X-rays and flares

Solar flares are the biggest explosions in our solar system. We see a sudden and huge increase in brightness on the Sun. The increase isn't just in white light, but in the ultraviolet, X-rays, gamma-rays, and radio waves as well.

Have a look at the image above - it's an X-ray picture of the Sun in reverse colour. Normally when we look at images of the Sun then we colour those bits of the Sun with more X-rays brighter than those with less X-rays. Sometimes, though, it is easier to see what is going on on the Sun if you colour those bits of the Sun with more X-rays darker than those with less X-rays (which, consequently look brighter). When we do this then we say the image is in reverse colour. Can you see a part which has lots of X-rays (remember this is a reverse colour image of the Sun)? Image copyright Yohkoh Data Archive Centre, MSSL, Department Space and Climate Physics, UCL.

Look at the black arch-shaped blob on the left hand side of the Sun - this is a solar flare, a huge explosion on the Sun. Remember the Earth's diameter is only around one hundreth of the Sun's diameter, so solar flares are generally much larger that the entire Earth!

We study the Sun in X-ray light with a Japanese satellite called YOHKOH, which means Sunbeam. It's easier for us to study flares in X-rays than it is in white-light because the increase in X-ray emission is much greater than in visible light for most flares. At the moment we usually only see an increase in visible light for really big flares, but this might change as our instruments get more sensitive.

The other reason that we show X-ray images of flares rather than white-light is that at the moment most white-light observations are from ground based telescopes and so our observations are by no means continuous!

When a flare goes off, the temperature can get up to and even above 10 million degrees.

	Here is a plot of X-ray brightness for two flares observed by a satellite called GOES (Geosynchronous Operational Environmental Satellite). Each of the horizontal lines indicates a level of X-rays 10 times as high the line directly beneath it. Therefore the scale on the y-axis shows us by how much the brightness has increased by factors of 10. We use letters to tell us how big the flare is. This flare was an M-class flare because its brightness went past the M level. Really big ones are X-class flares.

The first solar flare was seen in 1851, when astronomers were looking at sunspots in visible light. Of course this wasn't the first flare on the Sun, just the first one that we'd been able to see! Two astronomers, Carringtion and Hodgeson, happened to be observing the Sun independently at the time - they were lucky that the flare they saw was a large one and clearly observable in visible light!