Solar Flares

Type of physical science: Astronomy; Astrophysics

Field of study: Stars

A solar flare is a high-energy outburst in the chromosphere of the sun that emits a variety of electromagnetic radiation, ranging from energetic γ (gamma) rays to radio wavelengths. The emissions cause modification of the earth's ionosphere, which adversely affects communications and various forms of technology.

Overview

Solar flares are among the most spectacular directly observable dynamic sights in the heavens. These violent phenomena are insignificant compared to quasars and black holes, but the latter are not readily observable, while solar flares are immense and nearby. Flares are sudden brightenings of the sun that emit bursts of increased radiation. They occur in the lower atmosphere of the sun (chromosphere), where there are strong magnetic fields. The strongest magnetic fields undergoing the greatest change are associated with and located near the center of sunspot groups.

Sunspots are cyclical. They appear and disappear with varying frequency, though on average solar cycles last for approximately eleven years. In the nineteenth century, Swiss astronomer Johann Rudolf Wolf established a numbering system for solar cycles, designating the 1755–66 cycle as the first; solar cycle 24 began in January 2008.

Solar flares are intense and relatively short lived. The intensity of emission rises in a few minutes, increasing more than ten times in brightness in the visible range alone and often resulting in catastrophic eruptions. In those few minutes, the brightening may include a billion square kilometers of the sun's surface (up to a thousandth of the entire solar disk). This explosive development frequently begins in the upper portion of the chromosphere and then moves upward through the sun's atmosphere at a rate of up to one hundred kilometers per second, often reaching a height between seven thousand and sixteen thousand kilometers above the photosphere. The expanding edge moves outward at velocities approaching one hundred thousand kilometers per second.

Flares are commonly associated with fast movements in the corona, the tenuous magnetized envelope around the sun. The brightest flares tend to be the most explosive, reaching a peak in five to ten minutes and then fading over a period of up to two hours. Large flares are more magnetically complex and may have a visible filamented structure. The more magnetically complex the solar environment, the more likely flares are to occur, since colliding magnetic fields are the most common cause of flares. The absence of disturbances in the photosphere during flares has led to the assumption that energy is stored in the magnetic fields anchored in the photosphere. The closer the energy emission is to the photosphere, the broader the spectrum of radiation and the more likely the flare is to be visible in white light.

Flares emit a wide variety of electromagnetic radiation, generally created by nonthermal mechanisms. They emit energy at wavelengths ranging from one hundred millionth of a centimeter (gamma rays and x-rays) to radio wavelengths of up to ten kilometers, covering the spectrum of ultraviolet, visible, and infrared energy in between. The different kinds of radiation come from different portions of the flare and from different altitudes within it. There is immense variation in the energy levels of the emissions, ranging from approximately one thousand to fifteen billion electronvolts. An astounding amount of energy is involved in a large flare; estimates range as high as 10³³ ergs, or more than the kinetic energy of the entire solar atmosphere in the visible range.

In addition to radiation, high-energy particles are also emitted during the flare. The emission of particles causes disturbances in the corona called flare surges, which frequently follow the magnetic lines of force in the region. Shock waves associated with this phenomenon may be sufficiently strong to be observed on the sun's surface. At times, the effect of a flare can be traced as far out as six hundred thousand kilometers by the effect of the shock wave on thin gaseous filaments in the sun's atmosphere. The magnetic field is often significantly altered or dissipated after the flare, although there is a gradual recovery of the magnetic field and the potential for further flares in the same area.

Flares emit great quantities of matter into space at velocities of up to five hundred kilometers per second. One of the major characteristics of flares is rotational motion of masses of gas, a result of the magnetic-field changes, which leads to the acceleration of particles in the solar atmosphere during the flare. Some of these particles are accelerated vertically beyond the escape velocity for the sun and are thus driven into interplanetary space.

Eruptions eject as much as a billion tons of gas at velocities approaching 1.6 million kilometers per hour. Waves of such particle plasmas generate shocks at distances of up to several astronomical units (AUs) from the sun. Because Earth's orbit is only one AU from the sun, these ejections into space are associated with various effects on the planet's atmosphere. The fastest particles are solar cosmic rays, which are high-energy charged protons moving at an appreciable fraction of the speed of light. During a flare, the sun may emit enough protons to raise the cosmic-ray background on Earth to 180 percent of its normal level.

Solar flares occur most frequently near or within rapidly developing sunspot groups, usually within the first ten to fifteen days of the life of the group, as this is when the most complex magnetic phase of a sunspot group takes place. The flares tend to appear and reappear in association with the same active sunspot regions. Although flares alter the magnetic field, multiple reappearances are possible, which implies that the original magnetic configuration returns between flares. There are thousands of flares during each eleven-year sunspot cycle. Less common are the flares visible in the white- or integrated-light portion of the spectrum.

