Electrically charged gas is a special form of matter called a plasma. This roiling, boiling plasma generates the Sun's powerful magnetic field. Like Earth's magnetic field, the Sun's magnetic field has a north pole and a south pole. On the Sun, however, the magnetic fields are much messier and more disorganized than on Earth. About every 11 years, the Sun's magnetic field does a flip. In other words, the north pole becomes the south pole, and vice versa.
This flip is one aspect of the roughly year activity cycle the Sun experiences as its magnetic field evolves slowly over time. As the cycle progresses, the Sun's stormy behavior builds to a maximum, and that's when the magnetic field reverses. Then the Sun settles back down to a minimum, only to start another cycle. Evolution of the Sun in extreme ultraviolet light from through , as seen from the telescope aboard Europe's PROBA2 spacecraft.
Sunspots are areas of particularly strong magnetic forces on the Sun's surface. They appear darker than their surroundings because they are cooler. Even so, scientists have discovered that when there are lots of sunspots, the Sun is actually putting out MORE energy than when there are fewer sunspots. During solar maximum, there are the most sunspots, and during solar minimum, the fewest. Through special filters, sunspots may look like the picture on the left.
The sunspot groups are as big as the giant planet Jupiter! On the right is a closeup of some other sunspots. The larger sunspot on the right is bigger than Earth! In the second or impulsive stage, protons and electrons are accelerated to energies exceeding 1 MeV. During the impulsive stage, radio waves, hard x-rays, and gamma rays are emitted.
The gradual build up and decay of soft x-rays can be detected in the third, decay stage. The duration of these stages can be as short as a few seconds or as long as an hour. Solar flares extend out to the layer of the Sun called the corona. The corona is the outermost atmosphere of the Sun, consisting of highly rarefied gas. This gas normally has a temperature of a few million degrees Kelvin.
Inside a flare, the temperature typically reaches 10 or 20 million degrees Kelvin, and can be as high as million degrees Kelvin. The corona is visible in soft x-rays, as in the above image. Notice that the corona is not uniformly bright, but is concentrated around the solar equator in loop-shaped features. These bright loops are located within and connect areas of strong magnetic field called active regions. Sunspots are located within these active regions. Solar flares occur in active regions.
The frequency of flares coincides with the Sun's eleven year cycle. They expand in size as they propagate away from the Sun and larger CMEs can reach a size comprising nearly a quarter of the space between Earth and the Sun by the time it reaches our planet. This can result in the sudden release of electromagnetic energy in the form of a solar flare; which typically accompanies the explosive acceleration of plasma away from the Sun — the CME.
These types of CMEs usually take place from areas of the Sun with localized fields of strong and stressed magnetic flux; such as active regions associated with sunspot groups. CMEs can also occur from locations where relatively cool and denser plasma is trapped and suspended by magnetic flux extending up to the inner corona - filaments and prominences. When these flux ropes reconfigure, the denser filament or prominence can collapse back to the solar surface and be quietly reabsorbed, or a CME may result.
CMEs travelling faster than the background solar wind speed can generate a shock wave. These shock waves can accelerate charged particles ahead of them — causing increased radiation storm potential or intensity. Important CME parameters used in analysis are size, speed, and direction.
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