An Unreported White-light Prominence

Nugget
Number:	270
1st Author:	Matt Penn
2nd Author:	Hugh Hudson
Published:	28 March 2016
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What is a "white-light prominence?

A white-light flare, as originally discovered in 1859 by Carrington and Hodgson, clearly shows us the powerful energy release at the onset of a solar flare. In many cases this "impulsive phase" produces most of the radiated energy of the flare, and it also coincides with mass elevation into the corona as the Neupert effect, and further into the heliosphere as a coronal mass ejection.

But the true white-light flare emissions come from the lower solar atmosphere as footpoint sources [1]. In rare cases, one also sees continuum emission from the low corona, and in such a case we call the phenomenon a "white-light prominence." Such a phenomenon can be observed visually as a protuberance above the limb, with a brightness sufficient to be noticeable. Until recently, there has been almost no literature on this topic, although there have been anecdotal reports (for example, a paper on solar gamma-ray observations [2] described an unpublished observations by J. W. Harvey and collaborators, at the McMath telescope, of SOL1980-06-21.

A recent description of the white-light prominence associated with SOL1998-11-22 called such a occurrence "incredibly rare." We pirate their graphics here as Figure~1, noting that it was observed by TRACE and discovered in that database by Harry Warren.

Figure 1: The TRACE observation of SOL1998-11-22 ("incredibly rare.")

One clear description of such an event appeared in the literature in 1991 [3], and the general interpretation is a fairly straightforward one: intense ablation of the chromosphere during a flare can create dense loops. The electron densities of these regions can be as high as 10¹² cm^-3. These can drive H-alpha into emission when on the disk, and shine by Thomson scattering in the visible continuum, when projected against the dark sky.

SOL2005-09-07 (X17)

The purpose of this Nugget is to document another example observed only visually, and not reported in the literature elsewhere so far as we know. This event was observed by author Penn, Eric Galayda, Aimee Norton, and Claude Plymate. None of them could find a camera lying around, amazingly enough, and so only visual impressions could be captured:

 Eric:

 It was most certainly a limb event. Started as a "am I seeing this?" moment 
 but quickly grew, bulging off-limb as it brightened. We were between setups 
 and really were kicking our bad luck that there was nothing to use to image 
 the flare. I believe even our cheap digital camera was without batteries.

 Claude:

 I was setting up the NAC for Matt and was in the early process of optical
 alignment. To check focus, I climbed up on top of the spectrograph guider
 platform. While trying to determine best focus, I noticed and odd "glint"
 projecting maybe a couple cm above the side of the solar disk imaged onto
 the table. It looked like some stray light reflecting from somewhere. I
 used a piece of paper to try and trace where the stray light was coming
 from. To my surprise, it seemed to be on axis and part of the image. I
 then noticed that the "glint" seemed to have risen a bit further above the
 limb. Perplexed, I thought about it for a moment, then yelled over at Eric
 to check the GOES X-ray monitor. The plot he pulled up showed an above
 X-class flare in progress!!! Holy $#&%!!! That's when I realized that we
 were actually seeing a corona mass ejection in white light!
 
 ...an above X-class flare in progress!!! Holy $#&%!!!

 Looking closely at the image, I could see a brightening at the foot point
 and was able to discern that the color wasn't quite white but more a pale
 purple. My guess at the time was that the purple was coming from a mix of
 both H-alpha and H-beta. About then was when Aimee walked in for her tour
 of the telescope. As you might imagine, we were all going ape and I can
 only guess her first impression of us. By that time, the CME had continued
 to move higher, had disconnected from the solar disk and was fading. I
 think the entire incident only lasted about 20 minutes.

 Aimee:

 I mostly remember Claude being very animated, literally jumping about 
 while  a dynamic feature writhed just off the limb of the Sun ... I think 
 Claude is wrong that I walked in on while it was going on.  I was there 
 from the start.  I distinctly remember him going through the phase of 
 thinking it was scattered light, placing the paper down and then starting 
 to swear.

 Matt:

 I remember [the flare] appearing blue-ish white to me, like a fluorescent 
 bulb; two bright regions maybe 30-50% brighter than the quiet Sun, maybe 
 each about 5mm across in size [at 2.4 arc sec/mm scale]. I remember 
 asking if we had any sort of digital camera around and we didn't... it was in 
 the days before we carried cell phones with cameras up there I guess.

 We left the image and continued with the tour, but a few minutes later Claude 
 called us back, saying that there was a prominence visible in white light.  
 We went back to the projected image and there was a prominence off the limb 
 of the Sun as plain as day.  It was bright, maybe 5-8 cm off the limb and 
 5-8cm long.  It was very thin, but showed structures, in particular, very 
 bright knots.  The knots seemed like point sources, and either were flickering 
 in the seeing or showed real time evolution.  I think the clouds were getting 
 worse (they were thick cumulus blowing by the summit) and we watched the 
 prominence for a few more minutes.  It stayed visible for that time but was 
 eventually covered by clouds.

There are definitely some scientific results in these commentaries, for example the idea that there might be bright points ("maybe 30-50% brighter than the quiet Sun") that might not have been resolved in other observations. Don't forget the large (1.6 m) aperture of the McMath-Pierce telescope, with a diffraction limit near 0.1 arc sec. Also the color remarks ("pale purple" and "blue-ish white, like a fluorescent bulb") call to mind the comparison of Carrington's flare with α Lyrae (Vega).

As regards other data on this fine X17 event of September 05, 2005 17:43 , Figure 2 shows RHESSI and LASCO file information here.

Figure 1: File images of the event: left, a RHESSI 25-keV image; right, the CME aftermath (a bright ray and fainter structure_ recorded by the Mauna Loa K-coronameter.

Conclusion

This event adds to our historical knowledge, albeit informal, of white-light prominences. As the Hiei event showed ([3]), though, and Ref. [4] confirmed, one can see these things if one looks for them (for example, in the data from HMI). They are not in fact "incredibly rare", though they do require exceptional circumstances.