An Unreported White-light Prominence

From RHESSI Wiki

Nugget
Number:	270
1st Author:	Matt Penn
2nd Author:	Hugh Hudson
Published:	28 March 2016
Next Nugget:	A meritorious index
Previous Nugget:	RHESSI's 5th Anneal
List all

Contents 1 What is a "white-light prominence? 2 SOL2005-09-07 (X17) 3 Conclusion 4 References

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although there have been anecdotal reports (for example, a paper on solar gamma-ray observations [2] described an unpublished observations by J. W. Harvey and collaborators, at the McMath telescope, of SOL1980-06-21.

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.

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 density can be of 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.

As regards the flare, an X17 event, Figure 2 shows RHESSI file information here.

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. The are not in fact "incredibly rare", though they do require exceptional circumstances.

References

[1] "Description of a Singular Appearance seen in the Sun on September 1, 1859"

[2] "Emission characteristics of three intense solar flares observed in cycle 21"

[3] "White-light flare observed at the solar limb"

[4] "Chromospheric and Coronal Observations of Solar Flares with the Helioseismic and Magnetic Imager"