Solar radio emission refers to
radio wave
Radio waves are a type of electromagnetic radiation with the longest wavelengths in the electromagnetic spectrum, typically with frequencies of 300 gigahertz (GHz) and below. At 300 GHz, the corresponding wavelength is 1 mm (short ...
s that are naturally produced by the
Sun
The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...
, primarily from the lower and upper layers of the atmosphere called the
chromosphere
A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively.
In the Su ...
and
corona
Corona (from the Latin for 'crown') most commonly refers to:
* Stellar corona, the outer atmosphere of the Sun or another star
* Corona (beer), a Mexican beer
* Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
, respectively. The Sun produces radio emissions through four known mechanisms, each of which operates primarily by converting the energy of moving
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
s into radiation. The four emission mechanisms are thermal
bremsstrahlung
''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
(free-free) 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 temperature
Brightness temperature or radiance temperature is the temperature at which a black body in thermal equilibrium with its surroundings would have to be in order to duplicate the observed intensity of a grey body object at a frequency \nu.
This con ...
s (intensities) 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 flare
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s and
coronal mass ejection
A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted ...
s.
History and observations
![Low-Frequency Solar Radio Telescopes](https://upload.wikimedia.org/wikipedia/commons/5/5d/Low-Frequency_Solar_Radio_Telescopes.jpg)
Radio emission from the Sun was first reported in the scientific literature by
Grote Reber
Grote Reber (December 22, 1911 – December 20, 2002) was an American pioneer of radio astronomy, which combined his interests in amateur radio and amateur astronomy. He was instrumental in investigating and extending Karl Jansky's pioneering wo ...
in 1944. Those were observations of 160 MHz frequency (2 meters wavelength) microwave emission emanating from the
chromosphere
A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively.
In the Su ...
. However, the earliest known observation was in 1942 during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
by British
radar
Radar is a detection system that uses radio waves to determine the distance (''ranging''), angle, and radial velocity of objects relative to the site. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, w ...
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
Joseph Lade Pawsey (14 May 1908 – 30 November 1962) was an Australian scientist, radiophysicist and radio astronomer.
Education
Pawsey was born in Ararat, Victoria to a family of farmers. At the age of 14 he was awarded a government schol ...
was that the Sun produces much more radio emission than expected from standard
black body radiation
Black-body radiation is the thermal electromagnetic radiation within, or surrounding, a body in thermodynamic equilibrium with its environment, emitted by a black body (an idealized opaque, non-reflective body). It has a specific, continuous spec ...
. The explanation for this was proposed by
Vitaly Ginzburg
Vitaly Lazarevich Ginzburg, ForMemRS (russian: Вита́лий Ла́заревич Ги́нзбург, link=no; 4 October 1916 – 8 November 2009) was a Russian physicist who was honored with the Nobel Prize in Physics in 2003, together with ...
in 1946, who suggested that thermal bremsstrahlung emission from a million-degree
corona
Corona (from the Latin for 'crown') most commonly refers to:
* Stellar corona, the outer atmosphere of the Sun or another star
* Corona (beer), a Mexican beer
* Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
was responsible. The existence of such extraordinarily high temperatures in the corona had previously been indicated by optical
spectroscopy
Spectroscopy is the field of study that measures and interprets the electromagnetic spectra that result from the interaction between electromagnetic radiation and matter as a function of the wavelength or frequency of the radiation. Matter wa ...
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
Ruby Violet Payne-Scott, BSc (Phys) MSc DipEd (Syd) (28 May 1912 – 25 May 1981) was an Australian pioneer in radiophysics and radio astronomy, and was one of two Antipodean women pioneers in radio astronomy and radio physics at the end of the ...
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 {{unreferenced, date=August 2013
The Radio Solar Telescope Network (RSTN) is a network of solar observatories maintained and operated by the 557th Weather Wing, ACC. The RSTN consists of ground-based observatories in Australia, Italy, Massachusetts ...