Flares are associated with various different solar phenomena, the most significant of which is sunspots. The intense magnetic fields impede radiation from below the photosphere, creating an area into which less heat is carried—a sunspot. Each sunspot has a dark nucleus, called an umbra, that ranges from two thousand to twenty thousand kilometers in diameter. The umbra is surrounded by a somewhat brighter penumbra of about four thousand to fifty thousand kilometers in diameter. Sunspots most commonly form bipolar sunspot groups, in which two sunspots with opposite charges are surrounded by many smaller spots.

Solar flares are also associated with prominences, which are cool gaseous formations that stand vertical to the sun's surface. Prominences, like flares, are caused by the magnetic activity of sunspots. They frequently form loops in the solar atmosphere along the lines of magnetic force, reaching as high as fifty thousand kilometers above the sun's surface. Other prominences are filaments of gas that stream away from sunspots. Prominences, like flares, are formed in sunspot belts. When they are near sunspots, they tend to vary rapidly; otherwise, prominences may be quite stable and last up to three hundred days. Prominences are usually not strong enough to eject matter into space, but eruptive prominences may force material to such great heights that escape occurs. Prominences are a more modest source of x-rays than flares, providing some of the particles that cause magnetic storms and auroras.

Scientists are interested in flares because of the fascinating physics associated with their behavior, the grandeur of the size of the phenomena, and the intense dynamism they represent. Their main concern with flares, however, results from the fact that these outbursts have a significant effect upon human communications and other forms of complex technology.

Applications

The effects of greatest importance to astronomers are the results of flare activity on the earth. There are two basic types of effect observable: those that are nearly simultaneous with the flare and those that are delayed by a day or more. The simultaneous effects can be observed roughly eight and a half minutes after the flare begins, as that is the amount of time it takes for light and other radiation to reach the earth from the sun. The delayed effects are caused by the arrival of the particles that were ejected from the sun, which are traveling more slowly and reach the earth a day or so later.

During a flare, the most notable effects occur in the upper portion of the earth's atmosphere, known as the ionosphere. In this very tenuous gaseous region, the number of free electrons increases, and the amount of electrical charge or current increases in an abrupt fashion, called either a "kick" or a "crochet." There is an immediate shortwave-radio fade-out that begins with the peak of the flare. The fade-out lasts for twenty-five minutes or so, after which the signal begins to recover. During the fade-out, the radio signal drops to as low as one-tenth of its normal level. On the other hand, the reflectivity of the ionosphere for long radio wavelengths is enhanced. This increased reflectivity also occurs at a lower level in the ionosphere and produces an anomaly in the phasing of waves received horizontally and by reflection. The changes in the ionosphere tend to suppress the normal background radio noise.

Among the delayed effects, changes in the earth's magnetic field are the most prominent. An increase in the strength of the magnetic field, combined with the increased ionization of the earth's atmosphere, leads to the brilliant displays of the aurora borealis and aurora australis, also known as the northern and southern lights. The auroral displays are not only caused by flares, since the sun emits particles on a regular basis, but the flares enhance the effects observed on the earth. The shimmering displays of light become prominent to greater altitudes, may increase in brightness, and may be seen from closer to the equator than usual; during such periods, the aurora borealis can sometimes be seen as far south as the US-Mexican border. The heightened aurora indicates the arrival of charged particles from the sun. Flares send particles in all directions, but Earth receives the most particles when it is directly above the flare—that is, when the flare is near the center of the solar disk. The most intense magnetic storms follow such an event.

While flares were visually observed in the 1860s and many useful discoveries and associations were made, the systematic study of flares began in 1891, with the invention of the spectrohelioscope by George Ellery Hale. The sunspot cycles of 1936, 1947, and 1958 were studied from ground-based observatories, with the most intense effort made during the International Geophysical Year, which lasted from July 1957 to December 1958. Various filters and spectrographs were used to accumulate information, and dramatic advances were made. These studies tended to focus on some narrow portion of the electromagnetic spectrum, with an increasing awareness that there were probably emissions in other parts of the spectrum that could not be measured from Earth because the radiation was blocked by the planet's atmosphere. Later discoveries confirmed that the emissions were much stronger in the x-ray and radio portions of the spectrum than in the ultraviolet and visible portions.

While instruments borne above the bulk of the earth's atmosphere by balloons were an aid to research, the advent of rocketry was a significant development. Rockets allowed exploration of the shortest wavelengths for the first time, which led to major revisions in the physical theories used to explain flares. From 1949 to 1960, Herbert Friedman and a team of scientists used rockets to observe x-rays from the sun through a complete sunspot cycle. The rocketry of the late 1950s and early 1960s led to only limited advances, however, because the time rockets could spend above the atmosphere was so limited.

