Solar magnetic field

The solar magnetic field is a powerful force throughout the Sun, causing flares, prominences, sunspots, and other phenomena. Charged particles from the Sun and the solar magnetic field interact with the terrestrial magnetosphere, which affects life on Earth.

Overview

A magnetic field is produced by, and associated with, moving charges (electrical currents). In a simple ring current, a dipolar field resembling the field of a bar magnet is what results. Complex interactions of charges occur everywhere in nature, and the large-scale motion of charges is extensive and intricate. Thus, depending on the current, magnetic fields can be found almost anywhere in various strengths. As an example, Earth’s dipolar magnetic field is about 0.6 gauss, rather weak compared to solar active regions with field strengths of a few thousand gauss and pulsars with field strengths of 1 trillion gauss.) Interactions between magnetic fields and charged particles result in the emission of X-rays, the energizing and modulation of cosmic rays, and the acceleration of charged particles. This is seen when a strong solar magnetic field expels charged particles from the Sun, while the weaker terrestrial magnetic field interacts with these particles. This forms a protective shield around Earth by deflecting many of them and letting some into the upper atmosphere producing the auroras (the northern and southern lights).

On a laboratory scale, the properties of magnetic fields are well known. The solar magnetic field, however, is much larger and more complex. The solar magnetic field appears to be generated and sustained by the Sun’s rotation and the turbulent convective motion of charged particles below the photosphere (the Sun’s visible surface). The development of improved observing techniques and instruments, including the use of space probes and space telescopes, has yielded much information on the Sun’s magnetic field and its effects.

In addition to the fascinating nature of the Sun's magnetic field, and many of its key components such as sunspots, there are very practical reasons in seeking to understand the workings of this phenomena. Chief among them is that in addition to the benefits the Sun renders to the Earth, it is also the source of often-times catastrophic damages. These include the generation of radiation that can be harmful to humans and to critical infrastructure such as power grids.

Sunspots have been seen from Earth since at least the fourth century B.C.E., as evidenced by early descriptions of them. They appear as darker spots of various sizes on the brighter solar photosphere. The temperature of the photosphere is about 5,800 kelvins, while sunspots are cooler by about 1,000 to 1,500 kelvins, thus making sunspots about one-third as bright as the rest of the photosphere. The spots have diameters ranging from a few thousand kilometers to more than 150,000 kilometers. Generally, smaller spots last only a few days, while larger ones linger for several weeks.

The solar sunspot cycle was discovered in 1843 by Samuel Heinrich Schwabe, who observed that the average number of sunspots varies systematically with an approximate periodicity of eleven years. At the beginning of a sunspot cycle, the spots appear symmetrically at solar latitudes of about 30° to 40° north and south, gradually working their way toward the Sun’s equator by the end of the eleven-year period. Then the pattern repeats during the next sunspot cycle.

In 1908, George Ellery Hale—the American astronomer responsible for establishing the observatories at Yerkes, Mount Wilson, and Palomar—observed that lines in the spectra of sunspots were split into several components. Earlier, in 1896, Pieter Zeeman, a Dutch physicist, had shown that such splitting occurs in the presence of a strong magnetic field. Hale thus established that the sunspots had intense magnetic fields (up to 5,000 gauss). Since the Sun is a sphere of hot gases, such magnetic fields can exist only as a result of powerful convective currents. Hale and his colleagues at Mount Wilson Observatory tried to measure a general solar dipolar magnetic field, similar to that of Earth (whose magnetic field resembles that of a huge bar magnet), but it was not until 1953 that Horace Babcock, using a specially designed solar magnetograph, succeeded. He showed that a weak, periodically varying dipolar field of 2 to 7 gauss exists over the entire Sun.

The solar magnetic field changes over a twenty-two-year cycle, twice the eleven-year periodicity of the sunspot cycle. Sunspots appear either in pairs or in clusters, but they are always organized in coherent pairs of opposite magnetic polarity. During a particular eleven-year sunspot cycle, for example, the leading members of the sunspot pairs may display positive polarity in the Sun’s northern hemisphere and negative polarity in the Sun’s southern hemisphere. At the end of the eleven-year sunspot activity cycle, the polarities in the two hemispheres reverse. The magnetic cycle then repeats after two sunspot cycles.

The Sun’s diffuse, dipolar magnetic field also undergoes a reversal of polarity with a periodicity of twenty-two years. A positive magnetic pole appears around the solar north rotational pole at the peak of every other sunspot cycle, while a negative magnetic pole is found around the solar south rotational pole at that time. The polar fields subsequently expand to latitudes of 50° to 60° while the sunspot numbers decline. After the sunspot minimum passes, the increase in number of sunspots with their strong local magnetic fields forces the global field back toward the rotational poles, there to coalesce and form opposite poles from those of the previous cycle.

