Solar radio emission

Solar radio emission refers to radio waves that are naturally produced by the Sun, primarily from the lower and upper layers of the atmosphere called the chromosphere and corona, respectively. The Sun produces radio emissions through four known mechanisms, each of which operates primarily by converting the energy of moving electrons into electromagnetic radiation. The four emission mechanisms are thermal bremsstrahlung emission, gyromagnetic emission, plasma emission, and electron-cyclotron maser emission. The first two are incoherent mechanisms, which means that they are the summation of radiation generated independently by many individual particles. These mechanisms are primarily responsible for the persistent "background" emissions that slowly vary as structures in the atmosphere evolve. The latter two processes are coherent mechanisms, which refers to special cases where radiation is efficiently produced at a particular set of frequencies. Coherent mechanisms can produce much larger brightness temperatures and are primarily responsible for the intense spikes of radiation called solar radio bursts, which are byproducts of the same processes that lead to other forms of solar activity like solar flares and coronal mass ejections.

History and observations

Radio emission from the Sun was first reported in the scientific literature by Grote Reber in 1944. Those were observations of 160 MHz frequency microwave emission emanating from the chromosphere. However, the earliest known observation was in 1942 during World War II by British radar operators who detected an intense low-frequency solar radio burst; that information was kept secret as potentially useful in evading enemy radar, but was later described in a scientific journal after the war. One of the most significant discoveries from early solar radio astronomers such as Joseph Pawsey was that the Sun produces much more radio emission than expected from standard black body radiation. The explanation for this was proposed by Vitaly Ginzburg in 1946, who suggested that thermal bremsstrahlung emission from a million-degree corona was responsible. The existence of such extraordinarily high temperatures in the corona had previously been indicated by optical spectroscopy observations, but the idea remained controversial until it was later confirmed by the radio data.
Prior to 1950, observations were conducted mainly using antennas that recorded the intensity of the whole Sun at a single radio frequency. Observers such as Ruby Payne-Scott and Paul Wild used simultaneous observations at numerous frequencies to find that the onset times of radio bursts varied depending on frequency, suggesting that radio bursts were related to disturbances that propagate outward, away from the Sun, through different layers of plasma with different densities. These findings motivated the development of radiospectrographs that were capable of continuously observing the Sun over a range of frequencies. This type of observation is called a dynamic spectrum, and much of the terminology used to describe solar radio emission relates to features observed in dynamic spectra, such as the classification of solar radio bursts. Examples of dynamic spectra are shown below in the radio burst section. Notable contemporary solar radiospectrographs include the Radio Solar Telescope Network, the network, and the WAVES instrument on-board the Wind spacecraft.
Radiospectrographs do not produce images, however, and so they cannot be used to locate features spatially. This can make it very difficult to understand where a specific component of the solar radio emission is coming from and how it relates to features seen at other wavelengths. Producing a radio image of the Sun requires an interferometer, which in radio astronomy means an array of many telescopes that operate together as a single telescope to produce an image. This technique is a sub-type of interferometry called aperture synthesis. Beginning in the 1950s, a number of simple interferometers were developed that could provide limited tracking of radio bursts. This also included the invention of sea interferometry, which was used to associate radio activity with sunspots.
Routine imaging of the radio Sun began in 1967 with the commissioning of the Culgoora Radioheliograph, which operated until 1986. A radioheliograph is simply an interferometer that is dedicated to observing the Sun. In addition to Culgoora, notable examples include the Clark Lake Radioheliograph, Nançay Radioheliograph, Nobeyama Radioheliograph, Gauribidanur Radioheliograph, Siberian Radioheliograph, and Chinese Spectral Radioheliograph. Additionally, interferometers that are used for other astrophysical observations can also be used to observe the Sun. General-purpose radio telescopes that also perform solar observations include the Very Large Array, Atacama Large Millimeter Array, Murchison Widefield Array, and Low-Frequency Array. The collage above shows antennas from several low-frequency radio telescopes used to observe the Sun.

