Solar radio emission

The Sun produces radio emissions through four known mechanisms, each of which operates primarily by converting the energy of moving electrons into electromagnetic radiation.

[4] 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.

[6] 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.

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.

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.

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.

[18] 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.

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.

[16] 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.

[22] Plasma emission refers to a set of related process that partially convert the energy of Langmuir waves into radiation.

Such population inversions can occur naturally to produce astrophysical masers, which are sources of very intense radiation of specific spectral lines.

[35][36] Magnetoionic theory describes the propagation of electromagnetic waves in environments where an ionized plasma is subjected to an external magnetic field, such as the solar corona and Earth's ionosphere.

[17][38] This assumption allows thermal effects to be neglected, and most approaches also ignore the motions of ions and assume that the particles do not interact through collisions.

-modes are produced at different rates depending on the emission mechanism and plasma parameters, which leads to a net circular polarization signal.

[29] This makes circular polarization an extremely important property for studies of solar radio emission, as it can be used to help understand how the radiation was produced.

[39] However, the presence of intense magnetic fields leads to Faraday rotation that distorts linearly-polarized signals, making them extremely difficult or impossible to detect.

[40] However, it is possible to detect linearly-polarized background astrophysical sources that are occulted by the corona,[41] in which case the impact of Faraday rotation can be used to measure the coronal magnetic field strength.

The density of the corona generally decreases with distance from the Sun, which causes radio waves to refract toward the radial direction.

[51][30] Finally, scattering tends to depolarize emission and is likely why radio bursts often exhibit much lower circular polarization fractions than standard theories predict.

[55] This concept is crucial to interpreting polarization observations of solar microwave radiation[56][57] and may also be important for certain low-frequency radio bursts.

The first three types, shown in the image on the right, were defined by Paul Wild and Lindsay McCready in 1950 using the earliest radiospectrograph observations of metric (low-frequency) bursts.

[59] They tend to occur in groups called noise storms that are often superimposed on enhanced continuum (broad-spectrum) emission with the same frequency range.

[68] Type II bursts exhibit a relatively slow drift from high to low frequencies of around 0.05 MHz per second,[69] typically over the course of a few minutes.

[71] Type II bursts are associated with coronal mass ejections (CMEs) and are produced at the leading edge of a CME, where a shock wave accelerates the electrons responsible for stimulating plasma emission.

[74] The reason for this is unknown, but a leading hypothesis is that the polarization level is suppressed by dispersion effects related to having an inhomogeneous magnetic field near a magnetohydrodynamic shock.

[80] However, small-to-moderate X-ray flares do not always exhibit Type III bursts and vice versa due to the somewhat different conditions that are required for the high- and low-energy emission to be produced and observed.

The electron beams that excite radiation travel along specific magnetic field lines that may be closed or open to interplanetary space.

[87] They are characterized by an outward-moving continuum source that is often preceded by a Type II burst in association with a coronal mass ejection (CME).

[90] They sometimes exhibit significant positional offsets from the Type III bursts, which may be due to the electrons traveling along somewhat different magnetic field structures.

[16] These objects have very high rotation rates, which leads to very intense magnetic fields that are capable of accelerating large amounts of particles to highly-relativistic speeds.

A collage of antennas from various low-frequency radio interferometry telescopes used to observe the Sun. From left to right, top to bottom: the Culgoora Radioheliograph, Clark Lake Radioheliograph, Guaribidanur Radioheliograph , Nancay Radioheliograph , Murchison Widefield Array , and Low Frequency Array .
Quiet Sun Radio Imaging in Multiple Frequencies
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 (NoRH), Very Large Array (VLA), Nançay Radioheliograph (NRH), Murchison Widefield Array (MWA) and Low-Frequency Array (LOFAR). The solid circles in the images on the right correspond to the size of the Sun seen in visible light.
Flowchart outlining the stages of plasma emission, which is responsible for most types of solar radio bursts. In the solar context, the electron beam is accelerated either by magnetic reconnection or a shock wave . Adapted from chart by Donald Melrose [ 23 ]
Solar radio bursts of Types I, II, and III as seen in dynamic spectrum observations from the Learmonth Solar radiospectrograph. The color corresponds to the intensity. The perfectly horizontal features seen at specific frequencies correspond to radio frequency interference from human-generated sources.
Murchison Widefield Array images of background radiation at 240 MHz (grayscale) with color contours showing Type III bursts at a range of frequencies. The bursts are hundreds of times brighter than the background, and the lower-frequency contours appear at greater heights because the bursts are produced by an electron beam that travels away from the Sun, exciting radio emission of decreasing frequency with increasing distance.