Cycles Of Magnetic Activity

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Samuel Heinrich Schwabe (1789-1875), a pharmacist and amateur astronomer of Dessau, Germany, diligently and meticulously observed the Sun day after day, and year after year, with his small 5-centimeter (2-inch) telescope. Schwabe was looking for the shadow of a hypothetical planet Vulcan that was predicted to revolve around the Sun inside the orbit of Mercury. His search included counting the number of sun-spots, which might be confused with Vulcan's shadow. Schwabe never found the planet, which does not exist, but he did unexpectedly discover that the total number of sun-spots visible on the Sun varies periodically, from a maximum to a minimum and back to a maximum, in about 10 years.

After 17 years of observations, Schwabe reported in 1843 that:

[tte total number] of sunspots has a period of about 10 years, "tte future will tell whether this period persists, whether the minimum activity of the Sun in producing spots lasts one or two years and whether this phenomenon takes longer to build up or longer to decline.22

Upon presenting a gold medal to Schwabe, the president of England's Royal Astronomical Society summed up the magnitude ofhis feat:

For thirty years never has the Sun exhibited his disk above the horizon of Dessau without being confronted by Schwabe's imperturbable telescope____"ttis is, I believe, an instance of devoted persistence unsurpassed in the annals of astronomy, "tte energy of one man has revealed a phenomenon that had eluded even the suspicion of astronomers for 200 years!23

tte sunspot cycle certainly does persist. Other astronomers have now compiled systematic records of the periodic variation in sunspot numbers for more than one hundred years (Fig. 5.15). ttey sometimes record the total area of the visible Sun covered by the spots, which rises and falls with the number of sunspots and is thought to be more physically significant.

Detailed observations of sunspots have been obtained at England's Royal Greenwich Observatory since 1874, including the numbers, sizes and positions of sunspots. tte maximum number of sunspots, at the peak of the cycle, can reach 200 per month, while fewer than a dozen spots are seen near the cycle minimum, five or six years later, tte data also show that the positions of sunspots and their associated active regions vary systematically during the sunspot cycle (Fig. 5.15).

With the passage of time during the 11-year activity cycle, new sunspots appear closer and closer to the solar equator, while streams of remnant magnetic flux spread poleward (north or south). In the early part of the cycle, when solar activity rises to its maximum, active regions break out in two belts at about 30 degrees latitude, one north and one south of the solar equator. As on the Earth, latitude is the angular distance north or south of the equator.

When the cycle progresses toward sunspot minimum, the active regions at mid-latitudes fade away, and new ones surface in belts that are closer and closer to the equator, tte drifting active-region belts, one in each hemisphere, describe a slow, 11-year churning motion that originates deep inside the Sun and sweeps down across the photosphere toward its equator, until the sunspots come together and disappear at sunspot minimum.

tten, out of the destruction, the cycle renews itself once more, and active regions emerge again at mid-latitudes about one-third of the way toward the poles. But no sun-spots are ever found at high latitudes near the polar regions of the Sun.

1880 1890 1900 1910 ] 920 1930 1940 1950 I 960 1970 [980 1990 2000 20I0

Year

FIG. 5.15 Sunspot cycle The location of sunspots (upperpanel) and their total area (bottom panel) have varied in an 11-year cycle for the past 130 years, but this magnetic activity cycle varies both in cycle length and maximum amplitude. As shown in the upper panel, the sunspots form at about 30 degrees latitude at the beginning of the cycle, within two bands of active latitudes (one in the north and one in the south) that migrate to near the Sun's equator at the end of the cycle. Such an illustration is sometimes called a "butterfly diagram" because of its resemblance to the wings of a butterfly. The upper panel also shows how the cycles overlap with spots from a new cycle appearing at high latitudes while spots from the old cycle are still present in the equatorial regions. The total area covered by sunspots (bottom panel), given as a percent of the visible hemisphere of the Sun, follows a similar 11-year cycle; during each cycle the total area often rises quickly from a minimum to a maximum and then drops back to a minimum at a slower rate. There are large variations in total sunspot area, and in solar activity, during each cycle, and from cycle to cycle. (Courtesy of David Hathaway, NASA/MSFC.)

