Sunspots And Stellar Structure

Type of physical science: Astronomy; Astrophysics

Field of study: Stars

The sunspots, the accompanying magnetic fields within and around them, and their periodicity are among some of the important observables associated with the internal structure, differential rotation, and convection zone of the sun.

Overview

The sun, a massive gaseous sphere with a radius of 696 million meters, some 150 billion meters away, is the only life-sustaining source of energy available to Earth. With 333,400 times the mass of Earth, 99.9 percent of the mass of the solar system resides in the sun. Almost 94 percent of the solar mass consists of hydrogen atoms; 5.9 percent is helium, and the remainder is composed of traces of the remaining ninety elements.

The most important properties of the sun are its great mass and the fact that one-half of this mass is inside a central spherical core of one-fourth the solar radius, which produces the energy emitted by the sun. The core temperature of the sun is 15 million Kelvins, and its pressure is estimated to be 250 atmospheres. At this temperature, despite the inordinately high density, the nuclei remain gaseous. Solar energy results from the fusion of four hydrogen nuclei to form helium, and in each case, 0.70 percent of the mass entering the reaction is converted into energy.

The sun generates 400 trillions of trillion watts of power, which amounts to burning 5 million tons of mass per second. The sun is 5 billion years old and is likely to shine at the present rate for several billion years to come. In due course, with depleted hydrogen supply, the core will consist mostly of helium.

The energy generated in the solar core is transported toward the surface through a series of radiative transfers. Thus, nuclei absorb the γ rays, re-emitting them as X rays, which are then converted to ultraviolet rays and finally to visible light. The zone of radiative diffusion, however, extends to only 85 percent of the solar radius, at which point the pressure of the gas drops so that a turbulent convective process takes over and transports the energy to the surface. The convective cells that form are responsible for the observed boiling and bubbling on the solar surface. The temperature at the outward surface of the convective zone--the photosphere--is closer to 6,000 Kelvins, and it is in the photosphere that the sunspots are found.

The sun was assumed to be a perfectly radiant sphere until 1610, when Galileo (1564-1642) reported sighting dark spots--sunspots--on the photosphere. In 1843, Heinrich Samuel Schwabe (1789-1875), a German amateur astronomer, announced after twenty years of observation that the sunspots appeared and decayed in an eleven-year cycle. It was realized later that, in reality, the cycle consists of twenty-two years, if the magnetic field reversal is taken into account. Furthermore, it was also discovered that there are considerable variations throughout the observed period. Since the beginning of the twentieth century, the last four periods have averaged only 10.4 years, with the rise time, which lasts only four years, being quicker than the duration of decay. Also, superimposed on the solar cycle may be an eighty-year cycle with episodes of waning and waxing sunspot incidences.

The sunspots appear at the beginning of the cycle at latitudes +/-40 degrees (+, north; -, south of the solar equator), progressing down to +/-5 degrees when the next cycle starts. An individual spot survives about twenty-seven days, roughly a solar rotation period, if it is small in size, while a larger one lasts more than a rotation. Sunspots are regions of powerful magnetic fields, and it will be seen that most of the surface activities of the sun, such as flares, solar wind, prominences, and the like, are related to intense, time-varying, and transient fields of photosphere and corona. Within the central region of the sunspot, a magnetic field on the order of 0.30 tesla is strong enough to exert pressure that exceeds the kinetic pressure, thus inhibiting the energy transporting gas flow from the lower level of the convective zone and making it darker by contrast to the photosphere. Also, the lack of flow of hotter gas facilitates expansion, aided by fanning out magnetic fields, leading to some 2,000 Kelvins cooler than the surrounding photospheric temperature.

The diameters of the umbrae, the darkest regions of the sunspots, seldom exceed 20,000 kilometers, limiting their depth to about the same distance and large-scale mass movement to about half their size. Sunspots and their associated fields appear to be anchored at this depth. Vertical flow of matter in sunspots is found to be limited to 25 meters per second, while the horizontal flow does not exceed 50 meters per second within the umbrae. Sunspots also show irregular patterns of bright points called "umbral dots" or "umbral granulation," in addition to solar filigree caused by delicate small-scale movement of magnetic field elements. Such movements are studied by spectral analysis, just as the strength of the magnetic field is determined by Zeeman spectral shift and the velocity fields are determined by the Doppler effect.

