Explosions: Solar Flares and
Coronal Mass Ejections

What is a flare?

Solar flares are the biggest explosions in the solar system. We say that we've seen a solar flare when we see a huge and sudden increase in brightness on the Sun. The increase in brightness isn't just seen in visible light (in fact it often isn't!) but in lots of other parts of the electromagnetic spectrum as well - in X-rays, gamma-rays, UV and radio waves.

Flares happen in active regions. These are parts of the Sun where the magnetic field is particularly strong and complicated and because of this there is a lot of magnetic energy associated with these regions. When the energy that has built up in the active region is released we see a flare.

This is what a flare looks like when we look at it in X-rays with the Yohkoh satellite. The image of the whole Sun on the left is a soft X-ray image taken with the Soft X-ray Telescope (SXT) on Yohkoh. We can see the bright flare on the West limb of the Sun - East and West are swapped on the Sun because we're looking at it rather than actually being on the Sun! The image on the right is a close up of the flare loop that we see in soft X-rays. The colours are showing us where the hottest parts are, so the hottest bit is the bright yellow at the top of the loop. The blue lines are a sketch of how we think the magnetic field lines look and where we think the energy is being released. The white contours (circles) are telling us that there is more energetic emission than the soft X-rays happening as well-we call this hard X-ray emission. This particular image came from the GSFC solar flare theory pages

The first solar flare was seen in 1851 when Carrington and Hodgson were independently monitoring sunspots. This one actually was seen in visible light and they saw a sudden brightening. Of course this wasn't the first flare the Sun had actually produced, just the first one that we'd ever been able to see!

How much energy do they produce?

The amount of energy that's released in a typical flare is about the same as 100 megaton bombs exploding all at once. That's about 10 million times more than the energy released by a volcanic explosion! But compared to the amount of total energy the Sun emits every second it's pretty small- only about 1/10.

How long do they last?

Most flares are quite short really, less than hour. The longest flare that we've seen with the Japanese Yohkoh satellite was 12 hours though. Compared to flares on other stars though the Sun is a bit of a wimp - some of those flares are a thousand times more energetic than the Sun and can last up to 10 days! 

How often do they happen?

The number of flares that we see is related to the Solar Cycle. When we're near the peak of the 11 year cycle then there are lots of active regions on the Sun and there are lots flares - multiple ones a day around the maximum. When the Sun's quiet, near the minimum of the cycle then there are much fewer flares. Sometimes there are periods when there are no active regions at all visible.

How do they affect us?

The energy that's released by a flare heats up and accelerates charged particles i.e. electrons, protons and nuclei. These energetic particles produce the X-ray, gamma-ray and radio emission that we see during the flare but some of them can also escape into interplanetary space where they can affect astronauts and electronic instruments in space. For spacecraft in low Earth orbit the Earth's magnetic field provides some protection, as does the body of the spacecraft, but outside the protective influence of the Earth's magnetic field the risks are much greater. Even in low Earth orbit astronauts can still be exposed to large radiation doses though. This is most likely in polar orbits becuase it's easiest for energetic particles to penetrate the Earth's magnetosphere here. That's also why we see the spectacular aurora borealis (northern lights) though so there is a plus side!

The other effect that flares have on us is that the UV radiation from the flare can heat up the upper atmosphere making it expand. When this happens there's an extra drag force on spacecraft which makes them lose height. The spaceraft thrusters would then need to be fired to get it back into the correct orbit. 
 

What is a Coronal Mass Ejection?

Coronal mass ejections (CMEs) are huge bubbles of the solar atmosphere that are thrown out into interplanetary space. We see them by using instruments which occult the photosphere, in effect they are creating an artificial solar eclipse. When this is done the corona can be seen and the density variations of the corona are mapped out by radiation which is scattered off coronal electrons. The corona appears brighter where there is more coronal material along the observers line of sight. A time series of images shows the coronal mass ejection to be a moving bubble of bright material.

The image on the left is taken with a coronagraph called C2 onboard the SOHO spacecraft and shows a bubble of plasma ejection off the Sun's east limb. The movie on the right is taken with C3 a coronagraph with a larger field of view and shows two CMEs. The first is a bubble ejected form the north pole of the Sun and the second a 'halo' CME which can bee seen surrounding the occulting disc and increasing in size as it moves away from the Sun.

CMEs can happen anywhere on the Sun not just in active regions where the magnetic field is concentrated. Their occurence is still thought to be linked to the solar magnetic field though and it's long term evolution. The Sun's magnetic field is generated via a dynamo action deep in the interior and the magnetic field lines then rise and emerge through the photosphere. This magnetic flux, which is constantly being injected into the solar atmosphere, needs to be removed and CMEs are the process by which this happens.

How much energy do they produce?

The energy of of a CME is mostly kinetic, that is due to it's motion. The amount is comparable to the energy released during a large flare.

How often do they happen?

The number of CMEs rises and falls with the solar cycle. About 1 CME occurs every 3 days at solar minimum and up to about 3 CMEs a day at solar maximum! They can be launched at any location on the Sun but during solar minimum they are most commonly produced at the equator. As solar activity increases they become more common at higher latitudes indicating the changing of the large scale magnetic field with solar cycle.

How do they affect us?

CMEs carry a huge amount of solar plasma threaded with magnetic field lines and can effect the Earth environment in a variety of ways. The events which are Earth directed, and appear as halos of bright material around the Sun, are the most likely to be detected at Earth. Luckily the Earth has a protective magnetic field around it, like that of a bar magnet, which protects us from most of the material the Sun throws at us. The magnetic field in the CMEs can have any orientation but it is those with southward directed fields that effect Earth the most. When this happens the magnetic field of the plasma and the Earth's magnetic field are oppositely aligned and a process called reconnection can occur allowing the solar plasma into the Earth's normally protective environment. As the name reconnection implies the connectivity of the magnetic field lines are changed and lines originally only connected to the Sun join those that are connected to the Earth. For Earth effects of CMEs see the Why study the Sun section.