THE ROTATION OF THE SUN The sun turns once every 27 days, but some parts turn faster than others. Such variations are clues to interrelated phenomena from the dense core outward to the solar wind that envelops the earth by Robert Howard hen Galileo turned his tele­ cause the gas that gives rise to them is sun's rotation. When sunspots are ob­ scope on the sun in 1610, he moving, and the amount of the shift is served with a small telescope, they ap­ Wdiscovered that there were proportional to the velOCity of the gas in pear simply as spots on the photosphere: dark spots on the solar disk. Observing the line of sight. Unfortunately for solar the bright surface of the sun that is seen the motion of the spots across the disk, observers each method has disadvan­ in ordinary white-light photographs. In he deduced that the sun rotated once in tages that limit its usefulness. larger telescopes they may appear as about 27 days. One of his contempo­ saucer-shaped depressions near the edge raries, Christoph Scheiner, undertook a iming a tracer as it crosses the disk of of the sun. Therefore it is hard to be systematic program of observing the the sun is a good technique for de­ sure that the same point in the sunspot is spots. From these observations Scheiner terminingT the solar rotation rate only if . being used for the purpose of measure­ found that the rotation period of the the tracer meets certain requirements. ment as the spot traverses the solar disk spots at higher solar latitudes was slight­ First, it must survive long enough un­ and is seen from a different angle each ly longer than that of the spots at lower changed in its appearance to be useful day. In order to avoid the problem, stu­ latitudes. In the middle of the 19th cen­ over some reasonable period of time. dents of the sun's rotation prefer to use tury R. C. Carrington, a wealthy English Second, it must have a clearly defined long-lived spots that cross the central brewer and amateur astronomer, deter­ structure, so that its position can be ac­ meridian of the sun more than once. The mined with good precision the amount curately determined. Third, it must be at central meridian is defined as the inter­ of the variation of the sun's rotation rate the same altitude in the sun's atmo­ section of the sun's visible surface with with latitude. sphere when it is seen at the center of the plane that passes through the sun's Scheiner's observations suggested that the solar disk and when it is seen at the axis of rotation and the earth; when a the sun does not rotate as a simple solid edge. Last, it must not wander around sunspot crosses the central meridian, the body. Since we now know that the sun the solar disk in some systematic way effect of its three-dimensional form is is a gas throughout, it does not surprise not directly related to the sun's rotation. minimized and its position can be most us that it does not rotate as a solid. One Although no features on the sun meet precisely measured. might have expected, however, that the all these requirements, sunspots are still Only a small fraction of sunspots live spots closer to the sun's equator would the easiest to observe, and they have long enough to cross the central merid­ move slower than the spots at the higher been studied more than any other trac­ ian more than once. The spots with the latitudes, just as the outer planets of the er. Thus they have probably provided longest lifetime are the ones found on solar system move slower in their orbits . the most accurate determinations of the the leading edge of groups of sunspots, than the inner ones. The fact that the equatorial regions rotate faster than the rest of the sun implies that an excess of angular momentum-the momentum of the sun's rotation-must be transferred SUN'S ROTATION can be monitored by noting the daily progress across the solar disk of from the higher solar latitudes to the a tracer such as a sunspot or a gaseous filament. Such features are associated with magnetic equatorial regions. This fact and its im­ fields, which can be detected directly by means of a solar magneto graph. The six magneto. plications yield information about the grams on the opposite page were made with the 150·foot tower telescope at the Mount structure of the sun's interior and the Wilson Observatory. Tbe sequence reads from top to bottom and shows the magnetic activ­ nature of its activity. ity on the sun at intervals of every other day from May 30, 1974, through June 9, 1974. The There are two basic methods for pre­ contours on the magneto grams represent equal-strength areas in