Electromagnetic radiation: nonthermal emissions

Objects emit light in many ways. Different conditions cause different forms of light to be emitted. Understanding the various forms of light emission can help understand the conditions where the light is emitted by studying the light itself.

Overview

Electromagnetic radiation is the name given to the phenomenon of waves of coupled electric and magnetic fields. Visible light is one form of electromagnetic radiation. Other forms, such as radio waves and X-rays, are different solely in the Frequency at which they oscillate and their wavelengths. The energy of electromagnetic radiation is linearly proportional to the frequency of the radiation. Various physical processes produce different forms of radiation. Thermal radiation is the most commonly produced electromagnetic radiation (sometimes called blackbody radiation). Thermal radiation is produced by the molecular motion of anything not at absolute zero temperature. A plot of intensity versus wavelength for thermal radiation produces a well-defined characteristic spectral profile. However, many other physical processes produce electromagnetic radiation other than through thermal means.

One of the most common forms of nonthermal electromagnetic radiation is emission spectra. When an electron in an atom jumps from a higher-energy state to a lower one, it emits electromagnetic radiation. The particular wavelength of radiation emitted depends on just the energy states in which the electron started and finished its transition. The greater the energy difference, the higher the frequency and the shorter the radiation wavelength. Different atoms have particular permitted energy levels for their electrons, and therefore, they emit only specific radiative wavelengths. This is unlike thermal radiation, which is continuous across many wavelengths. All atoms of a particular chemical element have the same possible energy levels, so they will all have the same possible spectral line emissions, or spectral “signature” or profile. Thus, an object's spectral lines can be analyzed to determine its chemical composition.

Another form of electromagnetic radiation emission that is similar to the emission spectra from electron transitions occurs through electron spin-flip transitions. Electrons have a small angular momentum called the spin of the electron. This gives the electron a small magnetic moment. If that magnetic moment is aligned with that of the atomic nucleus, then the atom has more energy than if it were aligned in the opposite direction. Therefore, if the electron flips its spin, the atom must emit a photon of light having energy equal to the difference in energy between the two states. One of the most important examples of this type of transition is the spin flip of hydrogen, which produces electromagnetic radiation with a characteristic wavelength of twenty-one centimeters.

Electromagnetic radiation comes from changing electric and magnetic fields, so any motion by a charged particle can produce electromagnetic radiation. In particular, certain types of electromagnetic radiation are produced from accelerated charges. One example of this type of radiation is synchrotron radiation. Acceleration is a change in velocity. That change can be from speeding up or slowing down, or it can be from a change in the direction of the velocity. Objects moving around in circular or other curved paths change direction even if they do not change speed. If these objects are charged particles, then this curved motion results in the emission of electromagnetic radiation. This radiation is called curvature radiation or cyclotron radiation. However, if the charged particles are moving at or near the speed of light, the radiation is called synchrotron radiation, and it takes on particular properties. The synchrotron radiation is beamed along the path of the electrons, which amplifies its intensity, and it is polarized (meaning that the electric fields of the various electromagnetic waves are aligned). A synchrotron radiation spectrum shows a drop in intensity with frequency, making an easily recognized spectral profile. A common synchrotron radiation source is electrons or other charged particles moving in a magnetic field. Jupiter’s magnetosphere emits synchrotron radiation.

Another form of radiation produced by accelerated electrical charges is Bremsstrahlung radiation. The name comes from the German word meaning “braking radiation” because it is produced when fast-moving charged particles abruptly slow down. Technically, the term Bremsstrahlung radiation can be applied to any radiation emitted by an accelerating charged particle, including synchrotron radiation, but in practice, the term is frequently limited to cases where the particles slow down (negative acceleration). Bremsstrahlung radiation can be generated, for example, during solar flares. The energy released in the flare event can accelerate electrons near the surface of the Sun to almost the speed of light. The electrons then interact with the outer layers of the Sun to slow down, producing a burst of X-rays from the Bremsstrahlung radiation of their deceleration. Like synchrotron radiation, Bremsstrahlung radiation has a spectral profile that decreases in intensity with increasing radiation frequency.

