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In the last few years amazing images of the sun have been around. They are from the Solar Dynamics Observatory's AIA instrument. The one below shows super-hot iron atoms. So, am I looking at field lines of the sun's magnetosphere?

enter image description here

The light is in the extreme ultra-violet range and was collected through filters. What are those filters made of? What is the exposure time for these photos? Does the unit use the kind of light recording surface a digital camera does, or is it a surface capable of recording in the EUV range?

The whole thing stirs a bunch of other questions, I thought I'd just test the waters...

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  • $\begingroup$ You can take a look at the first and third link in my old question also. One perfectly good purpose for asking a question is to get an authoritative and/or good answer generated in stackexchange for everyone else to enjoy and learn from. Seeing as how you've done that so nicely here, I don't think anyone will object! $\endgroup$
    – uhoh
    Commented Jan 31, 2017 at 4:03

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The composite image cannot be made from our standard jpegs as they are log-scaled to improve the contrast. You put the linearly-scaled data into the color channels of an sRBG image and then log scale the result.

The AIA instrument filters are multi-layer metallic filters deposited on glass mirrors that were ground to the right shape. All is explained in Lemen's article that is freely available at http://adsabs.harvard.edu/abs/2012SoPh..275...17L . Other articles describe the calibration and operations. A front filter keeps out the visible light in most channels. The CCD is not greatly heated by the radiation that gets through the filter but is cooled by a radiator pointing towards space.

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OK I'll bite, despite that Dean Pesnell of the SDO team must really love me by now. :)

Most of the info you're after is available on SDO Data pages, if you click on the info icon. Also note that the Fe (Iron) channel composite in your question is equipped with color-keyed channel info bottom left of it, tho it is rather difficult to see at the resolution you used. Here's the description for the composite image:

This image combines three images with different, but very similar, temperatures. The colors are assigned differently than in the single images. Here AIA 211 is red, AIA 193 is green, and AIA 171 is blue. Each highlights a different part of the corona.

So the composite is generated by taking the three different wavelength (given in their names in angstroms) images and combining them per color channel into a single color image. The three inputs are described as (using latest SDO data thumbnails as examples):

AIA 211 (Red channel in Fe composite)

enter image description here

This channel (as well as AIA 335) highlights the active region of the outer atmosphere of the Sun - the corona. Active regions, solar flares, and coronal mass ejections will appear bright here. The dark areas - called coronal holes - are places where very little radiation is emitted, yet are the main source of solar wind particles.

Where: Active regions of the corona
Wavelength: 211 angstroms (0.0000000211 m) = Extreme Ultraviolet
Primary ions seen: 13 times ionized iron (Fe XIV)
Characteristic temperature: 2 million K (3.6 million F)

AIA 193 (Green channel in Fe composite)

enter image description here

This channel highlights the outer atmosphere of the Sun - called the corona - as well as hot flare plasma. Hot active regions, solar flares, and coronal mass ejections will appear bright here. The dark areas - called coronal holes - are places where very little radiation is emitted, yet are the main source of solar wind particles.

Where: Corona and hot flare plasma
Wavelength: 193 angstroms (0.0000000193 m) = Extreme Ultraviolet
Primary ions seen: 11 times ionized iron (Fe XII)
Characteristic temperature: 1.25 million K (2.25 million F)

AIA 171 (Blue channel in Fe composite)

enter image description here

This channel is especially good at showing coronal loops - the arcs extending off of the Sun where plasma moves along magnetic field lines. The brightest spots seen here are locations where the magnetic field near the surface is exceptionally strong.

Where: Quiet corona and upper transition region
Wavelength: 171 angstroms (0.0000000171 m) = Extreme Ultraviolet
Primary ions seen: 8 times ionized iron (Fe IX)
Characteristic temperature: 1 million K (1.8 million F)

I've also tried to generate my own composites out of AIA 211, AIA 193, and AIA 171. These are the two tries:

     enter image description here

Image above is a RGB channel combine from direct grey scale conversion. It looks pretty nice, but too much red. OK, next try:

     enter image description here

That's better, but still not as good as from the SDO team of course. What I did here is to convert input channels into B&W by using the portrait profile in Photoshop, then recombined as RGB components just like before, and also adjusted output color balance a bit. I opted for 6500 K color temperature. And there was a bit of contrast adjustment too. It's still not exactly like the composites from SDO tho:

     enter image description here

OK, enough of playing with data. Let's try to answer your questions more directly:

What are those filters made of?

I have no idea. They're multilayer-coated optical filters that let through incident light in wavelengths as described above. I presume they're high quality and were really expensive though. :) My best guess would be they were made by multiple vacuum deposition layers of partially reflecting dielectric compounds on a glass substrate. This would be the so-called "traditional coating", but there are other processes that could have been uses for AIA.

What is the exposure time for these photos?

If you look at individual channel timestamps in the image, they're at most a few seconds apart. AIA is capable of taking at least one image each second. But changing optical path (AIA uses four telescopes) and filters will take a few seconds, which will reflect in timestamps. And each filter will likely require slightly different exposure times. The browse data you're looking at (Level 2) are processed from raw (Level 0) data stream and corrected to a standardized exposure time. Refer to the Guide to SDO Data Analysis documentation for more info.

Does the unit use the kind of light recording surface a digital camera does, or is it a surface capable of recording in the EUV range?

AIA uses 4Kx4K resolution CCD (Charge-Doupled Device) sensors that operate at about -70° C (source). Keeping sensors at low enough temperatures enables them to record in visible and UV range with low thermal noise. CCDs are a bit expensive to make, and you'd usually find cheaper CMOS sensors in your consumer grade appliances. But they are widely used in various professional markets, including astronomy.

Some additional sources:

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  • $\begingroup$ I had sort of thought that because the telescope is pointed at the sun, the filters and exposure times must be pretty special, also because they have to keep the CCD cool while also making it take pictures of the sun every 10 seconds. And about the iron atoms thing - did they choose iron atoms because they'd tend to stick to the magnetic field lines more, or are those atoms more common in the corona, and why would they be more common because other wavelengths are looking at He and C, and then my brain just goes phthfplemeffth and i wonder how many pages of reading i've just added to my list... $\endgroup$
    – kim holder
    Commented Feb 22, 2015 at 20:50
  • $\begingroup$ @briligg if it helps, descriptions of spectral lines of the three filters used might be a bit easier to understand here: 211, 193 and 171. Each of them highlight different regions and activities of the Sun. On AIA, the magnetic structures you mention are best highlighted by 171 Ångstroms filter (17.1 nm). $\endgroup$
    – TildalWave
    Commented Feb 22, 2015 at 22:23
  • $\begingroup$ Iron? For those of us who have an interest without a lot of deep knowledge, I thought heavier atoms typically migrated to the core, and they'd only be present in an old star that's used up all the lighter elements. Also... aren't the areas showing up brightest in these images darkest in visible light - what we would see as sunspots? $\endgroup$
    – Anthony X
    Commented Feb 23, 2015 at 4:18
  • $\begingroup$ @AnthonyX Those are an altogether different questions that delve into the core of heliophysics and would be best asked as individual ones, perhaps on Astronomy. There's no way of addressing them in a concise manner here in the comments. But I would add that there is a lot of convective motion in photospheres of stars, including our Sun's. It's not as simple as saying that layers don't mix. Nor is there simple correlation between spectral emissions of Iron and visible light intensity (or indeed sunspots). $\endgroup$
    – TildalWave
    Commented Feb 23, 2015 at 5:01

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