Umbrae in sunspots are surrounded by lighter, and therefore hotter, regions with complex structures known as penumbrae. The penumbral magnetic field has horizontal fine structures, giving the region a filamentary overlapping white and gray appearance. The average field strength is assumed to be nearly one-half the umbral region. Penumbral matter flow is nearly horizontal, unlike that in umbrae, with progressively decreasing velocities.

Regions known as "pores" with dark umbrae also occur, having magnetic fields on the order of 0.20 to 0.25 tesla. A variety of structures occur near bright chromospheric regions, which are called faculae or, if more intricate, filigree, with field intensities on the order of 0.10 to 0.20 tesla. Also occurring are compact magnetic structures known as magnetic knots, having fields of up to 0.1 tesla.

The sun rotates eastward like Earth. It was discovered that the adjacent sunspots exhibit opposite magnetic polarities; that is, the field coming out of one returns into the other. Further, the polarity of the field of the leading sunspot in one solar hemisphere is always opposite to the polarity of the field of the leading sunspot in the other hemisphere. George Ellery Hale discovered in 1924 that the solar magnetic field reversed once every eleven years so that the sunspots in reality had a twenty-two-year cycle. The intense fields at adjacent sunspots in either solar hemisphere were also found to be solenoidal, so that they fanned out of or into the sunspots vertically to the plane of the photosphere.

Thus, solar activity is marked by the manifestation of periodic occurrences of sunspots, accompanied by complex variations in associated magnetic field intensities. The solar magnetic field is copious, dissipative, and forever rising to the surface, only to be diffused and dispersed through numerous active regions. Characteristically, the surface magnetic fields, weak or strong, are not continuous and occur in conjunction with the active regions of the sun. For example, among the granular boundaries, a basic feature of the photosphere, concentrated tubular fields emerge and disperse into the solar atmosphere. A global magnetism that is somewhat akin to Earth's or poloidal fields appears to coexist and evolve during the activity cycle, along with toroidal fields, which are found at the deeper levels of the convective zone, that surface through the sunspots at higher latitudes at the beginning of the periodicity.

The dynamo theory of solar magnetic fields developed by Eugene N. Parker, Horace W. Babcock, and others in the 1950's provides a framework of explanations to the observed sun's magnetism, the sunspot activity cycle, and other major features such as flares, prominences, and solar winds. There are compelling reasons to believe that the convective zone consists of highly ionized gas--a mixture of ions and electrons--which is called the solar plasma. The rotating plasma induces both a current and an intense magnetic field. The sun does not rotate like a solid body. The rotational velocity of the outer layers increases with latitude. The solar equatorial rotational period of twenty-five days progressively increases to thirty days at about latitude 60 degrees. This differential rotation of the convective zone is assumed to be responsible for driving the dynamo and amplifying the solar magnetic fields. Another aspect of the solar dynamo is the radial differential motion to a depth of 30 percent of the sun's radius. The implication is that a greater solar mass motion is involved in the generation process of the field, extending to deeper levels than previously believed. Another important velocity component affecting the field-generating mass movement is the Coriolis force, which imparts a westward drift to longitudinal motion. The Coriolis force thus modifies north-south movements of sunspots and other field-oriented large- and small-scale surface features.

At the beginning of an activity cycle, solar magnetic field lines at a depth of one-tenth of the sun's radius are considered dipolar, or poloidal, with field lines emerging from the south pole and entering at the north. The differential rotation of the sun, after a few rotations, stretches these flux tubes constrained to move along the plasma surface so that they wrap around parallel to the equator on either side. As the density of the field lines increases, the magnetic buoyancy of the field is elevated. The convective motion of the plasma further twists the flux lines like strands of rope. The enhanced tension in the magnetic field lines at higher latitudes finally breaks through the solar surface as sunspots roughly along the east-west direction. The mechanism satisfactorily explains the incidence of sunspots and their geometry in either solar hemisphere and the bipolar nature of the fields of adjacent spots. As the flux tubes weaken, as a result of diffusion of energy through the sunspots at higher latitudes with the progressing cycle, the spots migrate toward the equator until it is time for a new cycle to begin. The dispersal, eruption, and random walk processes of the field are brought on by the large-scale movement of the plasma in the convective zone below, weakening and finally leading to the reversal of the polarity of the poloidal field at the start of a new cycle. The complexity of the envisioned process, which leads to the twenty-two-year magnetic cycle, is a matter for further investigation. In view of the numerous physical processes involved in the complete breakup of poloidal fields and the weakening of the toroidal fields, the possibility of nonreversal of the polarity at the end of a definite time frame exists.