1880 1890 1900 1910 ] 920 1930 1940 1950 I 960 1970 [980 1990 2000 20I0

Year

FIG. 5.15 Sunspot cycle The location of sunspots (upperpanel) and their total area (bottom panel) have varied in an 11-year cycle for the past 130 years, but this magnetic activity cycle varies both in cycle length and maximum amplitude. As shown in the upper panel, the sunspots form at about 30 degrees latitude at the beginning of the cycle, within two bands of active latitudes (one in the north and one in the south) that migrate to near the Sun's equator at the end of the cycle. Such an illustration is sometimes called a "butterfly diagram" because of its resemblance to the wings of a butterfly. The upper panel also shows how the cycles overlap with spots from a new cycle appearing at high latitudes while spots from the old cycle are still present in the equatorial regions. The total area covered by sunspots (bottom panel), given as a percent of the visible hemisphere of the Sun, follows a similar 11-year cycle; during each cycle the total area often rises quickly from a minimum to a maximum and then drops back to a minimum at a slower rate. There are large variations in total sunspot area, and in solar activity, during each cycle, and from cycle to cycle. (Courtesy of David Hathaway, NASA/MSFC.)

ttis systematic, 11-year drift of sunspots toward the solar equator was initially noticed in 1858 by the English astronomer Richard C. Carrington (1826-1875), during his studies of differential solar rotation, and then described by the German astronomer GustavSporer (1822-1895). It is graphically represented in a plot of sunspot latitude as a function of time, first drawn by E. Walter Maunder (1851-1928) in 1922 and brought up to date in Fig. 5.15. Such an illustration is sometimes called a "butterfly diagram" because of its resemblance to the wings of a butterfly.

As old spots linger near the equator, new ones break out at mid-latitudes, but the magnetic polarities of the new spots are reversed with north becoming south and vice versa - as if the Sun had turned itself inside out. During one 11-year cycle, the magnetic polarity of all the leading (westernmost) spots in the northern solar hemisphere is the same, and is opposite to that of leading spots in the southern hemisphere, tte magnetic polarity of the leading spots reverses in each hemisphere at the beginning of the next 11-year cycle.

Remnants of old sunspots move to the polar regions to replenish the weaker dipolar (two poles) magnetic field of the Sun. It also reverses at the beginning of each cycle, so the magnetic north pole switches to a magnetic south pole and vice versa, tten, for the next 11 years, in the new cycle, all the polarities will be exchanged, including those of all the sunspots and that of the general polar magnetic field, ttus, the full magnetic cycle takes an average of 22 years.

Active regions contain twinned bipolar sunspots that are aligned roughly parallel to the equator, and describe a global pattern of magnetic polarity. All of the sunspot pairs in each active-region belt have the same orientation and polarity alignment, with an exactly opposite arrangement in the two hemispheres. According to Hale's law of polarity, the leading, or westernmost spots (leading in the sense rotation) of any sun-spot group in the northern belt of active regions have the same magnetic polarity, while the following (easternmost) spots have the opposite magnetic polarity. In the southern hemisphere, the leading and trailing sunspots of any sunspot group also exhibit opposite polarities, but the magnetic direction of the bipolar sunspots in the southern belt is the reverse of that in the northern one.

ttus, if the leading spot in the northern hemisphere has one magnetic polarity, the leading spot in the southern hemisphere will have the opposite polarity. It's as if men and women always walked down the street in couples, with the men preceding the women on one side of the street and the women leading the men on the other side. Moreover, the couples exactly reverse their orientation at sunspot minimum every 11 years, as if participating in a dance that has been carefully choreographed, tte bipolar sunspot magnets then flip and turn around, so the leading spots in each hemisphere have opposite magnetic polarities during successive 11-year cycles, tte leading spots resume their original magnetic polarity in approximately 22 years after reversing orientation twice.

Magnetograms indicate that there is still plenty of magnetism at the minimum of the solar cycle of magnetic activity, when there are no large sunspots present, tte magnetism then comes up in a large number of very small regions spread all over the Sun (Fig. 5.16). tte magnetic field averaged over vast areas of the Sun at activity minimum is only a few ten thousandths of a Tesla, or a few Gauss, but the averaging process conceals a host of fields of small size and large strength. When the telescope resolution is increased, finer and finer magnetic fields are found, with higher and higher field strengths.

tte magnetic fields are never smoothly distributed across the photosphere, either during activity minimum or maximum. Instead, they are highly concentrated, inho-mogeneous, and everywhere clumped together into intense bundles that cover only a few percent of the photosphere's surface area.

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