, th
e-CALLISTOnetwork, 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
Radio astronomy is a subfield of astronomy that studies celestial objects at radio frequencies. The first detection of radio waves from an astronomical object was in 1933, when Karl Jansky at Bell Telephone Laboratories reported radiation coming f ...
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
Interferometry is a technique which uses the ''interference'' of superimposed waves to extract information. Interferometry typically uses electromagnetic waves and is an important investigative technique in the fields of astronomy, fiber opt ...
called
aperture synthesis
Aperture synthesis or synthesis imaging is a type of interferometry that mixes signals from a collection of telescopes to produce images having the same angular resolution as an instrument the size of the entire collection. At each separation an ...
. 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 Sea interferometry, also known as sea-cliff interferometry, is a form of radio astronomy that uses radio waves reflected off the sea to produce an interference pattern. It is the radio wave analogue to Lloyd's mirror. The technique was invented and ...
, which was used to associate radio activity with
sunspot
Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...
s.
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
The Karl G. Jansky Very Large Array (VLA) is a centimeter-wavelength radio astronomy observatory located in central New Mexico on the Plains of San Agustin, between the towns of Magdalena and Datil, ~ west of Socorro. The VLA comprises twen ...
,
Atacama Large Millimeter Array
The Atacama Large Millimeter/submillimeter Array (ALMA) is an astronomical interferometer of 66 radio telescopes in the Atacama Desert of northern Chile, which observe electromagnetic radiation at millimeter and submillimeter wavelengths. The a ...
,
Murchison Widefield Array
The Murchison Widefield Array (MWA) is a joint project between an international consortium of organisations to construct and operate a low-frequency radio array. 'Widefield' refers to its very large field of view (on the order of 30 degrees ac ...
, and
Low-Frequency Array
The Low-Frequency Array, or LOFAR, is a large radio telescope, with an antenna network located mainly in the Netherlands, and spreading across 7 other European countries as of 2019. Originally designed and built by ASTRON, the Netherlands Institu ...
. 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
Plasma or plasm may refer to:
Science
* Plasma (physics), one of the four fundamental states of matter
* Plasma (mineral), a green translucent silica mineral
* Quark–gluon plasma, a state of matter in quantum chromodynamics
Biology
* Blood pla ...
where the radiation originates, particularly
electron density
In quantum chemistry, electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial va ...
and
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
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 quantum chemistry, electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial va ...
in cm
−3,
is the
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
strength in
Gauss
Johann Carl Friedrich Gauss (; german: Gauß ; la, Carolus Fridericus Gauss; 30 April 177723 February 1855) was a German mathematician and physicist who made significant contributions to many fields in mathematics and science. Sometimes refer ...
(G),
is the
electron charge
The elementary charge, usually denoted by is the electric charge carried by a single proton or, equivalently, the magnitude of the negative electric charge carried by a single electron, which has charge −1 . This elementary charge is a fundame ...
,
is the
electron mass
The electron mass (symbol: ''m''e) is the mass of a stationary electron, also known as the invariant mass of the electron. It is one of the fundamental constants of physics. It has a value of about or about , which has an energy-equivalent of a ...
, and
is the
speed of light
The speed of light in vacuum, commonly denoted , is a universal physical constant that is important in many areas of physics. The speed of light is exactly equal to ). According to the special theory of relativity, is the upper limit ...
. 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
A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively.
In the Su ...
, where the magnetic field strengths are comparatively large, whereas low-frequency thermal bremsstrahlung and plasma emission dominates in the
corona
Corona (from the Latin for 'crown') most commonly refers to:
* Stellar corona, the outer atmosphere of the Sun or another star
* Corona (beer), a Mexican beer
* Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
, 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.