the magnetic fields. The strength in gauss of the fields at successive contour intervals is five, 10, 20, 40 cisely determining the rotation of the and 80. (The strength of the earth's magnetic field is less than one gauss.) The solid contours rep. sun at a partkular latitude. The first is to resent positive magnetic fields; the broken contours represent negative fields.Th e life. time the passage of some tracer (such as times of the smallest magnetic features are less than Ii day, so that in general it is possible a sunspot) across the solar disk. The sec­ to trace the daily rotational movement of only the larger magnetic features. Strongest of ac· ond is to photograph the spectrum of the tive regions that are seen here are associated with sunspots; it is difficult to determine the sun; the spectral lines of the solar at­ exact location of spots, however, because angular resolution of magneto grams is low. Evo· mosphere will be Doppler-shifted be- lutionary changes within active regions somewbat alter their appearance from day to day. 106 © 1975 SCIENTIFIC AMERICAN, INC oJ , � J I> " " u \. "'!- "- ., 0 <) �, C> c, .Y� " '0 () o " ,;" 0 . � , �, , ., o . o Q:, 00 0 • ,� �.� . ��;� "% 0 �- <:?I z:) -:::> ......, " . Q , �.,- ., - ., . ' .. '. © 1975 SCIENTIFIC AMERICAN, INC and so these are the ones that are most of their own later in their lifetime. A fur­ A second kind of tracer is the long commonly monitored. That is actually ther disadvantage of using sunspots as filaments of gas in or above the solar unfortunate, because the preceding and rotation tracers is that they are rarely chromosphere. The chromosphere is the following members of a sunspot group seen above a solar latitude of 35 degrees. transitional layer between the photo­ tend to separate early in the lifetime of Thus they can be used only for deter­ sphere, which has a temperature of the group. In addition the preceding mining the rotation of the sun at rather 6,000 degrees Kelvin, and the solar co­ spots tend to exhibit systematic motions low latitudes. rona, which paradoxically is much hot­ ter: about two million degrees K. The chromosphere can be observed only at the wavelengths of certain spectral lines, notably the line designated hydrogen alpha in the red region of the spectrum. Photographs of the chromosphere made in the light of the hydrogen-alpha line show an abundance of lovely detail. Al­ though most of these features are too short-lived or too ill-defined to be used for accurately determining the rotation rate of the chromosphere at various lati­ tudes, large dark filaments are fairly re­ liable tracers. The filaments are actually clouds above the chromosphere that ap­ pear dark when they are seen projected against its bright surface but appear bright when they are seen extending be­ CENTRAL MERIDIAN OF THE SUN as it is seen from .the earth is defined as the inter. yond the edge of the solar disk. They are section of the portion of the sun visible from the earth (color) with the plane (gray) de· not ideal tracers because their appear­ fined by the sun's axis of rotation and the earth. Longitudinal positions of the various ance changes too rapidly. Their lifetime markers that are monitored to study sun's rotation are referred to sun's central meridian. is not negligible, however, and they are present at higher solar latitudes than N sunspots. For these reasons their rota­ tion characteristics have been studied in "/ detail. ( \ f the photosphere and the chromo- "-"" I sphere show rotation, what about the '" solar corona? This question is not easy " to answer. The corona is so faint com­ pared with the rest of the sun that it can be viewed only during a total eclipse of "- the sun or with the aid of the corona­ '\ graph: a telescope equipped with an oc­ I \ culting disk that blocks the light of the I \ photosphere as the moon does (although I \ not as effectively). In either case the E w exact connection between features of the \ solar disk and features of the corona is J hard to determine. Moreover, the coro­ / nal features are so poorly defined that it / is difficult to measure their size and po­ / sition. The rotation of the corona may therefore be measured accurately only / by correlating observations that extend / over at least several months. / Richard Hansen, S. F. Hansen and /" Harold C. Loomis have measured the / /' rotation period of the corona with a spe­ { cially designed instrument attached to a \ � corona graph on the mountain Haleakala on the Hawaiian island of Maui, and s they have found that the period varies somewhat from year to year.