Hi, my name is Lyndsay.

I want to tell you about solar loops and show you the latest images of the atmosphere of the Sun (the corona), made with a telescope called TRACE.

What is TRACE?

TRACE is the Transition Region And Coronal Explorer. It is a satellite carrying a telescope designed to look at the atmosphere of the Sun in great detail and to trace the magnetic field. It was designed, built, launched and is now operated by a group of scientists in the US, including some at the Lockheed Martin Solar and Astrophysics Laboratory in California, where I now work.

TRACE looks at the atmosphere of the sun... you might think that this is a rather dull place, just a fuzzy shell of hot gas, but no! Take a look at what TRACE has to show us......
 

The Tangled Sun

This image is the Sun as it looks in the extreme ultraviolet emission, produced by plasma at one million degrees Centigrade - click on it for a bigger version. What you'll see is a tangle of loops, as well as ribbons, bright dots; streamers and spikes. These bright structures are part of the corona Note, the sun isn't really blue! This is a false colour image.

But the visible surface of the sun looks relatively smooth, just a few sunspots here and there. Where did the loops come from?

What are coronal loops?

The answer is - loops are part of the sun's magnetic field which have popped up through the surface in a most unruly manner! The magnetic field guides gas in the corona into the strangest shapes! An example of how some materials respond to magnetic fields is the pattern of iron filings around a bar magnet. The image on the right (courtesy MPS Inc.) shows iron filings around a bar magnet. The filings line up into loops going from the north pole to the south pole of the magnet. The same happens with gas in loops on the sun.

Solar loops always run between north magnetic field and south magnetic field, often joining strong sunspot pairs. It is useful to think of the field in a single solar loop as being like that from an imaginary bar magnet underneath the surface. The magnetic field traps material, which can only move along the loop, not across it. So the loop has a sharply defined edge.
(graphic courtesy of Lee Slone/YPOP)

Zooming in on loops with TRACE

Several satellites have seen and helped us to understand solar loops. But TRACE  gives us the most detailed view yet of these pipes through the solar atmosphere! TRACE's powerful telescope enables us to see loops that are `only' 1,000 km across. OK, so that's pretty big - the distance between Aberdeen and London - but considering that the sun is 150,000,000 km away, it's not so bad!  Seeing these loops is equivalent to pointing a telescope on Earth at a lawn 1km away and seeing individual blades of grass!

Here are some amazing pictures of solar loops made by TRACE. The loops in these images are typically about 100,000 km long, that's about 8 times the diameter of the Earth! Click on the images below to get bigger versions.

Why do we see loops?

....because the magnetic field traps gas, and it is heated to temperatures of one million degrees, and more! At these high temperatures the loops shine brightly in extreme Ultraviolet or X-ray radiation. The instrument on TRACE is sensitive to this radiation. Sometimes all the loops light up in an arcade, but sometimes only one or two do, as in this picture, and we don't know why!!
(graphic courtesy of Lee Slone/YPOP)
 

Not all loops are at the same temperature, and neither are single loops all at one temperature. They tend to become hotter at the top. This image shows an overlay of two images of a group of loops. The blue areas show loops as they are seen by the TRACE telescope, which is sensitive to emission from gas at about one million degrees. The yellow shows the view that is seen by the Soft X-ray Telescope on Yohkoh - this is gas at three to five million degrees.

Hot and Cold running plasma?

So is anything actually flowing along these magnetic pipes? The answer is, yes, and there are a couple of ways to see it. Sometimes, one can actually see bright material moving along loops, as seen in this movie (7 Mb) made by the TRACE satellite.

But not all the gas in loops is hot. In this image of loops at the edge of the sun, there is also dark material present, curving high into the atmosphere and held there by magnetic forces, thousands of kilometres above the solar surface. This material is cooler than the bright gas around it, and is called a prominence. In a movie of this region, we can see the cool material being hurled back and forth along invisible field lines.

Another way to see moving gas in loops is using the Doppler effect. Like the rise and fall in pitch, or frequency, of a the sound from siren as it moves towards and then away from you, the frequency of the radiation from a moving blob of gas also rises and falls. This effect has been used to show that hot gas is moving along loops at a speed of up to 150,000 kilometres per hour!

So we know that that material is flowing along the loops - but we're not sure yet what turns the tap on!

But not only does material flow along the pipes, the pipes themselves are constantly moving.

Slowly going loopy...

The number of loops on the sun, like the number of sunspots, varies over the 22-year solar activity cycle.  Between solar maximum and solar minimum the number of loops slowly decreases. This picture, made with the Soft X-ray Telescope on the Japanese Satellite Yohkoh shows how the number and the size of loops (the bright areas) changes with the years.  The images were made every 120 days, between 1991 (near maximum, on the left) and 1995 (near minimum, on the right).

The solar coronal magnetic field can be very complicated.  When the sun is `quiet' it has quite a simple field with a north and south magnetic pole, like the Earth. At that time it has very few loops, and the ones it does have tend to be very small. But when the sun is `active', many large loops of magnetic field are pushed up from below,  into belts of `active regions'. These loops are constantly changing, because the field that we see in the corona is linked to strong fields running under the surface. These sub-surface fields are pushed and pulled about by forces in the turbulent interior of the sun, rather like spaghetti in a pot of boiling water.