17 at 13:04
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    $\begingroup$ potentially related artifact questions in Space SE: What are these dark lines in IRIS images of the sun? and What are these echo-like artifacts in some SDO images? and What is this white dot and strange line in SOHO image? $\endgroup$
    – uhoh
    Commented Jan 17 at 13:26
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    $\begingroup$ @uhoh But those (linked) artifacts looks nothing like those above... $\endgroup$
    – not2qubit
    Commented Jan 17 at 18:56
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    $\begingroup$ So what are solar scientists looking for when analyzing this image, that is not already available in the other images in OP link? $\endgroup$
    – not2qubit
    Commented Jan 17 at 18:59
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    $\begingroup$ @not2qubit the different filter bands show, to some differing degree, different depths into the Sun's atmosphere. So, from the 4500Å images you can track the location of sunspots on the solar surface (or photosphere), which points to the locations where magnetic flux lines enter the photosphere. $\endgroup$ Commented Jan 18 at 10:32
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The purpose of the Solar Dynamics Observatory is:

to understand the influence of the Sun on the Earth and near-Earth space by studying the solar atmosphere on small scales of space and time and in many wavelengths simultaneously.

That's pretty open-ended and can cover a lot of research areas. Indeed, it does:

SDO has been investigating how the Sun's magnetic field is generated and structured, how this stored magnetic energy is converted and released into the heliosphere and geospace in the form of solar wind, energetic particles, and variations in the solar irradiance.

So, you could say that one of the goals of SDO is to build better models of space weather so as to predict solar storms that could affect satellite operations. The SDO has 3 main instruments on board, but the Atmospheric Imaging Assembly (AIA) was designed and is operated by the Lockheed Martin Solar and Astrophysics Laboratory (LMSAL).

The AIA is responsible for the images you linked. While LMSAL is ostensibly a scientific research organization, it is notably a part of Lockheed Martin, a major defense contractor. It would be naive to ignore the fact that USSF is very interested in space weather and its impact on satellite operations. So while I am sure there is a lot of good science being performed on the data provided by the SDO, I am equally confident that some of that research is specifically aimed at protecting satellites from solar activity.

You can see that the majority of sensor channels in the AIA are tuned to specific elements because those lines are of interest to existing models. And while those frequencies are likely most predictive for modelling solar flares and the like, it would probably be very difficult to make a complete model that ignores the bulk of radiation emitted by the sun. It would be like trying to model the sound coming from a rock concert by sampling only 40 Hz and 4 kHz while ignoring the 80-200 Hz range.

So you can think of the AIA as a multi-channel instrument that listens for very precise frequencies like individual instruments in an orchestra, while also capturing the broadband range where most of the power is being emitted, so that the data can be correlated into a useful composite picture.

Also consider that this data is almost certainly consumed predominantly by algorithms, not human eyeballs. The algorithms most likely need the visible frequency images as a reference baseline for the rest of the data.

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  • $\begingroup$ The 4500 Å (very close to the so called "G-Band") image can be used for several purposed, for instance to precisely measure where the telescope is pointing (which is harder to do just with EUV images, as the "edge" of the sun is much less well defined there). Most solar telescopes operating in soft X-rays or EUV wavelength are designed with the option to also take a visible continuum image. G-Band is quite popular for that purpose. $\endgroup$
    – pygri
    Commented Jan 18 at 8:43
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There are several purposes that such an image can be used for.

Consider the fact that it uses the same CCD device that is used for the EUV images.

Therefore it can be used to (among other things):

  • Calibrate the CCD gain, since the Sun's luminosity at 450 nm is a lot more constant (outside of sunspots and faculaes) than the EUV luminosity of the corona and where its behavior (e.g. limb darkening) is well understood.

  • Calibrate the position and angle of where the telescope is pointing. The sharp boundary of the photosphere is well suited for that.

  • Obtain flat fields images

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  • $\begingroup$ Granulation and spots means it cannot be used for flat-fielding. $\endgroup$
    – ProfRob
    Commented Jan 18 at 10:31
  • $\begingroup$ Many good answers here, but this is the most specific directly addressing my question. $\endgroup$
    – not2qubit
    Commented Jan 18 at 18:08

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