Mechanisms

All of the processes described below produce radio frequencies that depend on the properties of the plasma where the radiation originates, particularly electron density and magnetic field strength. Two plasma physics parameters are particularly important in this context:
The electron plasma frequency,
and the electron gyrofrequency,
where is the electron density in cm⁻³, is the magnetic field strength in Gauss, is the electron charge, is the electron mass, and is the speed of light. The relative sizes of these two frequencies largely determine which emission mechanism will dominate in a particular environment. For example, high-frequency gyromagnetic emission dominates in the chromosphere, where the magnetic field strengths are comparatively large, whereas low-frequency thermal bremsstrahlung and plasma emission dominates in the corona, where the magnetic field strengths and densities are generally lower than in the chromosphere. In the images below, the first four on the upper left are dominated by gyromagnetic emission from the chromosphere, transition region, and low-corona, while the three images on the right are dominated by thermal bremsstrahlung emission from the corona, with lower frequencies being generated at larger heights above the surface.
File:QuietSun RadioImaging.jpg|alt=Quiet Sun Radio Imaging in Multiple Frequencies|center|thumb|800x800px|The Sun as seen in radio waves from 25.8 GHz down to 24.6 MHz. From upper-left to lower-right, the observations were recorded by the Nobeyama Radioheliograph, Very Large Array, Nançay Radioheliograph, Murchison Widefield Array and Low-Frequency Array. The solid circles in the images on the right correspond to the size of the Sun seen in visible light.

Thermal bremsstrahlung emission

emission, from the German "braking radiation", refers to electromagnetic waves produced when a charged particle accelerates and some of its kinetic energy is converted into radiation. Thermal bremsstrahlung refers to radiation from a plasma in thermal equilibrium and is primarily driven by Coulomb collisions where an electron is deflected by the electric field of an ion. This is often referred to as free-free emission for a fully ionized plasma like the solar corona because it involves collisions of "free" particles, as opposed to electrons transitioning between bound states in an atom. This is the main source of quiescent background emission from the corona, where quiescent means outside of radio burst periods.
The radio frequency of bremsstrahlung emission is related to a plasma's electron density through the electron plasma frequency from Equation. A plasma with a density can produce emission only at or below the corresponding. Density in the corona generally decreases with height above the visible "surface", or photosphere, meaning that lower-frequency emission is produced higher in the atmosphere, and the Sun appears larger at lower frequencies. This type of emission is most prominent below 300 MHz due to typical coronal densities, but particularly dense structures in the corona and chromosphere can generate bremsstrahlung emission with frequencies into the GHz range.

Gyromagnetic emission

Gyromagnetic emission is also produced from the kinetic energy of a charge particle, generally an electron. However in this case, an external magnetic field causes the particle's trajectory to exhibit a spiral gyromotion, resulting in a centripetal acceleration that in turn produces the electromagnetic waves. Different terminology is used for the same basic phenomenon depending on how fast the particle is spiraling around the magnetic field, which is due to the different mathematics required to describe the physics. Gyroresonance emission refers to slower, non-relativistic speeds and is also called magneto-bremsstrahlung or cyclotron emission. Gyrosynchrotron corresponds to the mildly relativistic case, where the particles rotate at a small but significant fraction of light speed, and synchrotron emission refers to the relativistic case where the speeds approach that of light.
Gyroresonance and gyrosynchrotron are most-important in the solar context, although there may be special cases in which synchrotron emission also operates. For any sub-type, gyromagnetic emission occurs near the electron gyrofrequency from Equation or one of its harmonics. This mechanism dominates when the magnetic field strengths are large such that >. This is mainly true in the chromosphere, where gyroresonance emission is the primary source of quiescent radio emission, producing microwave radiation in the GHz range. Gyroresonance emission can also be observed from the densest structures in the corona, where it can be used to measure the coronal magnetic field strength. Gyrosynchrotron emission is responsible for certain types of microwave radio bursts from the chromosphere and is also likely responsible for certain types of coronal radio bursts.