Applications

The sun is Earth's nearest star. Everything scientists know about the sun is applicable to most other stars. In addition to the obvious properties--namely, the mass and luminosity--the sunspots and the attendant magnetic cycle are aspects that can be observed readily. The sunspot cycle is related to long-term temperature fluctuations on Earth; variations in the charge-related properties of the interplanetary medium caused by the changing magnetic field and the consequent irregularity of the ejected mass in the form of flare, solar wind, and the like; and drastic variations in the electromagnetic induction properties of terrestrial installations brought on by so-called magnetic storms (which occur at peak sunspot activity cycle and are caused by a rapidly dissipating solar magnetic field). There appear to have been long episodes of solar inactivity, with the consequent drop in Earth's temperature. An accumulated body of information indicates that the sun may be a variable star, possibly with complex periodicity. Such a prospect for the solar luminosity changes would have far-reaching ramifications for the history of Earth's temperature.

The occurrence of the magnetic field is common in the universe, in the Milky Way galaxy, and in the solar system. The dynamo mechanism or its variant appears to be the most plausible mode of large-scale amplification of magnetic fields. A thorough knowledge of the solar dynamo will add to an understanding of the sunspot cycle and a host of other surface mass ejection processes. It is believed that the magnetic fields of the major planets, including those of Earth, are also products of the dynamo mechanism.

The study of sunspot cycles has necessitated sophisticated and precise instruments that help unravel the mysteries of smaller and more varied magnetic elements of the sun. The field of helioseismology has helped astronomers to probe in greater depth the convective and the radiative zones of the sun, enabling them to reexamine and refine the dynamo theory, which in turn has led to progress in the knowledge of planetary magnetic fields.

Spectroscopic studies of magnetic fields in other stars have revealed that the majority of stars possess both toroidal and poloidal fields similar to those found on the sun, with intensities ranging from 0.20 to 2.00 tesla; the observed poloidal fields appear to have periodicities ranging from one-half of a day to decades; and among many of the lower main sequence stars, "starspots" are suspected to exist.

Magnetic field activity, presumably similar to that of the sun, appears to be stronger in many younger stars, which rotate more rapidly in comparison with older and lower-angular-velocity stars. The younger stars in Orion have field intensities three times that of similar-sized older stars. It is evident from observations that starspots in relatively younger stars, which radiate intense X rays and ultraviolet rays, actually make them appear dimmer. It is plausible that the sunspot and the associated magnetic field intensity may have been stronger at an earlier epoch of the solar system, when presumably the sun was spinning more rapidly. Closer to home, the sunspot activity has left a permanent record, dating back thousands of years, in concentrations of the carbon 14 radioactive isotope, produced by the modulating effect of the solar magnetic field on cosmic rays reaching Earth. The reduced number of sunspots implies a weaker magnetic field. This, in turn, leads to a greater cosmic-ray flux that reaches Earth and produces higher concentrations of carbon 14. A measurement of concentrations of carbon 14 at various levels of the soil is easily correlated with sunspot activity.

Context

Humans have worshiped the sun from the dawn of history. With Galileo's discovery of the sunspots in 1610 began a new era of inquiry into the causes of the blemishes seen in the otherwise perfect photosphere. It was during the middle of the nineteenth century that Schwabe announced the discovery of the eleven-year sunspot cycle; a coherent picture of the phenomenon emerged much later. The current solar research is concerned with many broad areas, such as sunspots, their emergence, and the solar magnetic activity cycle, as well as the generation of thermonuclear energy, its transport mechanism to the surface, and the sun's atmospheric physics.

