Sunday, October 31, 2010

Solar flares

Most frequently at the peak of the sunspot cycle, violent eruptions of gas are ejected from the sun’s surface. The prominences and flares may rise to some 60,000 miles (100,000 km) and may be visible for weeks. Solar flares are more sudden and violent events than prominences. While they are thought to also be the result of magnetic kinks, they do not show the arcing or looping pattern characteristic of prominences. Flares are explosions of incredible power, bringing local temperatures to 100,000,000 K. Whereas prominences release their energy over days or weeks, flares explode in a flash of energy release that lasts a matter of minutes or, perhaps, hours.

Understanding Sunspot Cycles

Long before the magnetic nature of sunspots was perceived, astronomer Heinrich Schwabe, in 1843, announced his discovery of a solar cycle, in which the number of spots seen on the sun reaches a maximum about every 11 years (on average). In 1922, the British astronomer Annie Russel Maunder charted the latitude drift of sunspots during each solar cycle. She found that each cycle begins with the appearance of small spots in the middle latitudes of the sun, followed by spots appearing progressively closer to the solar equator until the cycle reaches its maximum level of activity. After this point, the number of spots begins to decline. The most recent maximum occured in early 2001.
Actually, the 11-year period is only half of a 22-year cycle that is more fundamental. Recall that the leading spots on one hemisphere exhibit the same polarity; that is, they are all either north magnetic poles or south (and the followers are the opposite of the leaders). At the end of the first 11 years of the cycle, polarities reverse. That is, if the leaders had north poles in the southern hemisphere, they become, as the second half of the cycle begins, south poles.
The cyclical nature of sunspot activity is very real, but not exact and inevitable. Studying historical data, Maunder discovered that the cycle had been apparently dormant from 1645 to 1715. At present, there is no explanation for this dormancy and other variations in the solar cycle.

Sunspots: What They Are

Sunspots are irregularly shaped dark areas on the face of the sun. They look dark because they are cooler than the surrounding material. The strong local magnetic fields push away some of the hot ionized material rising from lower in the photosphere. A sunspot is not uniformly dark. Its center, called the umbra, is darkest and is surrounded by a lighter penumbra. If you think of them as blemishes on the face of the sun, just remember that one such blemish may easily be the size of the earth or larger.
Sunspots may persist for months, and they may appear singly, although, usually, they are found in pairs or groups. Such typical groupings are related to the magnetic nature of the sunspots. Every pair of spots has a leader and a follower (with respect to the direction of the sun’s rotation), and the leader’s magnetic polarity is always the opposite of the follower. That is, if the leader is a north magnetic pole, the follower will be a south magnetic pole.
Sunspots are never seen exactly at the equator or near the solar poles, and leaders and followers in one hemisphere of the sun are almost always opposite in polarity from those across the equator. That is, if all the leaders in the northern hemisphere are south magnetic poles, all the leaders in the southern hemisphere will be north magnetic poles.
We have said that sunspots are thought to be associated with strong local magnetic fields. But why are the fields strong in certain regions of the photosphere? A meteorologist from Norway, Vilhelm Bjerknes (1862–1951) concluded in 1926 that sunspots are the erupting ends of magnetic field lines, which are distorted by the sun’s differential rotation. That is, like the gas giant jovian worlds, the sun does not rotate as a single, solid unit, but differentially, at different speeds for different latitudes. The sun spins fastest at its equator—the result being that the solar magnetic field becomes distorted. The field lines are most distorted at the equator, so that the north-south magnetic field is turned to an east-west orientation. In places where the field is sufficiently distorted, twisted like a knot, the field becomes locally very strong, powerful enough to escape the sun’s gravitational pull. Where this happens, field lines “pop” out of the photosphere, looping through the lower solar atmosphere and forming a sunspot pair at the two places where the field lines pass into the solar interior.