Thermal bremsstrahlung emission
Bremsstrahlung
''Bremsstrahlung'' (), from "to brake" and "radiation"; i.e., "braking radiation" or "deceleration radiation", is electromagnetic radiation produced by the deceleration of a charged particle when deflected by another charged particle, typicall ...
emission, from the German "braking radiation", refers to electromagnetic waves produced when a charged particle accelerates and some of its
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
is converted into radiation. ''Thermal'' bremsstrahlung refers to radiation from a plasma in
thermal equilibrium
Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in ...
and is primarily driven by
Coulomb collision A Coulomb collision is a binary elastic collision between two charged particles interacting through their own electric field. As with any inverse-square law, the resulting trajectories of the colliding particles is a hyperbolic Keplerian orbit. Th ...
s where an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
is deflected by the
electric field
An electric field (sometimes E-field) is the physical field that surrounds electrically charged particles and exerts force on all other charged particles in the field, either attracting or repelling them. It also refers to the physical field fo ...
of an
ion
An ion () is an atom or molecule with a net electrical charge.
The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conven ...
. 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
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
, 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
Corona (from the Latin for 'crown') most commonly refers to:
* Stellar corona, the outer atmosphere of the Sun or another star
* Corona (beer), a Mexican beer
* Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
and
chromosphere
A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively.
In the Su ...
can generate bremsstrahlung emission with frequencies into the GHz range.
Gyromagnetic emission
Gyromagnetic emission is also produced from the
kinetic energy
In physics, the kinetic energy of an object is the energy that it possesses due to its motion.
It is defined as the work needed to accelerate a body of a given mass from rest to its stated velocity. Having gained this energy during its accele ...
of a charge particle, generally an
electron
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no kn ...
. However in this case, an external
magnetic field
A magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. A moving charge in a magnetic field experiences a force perpendicular to its own velocity and to ...
causes the particle's
trajectory
A trajectory or flight path is the path that an object with mass in motion follows through space as a function of time. In classical mechanics, a trajectory is defined by Hamiltonian mechanics via canonical coordinates; hence, a complete traj ...
to exhibit a spiral gyromotion, resulting in a
centripetal acceleration
In mechanics, acceleration is the rate of change of the velocity of an object with respect to time. Accelerations are vector quantities (in that they have magnitude and direction). The orientation of an object's acceleration is given by the ...
that in turn produces the
electromagnetic waves
In physics, electromagnetic radiation (EMR) consists of waves of the electromagnetic (EM) field, which propagate through space and carry momentum and electromagnetic radiant energy. It includes radio waves, microwaves, infrared, (visible) lig ...
.
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
A synchrotron is a particular type of cyclic particle accelerator, descended from the cyclotron, in which the accelerating particle beam travels around a fixed closed-loop path. The magnetic field which bends the particle beam into its closed p ...
'' 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
harmonic
A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...
s. This mechanism dominates when the magnetic field strengths are large such that
>
. This is mainly true in the
chromosphere
A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively.
In the Su ...
, where gyroresonance emission is the primary source of quiescent (non-burst) radio emission, producing microwave radiation in the GHz range.
Gyroresonance emission can also be observed from the densest structures in the
corona
Corona (from the Latin for 'crown') most commonly refers to:
* Stellar corona, the outer atmosphere of the Sun or another star
* Corona (beer), a Mexican beer
* Corona, informal term for the coronavirus SARS-CoV-2, which causes the COVID-19 di ...
, 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.
Plasma emission
![Plasma Emission Flowchart](https://upload.wikimedia.org/wikipedia/commons/0/02/Plasma_Emission_Flowchart.svg)
Plasma emission refers to a set of related process that partially convert the energy of
Langmuir waves Plasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals in the ultraviolet region. The oscillations can be described as an instability i ...
into radiation.
It is the most common form of coherent radio emission from the Sun and is commonly accepted as the emission mechanism for most types of solar radio bursts, which can exceed the background radiation level by several
orders of magnitude
An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic dis ...
for brief periods.
''
Langmuir waves Plasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals in the ultraviolet region. The oscillations can be described as an instability i ...
