sunspots
sunspots,
dark, usually irregularly shaped spots on the sun's surface that are actually solar magnetic storms. The spots are darker because the temperature of the spots is lower than that of the surrounding photospherephotosphere,luminous, apparently opaque layer of gases that forms the visible surface of the sun or any other star. The photosphere lies between the dense interior gases and the more attenuated gases of the chromosphere.
..... Click the link for more information. (the visible surface of the sun). All but the smallest sunspots show a dark central portion (the umbra) with a lighter outer area (the penumbra). The spots often form in pairs or groups, with a large, long-lived leader spot matched with one or more smaller spots of opposite magnetic polarity.
Studies of the spectra of sunspots show evidence of the Zeeman effectZeeman effect,
splitting of a single spectral line (see spectrum) into a group of closely spaced lines when the substance producing the single line is subjected to a uniform magnetic field. The effect was discovered in 1896 by the Dutch physicist Pieter Zeeman.
..... Click the link for more information. , indicating the presence of a large magnetic field. In addition, measurements of the Doppler effectDoppler effect,
change in the wavelength (or frequency) of energy in the form of waves, e.g., sound or light, as a result of motion of either the source or the receiver of the waves; the effect is named for the Austrian scientist Christian Doppler, who demonstrated the effect
..... Click the link for more information. in the spectral lines show that there is a vortex motion in sunspots similar to that of a tornado on earth. The lower temperature of the gases constituting a sunspot results from the lower pressure due to the strong magnetic field.
Sunspots appear usually only between latitudes from 5° to 35° north and south of the sun's equator. They are not permanent since the sun's surface is gaseous. Because the sun rotates on its axis, a sunspot cannot be observed continuously for more than about two weeks.
An 11-year cycle from one period of maximum sunspot activity to the next is usually observed. However, a period during which most sunspots have one magnetic polarity is followed by another period during which most have the opposite magnetic polarity; thus, the cycle actually covers 22 years. During each 11-year period sunspots appear first at higher latitudes and later at latitudes closer to the solar equator as the period progresses. Periods in which an increase in sunspots is observed are called active periods.
A number of phenomena are associated with sunspots. Sunspot activity can produce solar flares and coronal mass ejections (eruptions of charged particles into space), which can cause various disturbances on earth—these include geomagnetic storms which manifest themselves as auroras, interference with radio reception and electric power grids, and disturbances of the magnetic compass (see also chromospherechromosphere
[Gr.,=color sphere], layer of rarefied, transparent gases in the solar atmosphere; it measures 6,000 mi (9,700 km) in thickness and lies between the photosphere (the sun's visible surface) and the corona (its outer atmosphere).
..... Click the link for more information. ). A storm in 1989 caused a blackout in Quebec prov., Canada, and a more extreme storm in 1859 affected the telegraph system, shocking operators. Evidence for even more intense geomagnetic storms caused by more powerful coronal mass ejections has been found by measuring radioactive isotopes contained in tree rings and ice cores. The peak flare rate, however, occurs several years after sunspot activity reaches its maximum, and the strongest flares typically occur as the cycle is nearing its minimum.
The Chinese recorded dark features on the sun seen with the naked eye in 28 B.C. Other observers including Kepler suspected that these events might be transits of Mercury or Venus. Galileo observed them systematically for several weeks before concluding that they had to be events taking place on the solar surface. In 1826 amateur astronomer Heinrich SchwabeSchwabe, Samuel Heinrich
, 1789–1875, German apothecary and amateur astronomer. In the hope of discovering a new planet between Mercury and the sun, he made daily observations and tallies of sunspots.
..... Click the link for more information. began a series of solar observations (in hopes of finding planet Vulcan). By 1843 he had collected enough data to announce the existence of the sunspot cycle. Reviewing historical records in 1890, E. Maunder noticed that sunspot counts fell drastically between 1645 and 1715. In 1976 J. Eddy correlated Maunder's data with a low frequency of aurorae and the reduced sizes of annual tree rings. This "Maunder Minimum" may have played a role in the unusually low temperatures in the Northern Hemisphere during this portion of the period (c.1550–1850) known as the Little Ice Age.
sunspots
(sun -spots) Comparatively dark markings in the granulation of the solar photosphere, ranging in size from pores, which are no larger than individual granules, to complex structures covering several thousand million square kilometers. All but the smallest spots consist of two distinct regions: a dark core or umbra and a lighter periphery or penumbra. The umbra usually appears featureless, while the penumbra – which can account for as much as 80% of the total area of a spot – may be resolved into delicate filaments, aligned radially in the case of regular spots. Most spots do not occur singly but tend to cluster in groups. The formation, development, and decay of large groups may occupy several weeks, or even months in exceptional cases, but the majority of groups are small, having lifetimes of not more than a couple of weeks.Sunspots are the centers of intense localized magnetic fields, which are thought to suppress the currents bringing hot gases from the convective zone before they reach the photosphere. In the umbra, where the field is strongest (0.2–0.4 tesla), convection is almost completely inhibited; in the penumbra, where the field is more horizontal, a radial flow of material takes place (see Evershed effect). The umbra is therefore much cooler than the penumbra, which is in turn cooler than the surrounding granulation, their temperatures being of the order of 4000, 5600, and 6000 K respectively. An alternative explanation for the lower temperature of sunspots suggests that far from inhibiting the flow of heat the magnetic field actually enhances it, converting at least three-quarters of the flux into magnetohydrodynamic waves (Alfvén waves) that propagate rapidly along the field lines without dissipation.
Sunspot groups exhibit a great diversity of structure and are most conveniently classified according to the configuration of their magnetic field as unipolar, bipolar, or complex. Bipolar groups are by far the most common. In their simplest form they consist of two main spots of opposite magnetic polarity, termed (with respect to the direction of the Sun's rotation) the preceding (p -) and following (f -) spots, of which the p -spot usually lies slightly closer to the equator. The line of inversion separating the regions of opposite polarity (where the vertical component of the magnetic field is zero) is often the location of a relatively stable filament and the scene of violent flares, both of which occur above the sunspot group in the upper chromosphere/inner corona.
The number of sunspots fluctuates over an average period of approximately 11 years – the so-called sunspot cycle – during which time there is a corresponding variation in the mean heliographic latitude of all groups. Moreover the respective polarities of the p - and f -spots, which are consistent within a particular hemisphere but opposite on the other side of the equator, reverse at sunspot minimum for the groups of the new cycle.
Sunspots are the most obvious manifestation of solar activity. Together with their associated faculaeplages, filaments (or prominences), and flares, they constitute active regions, whose influence extends from the photosphere through the chromosphere to the corona.