Podcaster: Cosmic
Title: The Apogee Podcast – Solar Chromospheric Umbral Fibrils
Links: astroandmusic.blogspot.com
Article: http://arxiv.org/abs/1412.6100
HelioViewer: http://helioviewer.org/
Description: In this Apogee Podcast, Cosmic discusses observations of some newly discovered and sofar enigmatic structures within sunspots.
Bio: Cosmic is a self- and crowd-funded independent research astronomer and space musician.
Today’s sponsor: This episode of “365 Days of Astronomy” is sponsored by — no one. We still need sponsors for many days in 2015, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.
Transcript:
Hello! This is Cosmic, and welcome to the Apogee podcast! The format of this podcast is myself describing, discussing, and critiquing an article of my choice from an astronomical journal. Sometimes topics within these articles send me off on temporary tangents, but they’re always relevant to the overall topic. These podcasts will take place at or near the date of the apogee which is when, along its orbit around the Earth, the Moon is furthest away.
If you have any articles to suggest for future podcasts, I would be happy to take a look at them. I can be reached at cosmiclettuce AT gmail DOT com.
The music you hear in the background are my own compositions. These and many others can be downloaded from my soundcloud channel ‘cosmiclettuce’. I hope you enjoy listening to space music as much as I do.
The apogee for this podcast will take place today, 09 January 2015, at 18:18 UTC. The lunar distance at that time will be 405410 km, which is 827 km further away than last apogee on 12 December, and 744 km closer than the next apogee on 6 February 2015.
Ok, on with the paper!
This apogee, I return to looking at the sun in a paper entitled “Stable Umbral Chromospheric Structures” by solar astronomers at Queen’s University in Belfast, Trinity College in Dublin, and Stockholm University in Sweden.
The first thing I need to do is break down the title of this paper a little bit. The chromosphere is a layer in the atmosphere of the sun just above the photosphere and below the transition region and corona. If you’ve ever looked at the sun through a hydrogen-alpha filter, you were looking at the chromosphere. Temperatures range from 6,000 C at the bottom of the chromosphere to about 20,000 C at the top. Sunspots reside primarily in the sun’s photosphere, but their structures extend up into the chromosphere.
Sunspots have two primary parts: the umbra and the penumbra. The umbra is the dark, cooler, homogeneous-looking area in the centers of sunspots. The penumbra is the area usually surrounding the darker umbra and characterized by a well organized radial distribution of, somewhat randomly alternating, bright and dark filaments.
It has always been thought that the strong magnetic fields within the umbra of sunspots are oriented vertically. When looking at material tracing these magnetic fields, we see this material falling onto the sunspots umbra normal to the surface of the sun (that is, moving straight down). But recent observations with very high resolution solar telescopes has revealed that it’s much more complicated than that. No surprise there.
The authors of this paper used the Swedish 1m Solar Telescope (the second largest refracting telescope in the world, located at La Palma observatory in the Canary Islands) to observe three different sunspots at three different wavelengths: a 10 angstrom filter centered on a wavelength of 3953.7 angstroms, a narrow 1.1 angstrom interference filter centered on the Calcium-II H line core at 3968.4 angstroms, and hydrogen-alpha at 6562.8 angstroms. This solar telescope also employs an adaptive optics system which greatly increases the spacial resolution of the images to 25-52 km per pixel by partially compensating for the blurring effects of the earth’s atmosphere.
The structures observed and reported on in this paper are what the authors call “Umbral Ca II H/K Fibrils”, or UCFs. UCFs are difficult to observe for two reasons:
1. they reside primarily within the dark umbra of the sunspot
2. they are narrow, pushing the limits of visibility even with the help of adaptive optics
Back in 1969, solar astronomers observed “umbral flashes” for the first time and noticed that they happened every few minutes. A flash will take place somewhere within the umbra, and then rapidly spread and fade out in a ring dispersing its energy to a larger area. Although umbral flashes have been observed and studied for 45 years, my review of the published papers (primarily focused on observations) shows that the nature and cause of the flashes is still a mystery. A great way to watch umbral flashes, and the sun in general, is with an online viewer called HelioViewer. I’ll include a link to this website in the podcast notes.
The cool thing is that umbral flashes can be used just like a camera flash is used to illuminate things in a dark room.
Are these UCFs being created by the complex shock fronts of the umbral flashes, or are they more stable structures mearly being illuminated by the flashes? The structures seem to last an unusually long period of time — tens of minutes and often over multiple flashes. Careful examination of the UCFs within the three sunspots studied in this paper resulted in the authors concluding the latter. Quoting the paper: “the probability that two very different flashes, minutes apart, would randomly produce visually horizontal structures with the degree of similarity shown in this paper, and at the same locations, in a layer of the atmosphere that would otherwise be nearly homogeneous and vertical, is indeed very unlikely.” You can see these stable UCFs, many so large as to extend into the penumbra, in Figure 3 of the paper.
This careful examination led to another surprise: the material in these structures sometimes traced magnetic field inclinations far from vertical in an area of the sunspot where only strong vertical fields were expected. The authors don’t offer an explanation, but they do make some excellent measurements showing that the angle of the magnetic field is steeper the deeper into the umbra you go. They suggest that more observations at different viewing angles (like when the sunspot is more towards the limb of the sun) could provide better constraints on the inclinations of these UCFs.
They also observed that many of the UCFs are made of two bright components and a central dark structure. They theorize, in agreement with other authors, that these components may be up-flows and down-flows of material. Taking high- resolution spectra of these UCFs and measuring their doppler velocities would be a method to confirm or deny this theory.
The authors conclude by asserting that UCFs are common in sunspots as long as there is sufficient spacial resolution to see them. The origin and nature of UCFs is unknown, but both large and small UCFs (the latter appearing small probably because of their more vertical orientation) have been observed. More observations looking specifically at UCFs are needed.
I had to chuckle at the end of the paper when it was stated that a larger telescope would be needed to observe these very faint UCFs especially when umbral flashes weren’t present to illuminate them. They need a larger telescope to observe by far the brightest object in the sky???? There’s a obvious explanation for this. These observations were made with very narrow band filters. Even though the sun is very very bright, very little light gets through these filters. A larger telescope will do two things: first, it will increase the spacial resolution of the images (especially while using adaptive optics) and the images will appear sharper. Second, telescopes are “light buckets” and the larger the bucket the more light you can collect and hence you can see fainter objects.
I think it would be worthwhile trying to observe the umbra and UCFs during and right after a flare event. Usually a lot of material is thrown up and it’d be a way to trace the magnetic fields down into the umbra and to see how these UCFs are effected by that material.
So until next apogee … I bid you Peace.
End of podcast:
365 Days of Astronomy
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