It has become possible to explore the eleven-year cycle in the ultraviolet region of the spectrum with the aid of the Nimbus 7 and Solar Mesosphere Explorer satellites. In the 1980's, the Solar Maximum Mission satellite carried out delicate experiments and simultaneously collected a vast amount of data on the solar atmosphere, enhancing the understanding of the structure and variable radiations that occur there. Research on Earth's radiation belt, combined with the deep-space planetary probe data, leads to the conclusion that the pervasive solar wind that creates a dynamic interplanetary medium is yet to be understood with respect to its interactive properties.

Solar physics, solar observation, and the complex aspect of the subject started with a chance sighting of sunspots by an amateur astronomer. The current high-resolution solar telescopes that use clusters of mirrors are able to provide considerably more detailed information on ever-smaller areas of the sun. Scientists have been able to scan the surface and the atmosphere of the sun from high-energy γ rays to low-frequency radio waves of the electromagnetic spectrum. With the advent of supercomputers and the availability of space shuttles to deploy specialized instruments, such as high-resolution solar telescopes, high-energy X-ray and γ-ray telescopes, and pinhole/Occulter facilities, the coming decades will witness spectacular progress in solar physics, which will be aided by advances in related fields.

Historically, the source of solar energy could be explained only in terms of thermonuclear fusion; the lessons of plasma physics and magnetohydrodynamics should help scientists to explain the solar dynamo mechanism and, hence, the related activity cycle, including sunspots.

Principal terms

CORIOLIS FORCE: deflective force caused by solar rotation on moving matter

FACULA: bright spots or streaks on the photosphere associated with the magnetic field

KINETIC PRESSURE: the average force per unit area produced by atoms and molecules

PENUMBRA: the lighter outer region surrounding the darker umbral region on sunspots

PHOTOSPHERE: a relatively thin surface layer of the sun from where the majority of photons (light considered as particles) are emitted

PROMINENCES: arches of glowing gases often seen in the vicinity of sunspots

SOLENOIDAL FIELD: the magnetic field similar to that of a bar magnet found when an electrical current is passed through a cylindrical coil of wire

UMBRA: the darkest inner region in the sunspot

ZEEMAN EFFECT: the broadening or splitting of spectral lines of light emitted by atoms caused by the presence of magnetic fields

Bibliography

Foukal, Peter V. "The Variable Sun." SCIENTIFIC AMERICAN 262 (February, 1990): 34-41. Written for the general reader, the article describes the history of sunspots and the variation in the solar luminosity. Presents the current findings, pointing to the possibility that the steady sun is more than likely a variable star. An interesting article for all levels of readers.

Gibson, Edward G. THE QUIET SUN. NASA SP-303. Washington, D.C.: Government Printing Office, 1973. Contains many photographs, illustrative graphs, and charts. Although technical, this volume is written so that a nonscience general reader will not experience any difficulty with the text.

Giovanelli, Ronald G. SECRETS OF THE SUN. New York: Cambridge University Press, 1984. This book, which contains many photographs of sunspots and their progression in the eleven-year cycle, is specially written for nonscientists, scientists, and lay readers alike. A rare example of an author's serious attempt to introduce the reader to a rather complex set of ideas.

Jordan, Stuart, ed. THE SUN AS A STAR. NASA SP-450. Washington, D.C.: Government Printing Office, 1981. Although technical, this volume is a complete source of solar physics. Includes a large number of references at the end of each chapter. A nontechnical reader can skip over the occasional mathematical equations without experiencing a sense of loss in logic or continuity.

Parker, E. N. "Magnetic Fields in the Cosmos." SCIENTIFIC AMERICAN 249 (August, 1983): 44-54. Parker describes the process of amplification of the existing fields. The solar field and the attendant sunspot cycles are described adequately by the dynamo mechanism insofar as the features are concerned. Describes the longitudinal or poloidal magnetic fields of major planets in the solar system. The article is directed toward an interested general reader.

Wentzel, G. Donat. THE RESTLESS SUN. Washington, D.C.: Smithsonian Institution Press, 1989. Written primarily for the general reader, this excellent volume contains photographs, illustrations, and information on all types of solar activities. Even a casual reader can get engrossed in reading this work because of the easy style and depth of material covered. Geared for a beginner.

A giant prominence leaps out from the sun

Solar flare seen from Skylab 4

The sun's vital statistics

Sunspots, cooler than the surrounding photosphere, are darker

Sunspot cycle from 1610 to 1976

Structure of the sun

The Measurement of Magnetic Fields