Starship Asterisk*

An Active Prominence on the Sun

Explanation: Sometimes the Sun's surface becomes a whirlwind of activity. Pictured is a time-lapse video of the Sun's surface taken over a two hour period in early May, run both forwards and backwards. The Sun's surface was blocked out so that details over the edge could be imaged in greater detail. Hot plasma is seen swirling over the solar limb in an ongoing battle between changing magnetic fields and constant gravity. The featured prominence rises about one Earth-diameter over the Sun's surface. Energetic events like this are becoming less common as the Sun nears a minimum in its 11-year activity cycle.

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This is really cool!

When I watch a prominence form and flow in a video such as this, it looks a lot like fluid material is making up exactly the portion that we see. In still photographs I used to see, I had no reason to doubt this: a vast arc of bright orange looked like some of the gas at the Sun's surface was following a bridged path, having jumped out of the Sun and arced outward until gravity pulled it back down. Prominences often had roughly the parabolic shapes of a ballistic trajectory.

I now think this is a poor description of the overall movement, which is more like a rope that has been "thrown away" from the surface ()perhaps there is a better term). Also, as I watch videos of prominences, it appears that we are only seeing part of the material flow visibly. I conclude that some of the material ejected with a prominence is unseen (to my eyes), but it may show itself as the display progresses, changing from unseen to light-emitting. Clearly, some of that material is giving off radiation/light above the amount of radiation from the corona. Whereas some things I have read talk about photons being generated deep in the Sun and working their way out to the surface, clearly the material itself in a prominence is giving off light, and some parts of it are much brighter than others.

What makes some material in a prominence visible to us? And within that, what causes some parts of a visible prominence to radiate a lot more light than the rest? Is this hydrogen gas giving off blackbody radiation? Are the brighter parts being energized from the magnetic field? Is there a bit of fusion happening way out in the middle of a prominence? Or are the visibly brighter parts actually only bright in the visible part of the spectrum -- perhaps they are less energetic than other parts of the prominence, which might be radiating at higher frequencies than I can see.

MarkBour wrote: ↑Mon Jun 18, 2018 4:00 pm When I watch a prominence form and flow in a video such as this, it looks a lot like fluid material is making up exactly the portion that we see.

Of course, this is true, as we are observing a plasma moving under the influence of various forces, and plasma is a fluid.

Clearly, some of that material is giving off radiation/light above the amount of radiation from the corona.

Well, in visible light the corona is a continuum. We are seeing it as scattered light from the Sun. (It also emits light, but almost all of it is in the x-ray and extreme UV spectrum.) The corona is brighter than the prominences, but since we're only observing a very narrow wavelength, that blocks most of the coronal light and greatly increases the contrast of the nearly monochromatic prominence.

What makes some material in a prominence visible to us? And within that, what causes some parts of a visible prominence to radiate a lot more light than the rest? Is this hydrogen gas giving off blackbody radiation?

The prominence is made up almost entirely of hydrogen. It is dimmer than the Sun's surface, and in order to see it we must use a very narrow filter that isolates just the Ha emission line. This is not blackbody radiation. The brightness depends on several things- the temperature, the density, and how well matched the Ha emission is to the filter. With a narrow enough filter, the movement of the plasma can Doppler shift the emission outside the filter range, making that gas appear dim. But mostly what I think we're seeing is hotter and cooler regions and denser and more rarified regions.

Is there a bit of fusion happening way out in the middle of a prominence?

No, the temperature is far too low.

Thanks for the help, Chris!

I appears to almost become a "swirl" rather than an "Arc"... almost a wide, low spinning vortex... is what it "appears" to do... very interesting prominence.

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Boomer12k wrote: ↑Mon Jun 18, 2018 9:15 pm I appears to almost become a "swirl" rather than an "Arc"... almost a wide, low spinning vortex... is what it "appears" to do... very interesting prominence.

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True. My earlier question ranged over what I was wondering about from all the prominence pictures I have seen, rather than what was really being shown in today's APOD video. I agree with your description of this one, and it is rather different than my usual idea of a prominence.