The Sun produces a steady stream of electromagnetic radiation from the photosphere, essentially unchanging with time. Superimposed on this steady, quiet process are several dynamic, active, or changing events and cycles that show the Sun also has some unpredictable and explosive behavioral traits. These features are not significant in terms of total solar energy output but do influence the electromagnetic radiation received on Earth. They include sunspots, solar flares, magnetic storms, the solar cycle, and changes in the solar corona.
The surface of the Sun is covered by a number of dark spots, typically about the size of Earth (6,000 miles or 10,000 km across), that appear, disappear, and change in size and shape over periods of a day to 100 days. In detail they show a change in color from darkest in the center (called the umbra) to an intermediate color around the edges (penumbra) to merge with the photosphere on their edges. The colors correspond to changes in temperature, with the darkest regions representing a drop in temperature from
6,000 K (10,340°F; 5,727°C) on the photosphere, to 5,500 K (9,440°F; 5,227°C) in the penumbra, to 4,500 (7,640°F; 4,227°C) in the umbra.
Sunspots are intricately related to the magnetic field of the Sun. The magnetic field strength is measured to be about 1,000 times stronger in the sun-spots than elsewhere on the photosphere. The strong magnetic field in these regions can block the normal convective flow from rising in these spots, explaining why they are cooler than their surroundings.
Sunspots typically occur in pairs, and the magnetic field has opposite polarity (positive or negative) in adjacent spots, with magnetic field lines looping out of one spot high into the solar atmosphere and into the other, forming a high-reaching arc in between. Another interesting observation is that at any time, all of the loops between sunspots have the same configuration in each hemisphere with respect to the rotation of the Sun. If the field lines loop from west to east in one pair, then all the other pairs in that hemisphere orient in the same direction, and all the pairs in the opposite hemisphere orient in the opposite direction. This phenomenon results from the differential rotation of the Sun. The different speeds of rotation between the poles and the equator cause the N-S magnetic axes to be distorted and sometimes reoriented to E-W, and the convection cells then distort these lines further forming the loops. These loops are occasionally so attenuated that they become shaped like narrow tubes which distort the convection cells and become manifested as sunspots.
The numbers and distributions of sunspots change in a regular fashion, a phenomenon known as the sunspot cycle. More than a century of observation shows that the number of spots peaks at about 100 per year every 11 years, and then decreases to about zero in between peaks. There is some variation in the cycle from 7-15 years, but it is a well-established cycle. Sunspots do not move in position, but during the course of a cycle, older spots at high latitudes are gradually replaced by more spots in more equatorial regions as the cycle reaches a climax every 11 years, with most spots appearing 15-20° from the equator. The solar minimum is when the spots are fewest in number, and the solar maximum is when most spots are observed, especially in equatorial regions. As the solar maximum grades into the next minimum, the highest latitude spots tend to disappear first, leaving the last few spots near the equator. As the next maximum approaches, the new spots will appear in high latitudes.
The sunspot cycle is complicated further by the fact that the entire magnetic field of the Sun reverses polarity every 11 years as part of a full 22-year cycle, so that the leading spots in both hemispheres switch from positive to negative, and negative to positive with each successive 11-year sunspot cycle. This cyclicity allows for a fairly regular interaction of the magnetic field with the convection in the outer layers of the Sun, in a manner similar to the generation of the Earth's geodynamo and magnetic field. The present 11-year cycle has not always been so regular, for instance, from the mid-1600s to the early 1700s, there were very few sunspots. This reflects the complex dynamics of the Sun's magnetic field and interaction with the convection system.
Although sunspots are dark, cool, and relatively inactive areas, they are sometimes surrounded by active regions that emit huge quantities of energetic particles into the surrounding corona. These active regions also follow the solar cycle and, like the sun-spots, are most abundant and active during the solar maximum.
Solar prominences are giant loops or sheets of glowing ionized gas that erupt from the photosphere and move through the lower corona under the influence of the magnetic field, following the loops between two sunspots. They may be caused by magnetic instabilities near the sunspots where the magnetic field lines are extremely concentrated and unstable. At times their height reaches about half the solar diameter. Some prominences last for days or weeks in a fairly stable configuration, whereas others surge and disappear in a matter of hours.
Active regions also release highly energetic flares that erupt and move around on the surface near the active regions, releasing huge amounts of energy, especially in the X-ray and ultraviolet wavelengths. Temperatures in the cores of flares can exceed 10,000,000 K (17,999,540°F; 9,999,726°C), and the amount of energy released in these flares in a few minutes can exceed that from the larger prominences over a period of weeks. The particles released in the flares are so energetic that they escape the Sun's gravity and are blasted into space where they disrupt communications and other electronics.
The sunspot cycle also affects the solar corona, which is smooth and uniform during solar minimums, and irregular, much larger, and contains many beams or streamers moving away from the Sun at solar maximums. The corona may be heated mostly by flares and prominences; during periods of more activity the corona becomes more active as well, and the solar wind increases.
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