Sunday, October 31, 2010

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.

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