Prominences and Filaments (photo of a prominence by David Evans)
by Jack Carlyle
Did you know that the Sun emits a lot of ultraviolet light, UV, as well as the visible light which we can see? Although our eyes can’t see UV radiation, you can make a camera that can “see” in UV light. If you use a normal camera to take a photo of the Sun (don’t ever look directly at the Sun btw, duh!) then it doesn’t look all that exciting – pretty much a smooth ball, with maybe a spot or two. But if you point one of these UV cameras at the Sun, things look a lot more exciting. While we’re at it, why not put one of these cameras in space? And if we’re launching a satellite, we may as well put on, like, ten cameras, all taking pictures at different UV wavelengths, in super high resolution (detail). Oh wait, NASA already did that, with a little bit of help from the Rutherford Appleton Laboratory, UK.
NASA’s Solar Dynamics Observatory, SDO, was launched in February 2010 and snaps a picture with all ten cameras every twelve seconds, orbiting the Earth in such a way that it almost always has a clear view. There are some incredible YouTube videos people have made with some of the images captured. You can look through them yourself on helioviewer.org – even ones that were taken earlier today. You can even make your own movies with the Helioviewer. I check it every morning (and I’m not that much of a nerd, I swear).
NASA SDO/AIA movie at 17.1nm
Anyway, when we look at these images, we can see a lot of detail on the Sun. There are lots of big bright loops poking out everywhere. If you look closely at the edge of the disc of the Sun, you might be able to see some bright features sticking out – these are called prominences.
If we look on the disc, we can see bright patches, called active regions. Sometimes we can see darker snake-like shapes running through these active regions, and we call these filaments. Sometimes you can see filaments in the quieter parts of the Sun, too.
Prominences, seen as bright emission on the solar limb, appear dark (and are called filaments!) against the solar disk.
Here is an image from Hinode/SOT in H-alpha emission at 656.3nm.
The dark filament seen here on the solar disk stretches across a huge distance. These images are pretty spectacular – and they can teach us a lot, too. We can’t see any other star in nearly this much detail, so we’ve got a lot we can learn. Prominences are ‘Cool’, see Sun|trek
To understand what’s going on, we have to understand what the Sun actually IS.
The Sun is a giant ball of plasma, a state of matter that’s a bit like gas, except this stuff is so hot it has become ionised. This means the electrons have separated from the atoms and the fluid can conduct electricity – which means the plasma “sticks” to magnetic fields. To put it properly, we say that the plasma is unable to move independently of the magnetic field lines, but this just means that the plasma can only flow up and down these (imaginary) lines, like water in a straw.
In places where the plasma is very dense, such as inside the Sun, the plasma motions will drag the magnetic field with it. Since some parts of the Sun rotate faster than others, the magnetic fields become a huge tangle, being “wound up” by this differential rotation. This results in magnetic field lines “popping out” of the surface into the solar atmosphere – a region where the plasma is much less dense, and so here the magnetic fields dominate the motions; whenever a field line moves from magnetic tension or reconfiguring, then any plasma on this line will move with it.
Here are some diagrams of how we think the Sun’s magnetic field behaves. Complicated, eh?
As these twisted bundles of magnetic field lines, or ‘flux ropes’ emerge, they lift cooler, denser material from the lowest part of the Sun’s atmosphere (the chromosphere), and this material is suspended higher in the atmosphere; the plasma here is very thin, or ‘diffuse’, but very hot (the corona). This means that it only emits a very weak light compared to the more dense plasma. So if a flux rope carrying some dense plasma occurs on the edge of the Sun (from our point-of-view, of course) we can see the dense plasma glowing, as the higher density means more light is emitted than by the surrounding corona. We call these prominences.
However, if this occurs on the front of the Sun as we see it, then this plasma suspended by the flux rope is slightly cooler than the background; this means it emits much less light and, if we look at the right wavelengths of light, we can see them as silhouettes. We call these filaments, but they’re exactly the same thing as prominences. The problem is that we’ve seen them for quite a bit longer than we’ve been able to explain them; when people first saw them, they thought that prominences and filaments were two completely different things. To be fair they do look pretty different, I guess…
Below is a computer simulation (from MacKay, 2010) of what we think goes on. This shows the complex magnetic field structures.
Go on, get onto Helioviewer and let me know (via iSun|trek ‘Contact us’) if you find any decent prominences or filaments.
Tell us a bit about yourself, Jack
Hello! I’m Jack and I’m a PhD student, studying solar physics. I grew up in London, and decided I quite liked it there, so I stayed to study astrophysics at University College London, UCL. I now work at the Mullard Space Science Laboratory, a department of UCL in Surrey, and also at the Max Planck Institute for Solar System Research in Germany. I study Coronal Mass Ejections, CME’s and the solar atmosphere.
How did you get interested in Astronomy/Solar Physics?
I first got interested in space when I was about 8 years old. I had a teacher who was really into astronomy, and used to bring in a small model planetarium and a telescope for viewing comets and the like. I absolutely loved this and the passion has stayed with me since. As I got older, I kept being captivated by spectacular astronomical pictures, like the ones at http://apod.nasa.gov/ , and so I decided that this was what I wanted to build a career in.
What else do you enjoy doing?
I like being creative in my free time; I play any musical instrument I can get my hands on and am always using my laptop to record weird songs. I’m also into taking photographs and making little books, and I’m a pretty avid YouTube-er. I like walking through forests and over hills during the day and through cities and over bridges at night. I also love Adventure Time, tea and travel!
What is the best astronomy/solar experience you’ve had?
Difficult choice, I’ve done so many amazing things…! Like in my third year of university, I was lucky enough to be able to take a trip to an observatory in the south of France. It was really far away from anything else and very high up, so the view of the stars at night was the most spectacular thing I’ve ever seen. We got to spend the week using the enormous telescopes, staying up all night every night. It was a lot of fun – and technically, it was work!