Most of what is known about the structure of flares and the mechanisms that cause them comes from satellite-based research. Satellites allow continuous observation, with the greatest advances coming since the August 1960 launch of Solrad I, the first satellite dedicated to observing solar radiation. The most significant early study was of x-ray emissions from the sun. The very first satellite results pointed to the areas around sunspots as strong x-ray-emitting regions. The flares were strong x-ray emitters, far above the normal soft or longer-wave x-rays routinely emitted by the solar corona. Because x-ray emissions are associated with solar flares, the amount of x-rays received by Earth varies dramatically over the eleven-year sunspot cycle.

Another significant accomplishment in the study of solar flares was the launch of the Solar Maximum Mission (SMM) satellite in 1980. Seven different instruments provided information from a variety of overlapping wavelengths, ranging from visible light to gamma rays. Although the satellite failed some ten months later, it was repaired in 1984 and provided a flow of useful information until it reentered the atmosphere in 1989.

One of the most difficult problems associated with flares has been their classification. In the past, some astronomers argued that there was no significant difference between flares; they simply represented different power levels of the same phenomenon. Strong flares allowed for the observation of all the phenomena, while weaker ones were beyond the ability of existing equipment to detect. Early classification systems simply arranged flares on a scale of one to four depending on brightness, usually in the visible spectrum. Some scientists used a fifth designation, negative one, to represent "subflares" (microflares) or exceedingly dim flares. Increased knowledge of the radiation at various wavelengths, especially gamma rays and hard x-rays, demonstrated that the brightest visible flares were not the brightest emitters at other wavelengths, and vice versa. This knowledge led to new classification systems based on other phenomena. The letters A, B, C, M, and X are used as classes, followed by a number—usually between one and nine, though X-class flares can take higher numbers. The strongest recorded flare to date was an X28, observed in November 2003.

In addition to the difficulties that flares cause for those involved in radio and television communications, they also pose a potentially lethal hazard to astronauts who venture beyond the earth's magnetic field, which deflects and provides protection from some of the high-energy particles ejected from the sun. Special shielding and a better ability to predict solar emissions are needed. Passengers and crew in high-altitude aircraft are at risk of radiation exposure as well, though at far lower levels.

With the development of ever smaller and denser microcircuitry, the effects of random incoming radiation have increased. These effects are called single-event upsets and may alter, reverse, or otherwise damage semiconductor circuits. High-energy particles can upset random-access memory (RAM) in computers, although the memories will usually reaccept information. Handheld electronic devices, such as smartphones and tablets, are vulnerable to magnetic energy and likely to malfunction. Flares can also affect satellite navigation systems, including aircraft navigation systems, especially at high altitudes or near the poles.

A more serious problem occurs when an electric charge is deposited, actually changing information. This has become an increasing problem for computerized control of aircraft and may eventually be addressed by the use protective shielding, as is done with satellites. Also vulnerable are long pipelines, for which the deposit of a charge can be both dangerous and corrosive; cathodic protection, in which the surface metal of the pipeline is made to function as the cathode of an electrochemical cell, is a common countermeasure. Induced ground currents may heat up power grids beyond their capacity, causing power fluctuations or even blackouts.

Finally, emissions from the sun can affect the orbits of satellites. The outgoing streams of particles and the magnetic fields together form a blast of solar wind that intensifies the density of the ionosphere, creating extra drag on satellites in orbit. One particularly strong flare on March 6, 1989, caused the SMM to drop a kilometer below its orbit.

Context

For a period of five minutes, at a little after 11:00 a.m. on September 1, 1859, while Richard C. Carrington was mapping sunspots, he saw two bright patches of light on the sun. They moved over the surface of one of the spots he was mapping, while the spot itself remained unchanged. Carrington reasoned that he had observed a solar atmospheric phenomenon that had occurred above the sunspot. Independently, R. Hodgson observed the same two spots and also reported them. This is the first recorded visual sighting of a solar flare in the astronomical literature. Carrington noted that the flare occurred at a time of intense magnetic storms, which lasted from August 28 to September 4 of that year.

From the 1870s on, spectroscopic flares were observed quite often by various astronomers, who all noted momentary bright reversals of the dark absorption lines in the spectra, indicating the presence of flares. The connection between flares and magnetic storms continued to be noted, and a sense of causation grew among the astronomers, although as late as 1892, the physicist Lord Kelvin was disputing the connection. Prior to the 1890s, the distinction between a solar prominence and a flare was not made.