The sunspot cycle and the twice-as-long magnetic cycle can be explained by the Solar Dynamo Theory, which involves interactions between the convective motions of plasma (ionized gas) beneath the photosphere and the Sun’s differential rotation. Energy produced by nuclear fusion in the Sun’s central core is transported slowly outward through the deep interior by the process of radiative diffusion (repeated absorption, emission, and scattering of photons by particles). Higher up, the energy is transported by radial convection currents and eddies (transport of energy by the actual motion of matter). It is generally believed that, within these Convection zones, powerful magnetic fields are generated by a dynamo process. Basically, a dynamo converts the energy of motion (kinetic energy) of an electrical conductor into electromagnetic field energy. The ionized matter (plasma) within the solar convection zone is a good conductor of electric current, with the ability to retain and “freeze” the magnetic field. The Sun rotates differentially, with a rotation period at the equator of about 25 days, increasing to 30 days above 60° latitude. This differential rotation modifies the magnetic fields generated by dynamo action in the convection zone.

According to the Dynamo Theory advanced by Babcock, Eugene Parker, and others, the solar magnetic cycle starts with the Sun’s global, dipolar magnetic field lines of force extending from pole to pole and frozen into the plasma a few hundred kilometers beneath the photosphere. The greater rotational speed at the lower latitudes means that the frozen-in magnetic field lines are carried faster and thus farther than those at higher latitudes. In due course, after many rotations, the magnetic field lines become tightly wrapped around the Sun, producing an intense east-west azimuthal magnetic field parallel to the equator. Adjacent magnetic field lines tend to repel one another, and when the field strength is a few thousand gauss, the repulsive force is strong enough that the field lines burst through the solar surface and loop back to reenter at a neighboring point, thus creating sunspots of opposite polarity and intense magnetic fields. The temperature around sunspots is lowered (dropping about 1,000 to 1,500 kelvins) as a result of lowered pressure in the region, caused by adjacent field lines repelling each other.

The Dynamo Theory successfully explains the reversed polarities of leading sunspots in terms of the opposite directions of the azimuthal field lines in the two hemispheres during a given sunspot cycle. The local azimuthal field gradually neutralizes the general dipole field, eventually reversing the polarity and starting the next sunspot activity cycle. The polarities, both of the Dipole field and of the leading sunspots, are reversed from those of the previous activity cycle, so this starts the second half of the magnetic cycle. After two sunspot cycles, the magnetic cycle repeats in polarity.

Sunspot pairs occur as a result of random bursting of magnetic field lines upward from within the photosphere and their reentering at a neighboring point to resume their path around the Sun. Each sunspot has a central darker umbra, with a temperature of about 4,500 kelvins, surrounded by a lighter penumbra, with a temperature of about 5,500 kelvins. Most sunspot umbrae have diameters ranging between 4,000 and 22,000 kilometers, and magnetic field strengths ranging between 2,500 and 3,500 gauss. Smaller sunspots, with umbrae ranging from 1,400 to 3,600 kilometers in diameter and field strengths averaging 2,000 gauss, are known as “pores.” Compact magnetic structures with diameters less than 1,000 kilometers, field strengths less than 1,500 gauss, and no discernible penumbral region are called “magnetic knots.” The lifetime of the sunspot and its associated magnetic field typically is days to weeks, and in general it is found to be proportional to the total field strength of a sunspot or an active region.

Large clusters of magnetically active regions tend to produce prominences and flares because of excessive magnetic buoyancy. Prominences are relatively cool masses of gases arching above the photosphere into the corona, following magnetic lines of force. According to the Skylab data of 1973, eruptive (or active) prominences, triggered by large-scale magnetic fields associated with major sunspot activity, widely dissipate the field and disperse charged particles out into interplanetary space. Flares (even more violent bursts of energetic charged particles and electromagnetic radiation) occur near complex groups of sunspots, and in the process, intense magnetic field lines emerge and dissipate into space.

When high-energy charged particles from large flares reach Earth, they interfere with Earth’s geomagnetic field and terrestrial communications systems. Coronal mass ejections (CMEs) are giant magnetic bubbles of ionized gas that carry enormous amounts of energy into space. If they encounter Earth, they can dump enough energy into its magnetosphere to cause disruptions of communication and electrical power distribution systems. Such phenomena gradually finetune the solar dynamo mechanism for the succeeding cycle. The solar wind (the continuous flow of charged particles from the corona into the far reaches of interplanetary space) carries a tenuous magnetic field with it as well. Fluctuations in the solar wind, as well as in X-ray and ultraviolet emissions from the Sun, are closely correlated with the magnetic cycle.

Evidence for long-term variability in solar activity and corresponding fluctuations in the solar magnetic field comes from the quantitative analysis of radioactive carbon-14 deposition in tree rings. Cosmic rays from various sources normally reach Earth’s upper atmosphere and produce several radioactive isotopes, including carbon 14, which results from a collision between energetic particles and nitrogen nuclei. During a prolonged period of reduced sunspot activity, the lack of turbulence in the Sun’s magnetic field will allow more intense cosmic rays to reach Earth, thus increasing the rate of production of carbon 14, which is absorbed by vegetation and eventually deposited in tree rings. Historical records (often indirect) show a good correlation between climate variations and various indicators of solar activity (such as sunspot numbers, auroras, and the size of the corona seen during solar eclipses). The Spörer minimum of the mid-fifteenth century (corroborated by the recorded paucity of auroras), the Maunder minimum (a seventy-year hiatus in sunspot activity, beginning about 1645), and the Little Maunder minimum between 1800 and 1830 are examples of periods of solar inactivity and colder climates on Earth.

