Solar X-ray emissions

The Sun emits X-rays and gamma rays, revealing the presence of extremely high temperatures (tens of millions of kelvins) and high-energy particles. Solar X-rays are produced in the solar corona (the hot, tenuous outer atmosphere of the Sun). Magnetic explosions or solar flares generate X-rays and gamma rays.

Overview

X-rays and gamma rays are forms of electromagnetic radiation with very short wavelengths and very high-energy photons. Electromagnetic radiation displays the properties of waves and particles; the term photon refers to electromagnetic radiation when it acts like a stream of particles. The wavelengths of X and gamma rays are less than ten nanometers compared to visible light, which ranges from about 400 nanometers (violet) to 700 nanometers (red). Their photons have energies ranging from about 100 electron volts (eV) upward; for comparison, visible light photons have energies from a little less than two to a little more than three eV. Because their photons are so energetic, X-rays and gamma rays generally indicate high temperatures and interactions of high-energy particles

The existence of temperatures of at least a million kelvins in the solar corona (the Sun’s outer atmosphere) has been known since the 1940s because of the emission lines of highly ionized elements in its optical spectrum. Why should the corona be so hot? The photosphere, or visible “surface” of the Sun, is much cooler (around 5,800 kelvins), and common sense had misled astronomers to expect a steady decrease of temperature outward rather than the precipitous temperature increase that occurs.

The physical cause of the high temperatures of the solar corona is thought to be the strong magnetic fields in the Sun. The Sun’s magnetic field is complex, with loops or arches extending beyond the photosphere into the corona. Charged particles flow along these loops, gain energy from them, and transfer it to the corona. Observations show that the corona is not uniform at X-ray wavelengths. The strongest brightest X-ray emission comes from regions where the magnetic field is the strongest, and the gas is the hottest. Coronal streamers of hot ionized gas follow the Sun’s magnetic field lines outward from these areas. Coronal holes are large, dark (in X-rays), cool regions with weak or absent magnetic fields.

Solar flares were discovered serendipitously in 1859, in ordinary visible light, by Richard Carrington, who had been making routine observations of sunspots. Subsequent observations at certain specific wavelengths (such as the red light at 656.3 nanometers emitted and absorbed by hydrogen atoms) disclosed flares more distinctly in the solar chromosphere (that part of the Sun’s atmosphere in between the photosphere and the corona), so before the 1950s, the general phenomenon was known as chromospheric flares. They were not assigned much importance physically.

X-ray and gamma-ray observations changed this perception of the importance of solar flares. Whereas solar flares had been perceived as complicated but otherwise insignificant features of the solar atmosphere—some kind of solar cloud—they were discovered to be a fundamentally important phenomenon, the prototype object of high-energy astrophysics. This branch, which involves observations of X-rays and gamma rays, brings together gravitational, atomic, nuclear, and plasma physics.

X-ray bursts accompany essentially every solar flare, and during times of great solar activity, this occurs many times per day. The “soft” (longer-wavelength, lower-energy) X-ray spectrum reveals the existence of dense plasmas, hot ionized gases composed of electrons and atomic nuclei with temperatures in the range of ten to twenty million kelvins, some ten times the typical coronal temperatures. X-ray images obtained with grazing-incidence telescopes show the hot plasmas to be trapped in the magnetic tubes, or loops, that may rise about 100,000 kilometers above the photosphere of the Sun. The energy trapped in the hot plasma flows down the magnetic field lines over a period of minutes, feeding into the chromosphere. The plasma, shown by its soft X-ray emission, appears to be the central agent in producing many of the classical effects of solar flares.

What initially produces the hot flare plasma? Its creation is usually marked by a “hard” X-ray burst (shorter-wavelength, higher-energy X-rays). These hard X-rays come from the interactions of fast electrons, with energies far above those of the particles in the hot plasma responsible for the soft X-rays. Furthermore, data from the Solar Maximum Mission (SMM) have shown that gamma-ray bursts also occur commonly in solar flares. The presence of gamma rays indicates that the high energy required to produce flares involves the acceleration of protons (and other ions) and electrons. X-rays and gamma rays have revealed many mechanisms involved in flare events. Magnetism plays a crucial but ill-understood role in causing the plasma instabilities that put on such spectacular and dramatic displays in solar flares. However, the initial cause of these events—and hence a general theory of flares—remains elusive.

In sum, a solar flare is believed to originate as an instability occurring in the solar atmosphere. This heterogeneous magnetized plasma extends upward from the photosphere (the Sun's visible surface) into interplanetary space. This instability results in the acceleration of high-energy particles, both electrons and ions, and the creation of plasmas with temperatures of tens of millions of kelvins. A flare is a magnetic explosion in the upper solar atmosphere, leading to rapid acceleration of high-energy particles and an outward eruption of denser solar material from near the photosphere. The flare features observable by ordinary techniques of optical spectroscopy from ground-based telescopes appear to be secondary products of this explosive release of magnetic energy.