Knowledge of the sun, its features, and its emissions continues to expand rapidly, especially since the launch of the SMM in 1980 and subsequent satellite-based observing devices. One notable accomplishment was the study of the great flare of March 6, 1989, the largest since satellite observations began. This particular flare released as much energy as ten trillion one-megaton hydrogen bombs. The great flare was followed by ten more extremely strong flares. The new technology made the observation of this significant event possible.

Research continues to be done on the particulate emissions of flares that have an impact on communications. The expansion of space-based flare sensing will allow warning of forthcoming communication problems, allowing satellites to switch to alternative links for improved continuity in communications. More study of solar magnetic fields is needed in order to better understand the processes that create solar flares and how they might affect life on Earth..

Principal terms

AURORA: a luminous display of streamers, or arches of light, caused by the influx of charged particles into the earth's magnetic field

CHROMOSPHERE: the lower atmosphere of the sun, just above the photosphere; has a temperature of about four thousand kelvins

CORONA: the thin upper atmosphere of the sun, above the chromosphere; reaches temperatures of one million kelvins or more

ELECTRONVOLT: the amount of energy acquired by an electron when it is accelerated by an electrical potential difference of one volt; one electronvolt is about two-trillionths of an erg

ERG: the amount of kinetic energy of a mass of two grams moving at one centimeter per second; a mosquito in flight possesses about one erg of energy

GAMMA RAYS: high-energy, short-wavelength emissions from the sun that affect the earth's ionosphere

IONOSPHERE: an upper layer of the earth's atmosphere, above sixty kilometers in altitude, that contains ionized particles and electrons

PHOTOSPHERE: the dense visible surface of the sun; has a temperature of about five thousand kelvins

Bibliography

Bai, T., and P. A. Sturrock. "Classification of Solar Flares." Annual Review of Astronomy and Astrophysics 27.1 (1989): 421–68. Print. While highly technical and rather narrowly defined topically, this article remains significant for the more than two hundred bibliographic entries it contains. There is enough description to give the general reader a sense of how complicated the problem of classifying flares remains.

Dance, Scott. "Brewing Solar Storm Looms over Technology." Baltimore Sun. Tribune Interactive, 23 June 2012. Web. 15 Jan. 2014.

Ellison, Mervyn Archdall. The Sun and Its Influence: An Introduction to the Study of Solar-Terrestrial Relations. London: Routledge, 1955. Print. Chapter 5 discusses what was known of the physics of flares in a narrative form. Primarily of historical interest.

Emslie, A. Gordon. "Explosions in the Solar Atmosphere." Astronomy Nov. 1987: 18–23. Print. This is an excellent popular survey of the kinds of research under way in the late 1980s, with clearly written background information on the nature of flares themselves.

Fan, Yuhong, and George Fisher, eds. Solar Flare Magnetic Fields and Plasmas. New York: Springer, 2012. Print.

Fuller-Wright, Liz. "Solar Flares: X-Class (Yes, That's a Real Thing)." Christian Science Monitor. Christian Sci. Monitor, 25 Oct. 2013. Web. 15 Jan. 2014.

Kundu, M. R., B. Woodgate, and E. J. Schmahl, eds. Energetic Phenomena of the Sun. Boston: Kluwer, 1989. Print. This volume in the Astrophysics and Space Science Library was the result of three meetings held at the Goddard Space Flight Center in January and June of 1983 and February of 1984. While it is highly technical, there are a few descriptive passages of interest to the general reader. Valuable for its bibliography.

Maxwell, Alan. "Solar Flares and Shock Waves." Sky and Telescope Oct. 1983: 285–88. Print. This article includes a summary of information about flare-generated shock waves as a source of acceleration for particles emitted by the sun. Considers some difficult concepts in a readable way.

Meadows, A. J. Early Solar Physics. Elmsford: Pergamon, 1970. Print. This brief historical account contains a fine summary of solar physics and significant information about related topics. Includes reprints of the original reports by Carrington and Hodgson in the appendices.

Rust, David M. "Solar Flares, Proton Showers, and the Space Shuttle." Science 216.4549 (1982): 939–46. Print. This article surveys the dangers of radiation from flares to occupants of the space shuttle, suggesting that there is no adequate level of shielding possible. Excellent diagrams, and only semitechnical.

Ryan, James M. "The Solar Maximum Mission." Astronomy May 1981: 6–16. Print. This is an excellent overview of the state of knowledge about solar flares in 1980.

Verschuur, Gerrit. "The Day the Sun Cut Loose." Astronomy Aug. 1989: 48–51. This is an excellent popular summary of the magnitude and effects of the great flare of March 6, 1989. Contains a reliable summary of information about the general nature of flares.

Vita-Finzi, Claudio. Solar History: An Introduction. Dordrecht: Springer, 2013. Print.

Sunspots and Stellar Structure

X-Ray and Gamma-Ray Astronomy