Searches for magnetic cycles in nearby Sun-like stars indicate that in earlier times, the Solar cycle may have been more irregular, erratic, and intense than it is now. Stronger magnetic fields, causing a significant loss of electromagnetic and particle energy from the Sun, may have contributed to a loss of angular momentum, thus decreasing the rotation rate. In any case, as stars age, their magnetic cycles tend to become more regular and well-established. The Sun’s magnetic field strength is expected to decline as it slowly enters the red giant stage. The expanding Sun’s rotation rate will slow down and the mass density of its convection zone will decrease, thus reducing the dynamo effect. Chromospheric emissions triggered by the magnetic field dissipation process are expected to subside to a low level; however, studies of chromospheric emission activities of some subgiants show a curious revival of phenomena associated with their magnetic fields.

Knowledge Gained

Clearly, there is still much to learn about solar magnetism. What is known has been gleaned by both earthbound and space-based observations from the mid-twentieth century onward. The solar magnetograph invented in 1952 by Horace Babcock and his father, Harold Babcock, marks a milestone in the observational study of the Sun’s magnetic field. Observations from space by the instruments onboard Skylab and the Solar Maximum Mission of regions of the electromagnetic spectrum blocked by Earth’s atmosphere from reaching the ground have enhanced the understanding of solar magnetic phenomena and their astrophysical implications.

The advent of new and improved magnetometers has yielded a wealth of data concerning the Sun’s magnetic field. Observations and measurements of small and large magnetic features and related solar atmospheric activities have led astrophysicists to develop an elaborate dynamo theory to explain the origin and variations of the solar magnetic field. Astrophysicists have established that the twenty-two-year magnetic cycle is responsible for most, if not all, of the phenomena collectively referred to as solar activity (such as sunspots, prominences, and flares), as well as the processes that result in the Solar wind and the formation of the interplanetary medium.

The Skylab data of 1973 amply demonstrates the link between the magnetic activity cycle and stress-relieving eruptive phenomena such as prominences, flares, and coronal emissions. Solar plasma, carrying with it extensive unstable magnetic fields, interacts with terrestrial magnetic fields to affect the size and shape of Earth’s magnetosphere and processes occurring in it. Data from the Solar Maximum Mission showed modulations in the output of solar energy attributable to the time-varying, churning action of the Sun’s magnetic activity cycle.

Questions have been raised concerning the magnetic stability of a convective shell dynamo such as the one envisioned for the Sun. Pointing out the difficulty of initiating in the solar atmosphere the large electric currents associated with the observed magnetic field, numerous theorists have suggested that there may be a potent, primordial magnetic field frozen within the solar core. It has been hypothesized that when interstellar nebulae with tenuous magnetic fields collapse to form stars, they can retain intense primordial fields over a relaxation time of some 5 billion years, which is the present age of the Sun.

Context

Almost half a century after Hale’s discovery of the strong magnetic fields associated with sunspots, in 1952 Babcock and Babcock devised the modern solar magnetograph, opening up a new era of solar observational physics. Knowledge of the Sun’s observed differential rotation, inferences about the Sun’s convective zone drawn from computer models of its interior, and the laws of electrodynamics from physics were combined to develop the theory of the solar dynamo to explain observed magnetic phenomena.

While the larger pieces of the puzzle appear to be in place, many missing elements blur the picture. The solar magnetic field is not uniform, but filamentary or tubular. The precise manner in which the large-scale field produces observed emission features and expels unstable field elements, seemingly perpetuating the solar dynamo and creating the solar wind, is not well understood. It is known that a large-scale solar magnetic field can lead to a variety of mechanical wave modes. These may contribute to a heating mechanism, but this notion requires further observational confirmation. Virtually all activity in the solar atmosphere (except for the granulation in the photosphere) owes its existence to and is orchestrated by the turbulent and somewhat unpredictable solar magnetic cycle, yet many astrophysically important details are not known. Numerous attempts at improving the dynamo model, including the invocation of a strong primordial magnetic field, have proved to be only marginally successful. Long-term, detailed observations of the finer magnetic elements and processes, possibly through remote sensors, will be required, along with concurrent theoretical refinements.

An area of immediate and compelling interest is the further study of the long-term effects of solar activity cycles on terrestrial weather patterns and climate trends. Cosmic rays and their modulation by magnetic fields play an important role in genetic mutation and the evolution of life in general. Quantitative observation in this area will aid in evaluating the extent to which the deflecting action of the solar magnetic field has affected life. Further probing in this area may bring to light relationships between the evolution of life on Earth and the solar magnetic cycle.

Finally, research on the Sun’s magnetic field has added to our insight into stellar magnetic fields in general. It has been firmly established that the Sun’s magnetic cycles, flare phenomena, and coronal properties are common at least among main sequence stars similar to the Sun. For astronomers, then, the Sun is a cosmic laboratory, a window to the realm beyond the solar system.

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