Knowledge Gained

Solar flares have connections to Earth that are both economically significant and scientifically important. Their high-energy radiation can perturb Earth’s ionosphere (a layer of ionized atoms in the upper atmosphere), inducing surge currents to flow in electrical power grids, causing failures and sometimes extensive blackouts. Similar disturbances of the ionosphere can also interrupt radio communications because the ionosphere reflects some types of radio communication. Unfortunately, solar physicists cannot predict the level of solar activity with much more precision than is afforded by the simple recognition of the well-known eleven-year sunspot cycle. They can predict large flares no better than seismologists can predict major earthquakes. The problem appears to lie in the complexity of the physics and the lack of adequate observations.

The high-energy radiation of a solar flare is produced in various ways. Soft X-rays are produced by hot plasmas (with temperatures exceeding ten million kelvins). In a major solar flare, the X-ray flux may reach as high as one-millionth of the total solar luminosity. Hard X-rays are intense flashes of higher-energy X-rays that occur near the onset of a solar flare, showing the acceleration of energetic electrons. Gamma rays are produced by nuclear interactions in solar flares, showing that particle acceleration in solar flares extends to protons and other heavier ionized particles. The spectra, time profiles, and spatial distributions of the high-energy radiation all guide the physics of the flare phenomenon. Fairly detailed observations of soft X-rays' properties have been obtained, but many observational gaps exist for hard X-rays and gamma rays.

The introduction of grazing-incidence X-ray optics, first from sounding rockets and later (in 1973) from the Skylab crewed space station, showed that the hot plasmas responsible for X-ray emission from solar flares were trapped in magnetic loops. These structures have their “footpoints” anchored in the solar photosphere but extend great distances into the corona. Since the Skylab era, numerous X-ray observatories have been used to study solar flares and the Sun’s corona, such as XMM-Newton and the Chandra X-Ray Observatory.

Context

The discovery of X-rays and gamma rays from the Sun led directly to a new branch of astronomy, high-energy astrophysics, and significant changes in understanding solar behavior. These discoveries resulted from using instruments designed to detect these high-energy emissions, mounted on instrument platforms ranging from the V-2 rockets captured from Germany during World War II to high-altitude uncrewed balloons and, eventually, artificial Earth satellites and deep space probes. These vehicles could place these detectors above Earth’s atmosphere, which blocks high-energy photons and opens a new view of the Sun and other stars.

After World War II, American researchers observed above Earth’s atmosphere, initially using V-2 rockets captured by the Germans. These observations showed the somewhat unexpected existence of “high-energy” radiation from the Sun, from ultraviolet (UV) to X-rays. Herbert Friedman’s group at the Naval Research Laboratory was instrumental in these observations, paralleling those of James Van Allen, the discoverer of Earth’s radiation belts. A most important discovery came in 1958 when Laurence E. Peterson and John Winckler flew a high-altitude balloon over Cuba for cosmic-ray studies and were able to detect gamma radiation from a solar flare that happened to occur during the balloon flight.

Over the decades since the 1950s, new launch vehicles, combined with remarkable progress in the technology of X-ray optics and X-ray and gamma-ray detectors, have led to a significant expansion of research in this area. Solar observations in the X-ray and gamma-ray spectral regions have considerably broadened our knowledge of the physics of solar flares and, hence, the Sun’s structure and physics.

Additionally, X-rays and gamma rays are observed from a variety of other astronomical sources: white dwarfs, neutron stars, black holes, supernova remnants, galaxies, and clusters of galaxies, to name some. Observatories specifically designed to detect the X-rays and gamma rays of such objects, including the Chandra X-Ray Observatory, the Compton Gamma Ray Observatory, XMM-Newton, the Swift mission, and Astro-E2, have been launched above Earth’s atmosphere and have returned data that have rendered startlingly beautiful images of some of these sources. It can be speculated that some of the same physics of magnetized plasmas in the Sun may underlie the high-energy emissions from these objects.

Bibliography

Chaisson, Eric, and Steve McMillan. Astronomy Today. 9th ed. Prentice Hall, 2020.

Darling, Susannah. "Solar X-Rays: How a CubeSat Sheds New Light on the Sun’s X-Ray Emissions." NASA, 2018, blogs.nasa.gov/sunspot/2018/12/21/solar-x-rays-how-a-cubesat-sheds-new-light-on-the-suns-x-ray-emissions. Accessed 20 Sept. 2023.

Eddy, John A. A New Sun: The Solar Results from Skylab. NASA SP-402. Government Printing Office, 1979.

Fraknoi, Andrew, David Morrison, and Sidney Wolff. Voyages to the Stars and Galaxies. Brooks/Cole-Thomson Learning, 2006.

Freedman, Roger A., and William J. Kaufmann III. Universe. 6th ed. W. H. Freeman, 2019.

Giovanelli, Ronald. Secrets of the Sun. Cambridge University Press, 1984.

Noyes, Robert W. The Sun: Our Star. Harvard University Press, 1982.

Schneider, Stephen E., and Thomas T. Arny. Pathways to Astronomy. 6th ed. McGraw-Hill, 2020.