Another form of nonthermal emission is Cherenkov radiation, which is also produced by ultrafast charged particles. Nothing can travel faster than light in a vacuum. However, light travels more slowly through a medium than through a vacuum. When a charged particle moving near the speed of light enters a medium in which the speed of light is slower than the particle, the changing electromagnetic fields produced by the particle’s motion augment each other to produce electromagnetic radiation moving in the same general direction as the particle’s motion. The particle slows as a result. While this sounds much like Bremsstrahlung radiation, the physical process is different. High-energy cosmic rays produce Cherenkov radiation when they enter Earth’s atmosphere.

Yet another nonthermal source of radiation is light amplification by stimulated emission of radiation (laser). An excited atom can be stimulated to emit radiation by the passage of a photon of light having the same wavelength as the light that would be emitted if the atom’s electron were spontaneously to go from a high energy level to a lower one. The stimulated radiation is in the same direction and in phase with the stimulating radiation, amplifying its intensity. The atmosphere of Mars is capable of lasing infrared light. Laser light is characterized by the light’s being coherent; that is, all the light is the same wavelength, moving parallel and in phase.

Pair annihilation can provide yet another form of nonthermal electromagnetic radiation. All particles of matter have a corresponding particle of antimatter. When a matter particle comes together with its antiparticle, the particle and the antiparticle annihilate each other. The combined mass of the particle and the antiparticle is converted into energy through Einstein’s famous relationship E = mc². The energy is realized by a pair of photons, or particles of electromagnetic radiation, moving in opposite directions from the annihilation site. The energy of the particles, and hence their wavelength and frequency, is determined by the particle's mass and the antiparticle.

Knowledge Gained

Electromagnetic radiation is produced in many different ways. The physical process determines the type of radiation produced. Measurement of that radiation and its characteristics can then be used to understand the source of the radiation.

For astronomers, one of the most essential forms of electromagnetic radiation is the emission from electron transitions between energy levels in an atom or molecule. The electromagnetic spectrum produced by a collection of atoms or molecules doing this is called its emission spectrum. The emission spectrum of a body can be used to determine its chemical composition. Furthermore, light shining on the body can excite electrons to higher energy levels if the electrons are in a low energy level. Only light having energy equal to the energy difference between energy levels can be absorbed. Thus, only specific wavelengths of light can be absorbed. These wavelengths are the same as those given off in emission spectra. Therefore, studies of the light absorbed and not reflected can also be used to determine the chemical composition of a body.

The twenty-one-centimeter radiation can detect the presence of hydrogen atoms in space. Synchrotron radiation can be used to probe the magnetospheres of planets. Bremsstrahlung radiation is essential in the study of planetary atmospheres and the Sun. Cherenkov telescopes have been constructed on Earth to study the nature of cosmic rays. Studies of the lasing properties of planetary atmospheres also lead to a better understanding of those atmospheres.

Though some are visible light, much of the nonthermal electromagnetic radiation is in forms other than visual light. Thus, studies of nonthermal radiation use techniques from all areas of astronomy: optical, radio, X-ray, infrared, and ultraviolet astronomy. Many forms of this radiation could only be studied once astronomers had developed the tools to study non-optical wavelengths of light. Our understanding of the universe has greatly improved since it is possible to study all these wavelengths and the character of many forms of nonthermal radiation.

Context

Astronomers study objects that are very far away. Unlike most other scientists, astronomers do not have the luxury of being able to have the object of their study in a laboratory setting but instead have to look at it from afar. This means primarily studying the electromagnetic radiation of the objects. While robotic spacecraft can conduct studies in situ on the surface of a world, only a very few planets have been probed in this manner. Other spacecraft have flown by planets or even been placed into orbit around those planets. However, most of these spacecraft have conducted their studies using electromagnetic radiation. Understanding the nature of this radiation and how it is created can lead to a far greater understanding of the bodies producing it.

Every object that is not at absolute zero temperature emits thermal radiation, and studying the thermal radiation emitted by a body is critical to understanding it. However, far more information can be gleaned from studies of the nonthermal radiation emitted. The tools and techniques of doing so have become among the most essential tools in the astronomer's arsenal for understanding celestial bodies.

Bibliography

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Kornecki, P., et al. "Exploring the Physics Behind the Non-Thermal Emission from Star-Forming Galaxies Detected in γ Rays." Astrophysical Processes, vol. 657, 5 Jan. 2022, doi.org/10.1051/0004-6361/202141295.

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Smith, Randall. "Non-Thermal Radiation Processes." University of Maryland, www.astro.umd.edu/~richard/ASTR680/AstroH-SS10-Nonthermal.pdf. Accessed 20 Sept. 2023.