'', also called ''electron plasma waves'' or simply ''
plasma oscillation Plasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals in the ultraviolet region. The oscillations can be described as an instability i ...
s'', are
electron density
In quantum chemistry, electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial va ...
oscillations that occur when a plasma is perturbed so that a population of electrons is displaced relative to the ions. Once displaced, the
Coloumb force pulls the electrons back toward and ultimately past the ions, leading them to oscillate back and forth.
Langmuir waves are produced in the solar corona by a plasma instability that occurs when a beam of nonthermal (fast-moving) electrons moves through the ambient plasma. The electron beam may be accelerated either by
magnetic reconnection
Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnectio ...
, the process that underpins
solar flare
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s, or by a
shock wave
In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
, and these two basic processes operate in different contexts to produce different types of solar radio bursts. The instability that generates Langmuir waves is the ''
two-stream instability
The two-stream instability is a very common instability in plasma physics. It can be induced by an energetic particle stream injected in a plasma, or setting a current along the plasma so different species (ions and electrons) can have different d ...
'', which is also called the ''beam'' or ''bump-on-tail'' instability in cases such as this where an electron beam is injected into a plasma, creating a "bump" on the high-energy tail of the plasma's particle velocity distribution.
This bump facilitates exponential Langmuir wave growth in the ambient plasma through the transfer of energy from the electron beam into specific Langmuir wave modes. A small fraction of the Langmuir wave energy can then be converted into electromagnetic radiation through interactions with other wave modes, namely
ion sound waves.
A flowchart of the plasma emission stages is shown on the right.
Depending on these wave interactions, coherent radio emission may be produced at the fundamental electron plasma frequency (
; Equation ) or its
harmonic
A harmonic is a wave with a frequency that is a positive integer multiple of the ''fundamental frequency'', the frequency of the original periodic signal, such as a sinusoidal wave. The original signal is also called the ''1st harmonic'', the ...
(2
). Emission at
is often referred to as ''fundamental plasma emission'', while emission at 2
is called ''harmonic plasma emission''. This distinction is important because the two types have different observed properties and imply different plasma conditions. For example, fundamental plasma emission exhibits a much larger
circular polarization
In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to t ...
fraction
and originates from plasma that is four times denser than harmonic plasma emission.
Electron-cyclotron maser emission
The final, and least common, solar radio emission mechanism is electron-cyclotron maser emission (ECME). ''
Maser
A maser (, an acronym for microwave amplification by stimulated emission of radiation) is a device that produces coherent electromagnetic waves through amplification by stimulated emission. The first maser was built by Charles H. Townes, Ja ...
'' is an acronym for "microwave amplification by stimulated emission of radiation", which originally referred to a laboratory device that can produce intense radiation of a specific frequency through
stimulated emission
Stimulated emission is the process by which an incoming photon of a specific frequency can interact with an excited atomic electron (or other excited molecular state), causing it to drop to a lower energy level. The liberated energy transfers to th ...
. Stimulated emission is a process by which a group of atoms are moved into higher energy levels (above
thermal equilibrium
Two physical systems are in thermal equilibrium if there is no net flow of thermal energy between them when they are connected by a path permeable to heat. Thermal equilibrium obeys the zeroth law of thermodynamics. A system is said to be in ...
) and then stimulated to release that extra energy all at once. Such population inversions can occur naturally to produce
astrophysical maser
An astrophysical maser is a naturally occurring source of stimulated spectral line emission, typically in the microwave portion of the electromagnetic spectrum. This emission may arise in molecular clouds, comets, planetary atmospheres, stellar at ...
s, which are sources of very intense radiation of specific
spectral line
A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to iden ...
s.
Electron-cyclotron maser emission, however, does not involve population inversions of atomic energy levels.
The term ''maser'' was adopted here as an analogy is somewhat of a
misnomer
A misnomer is a name that is incorrectly or unsuitably applied. Misnomers often arise because something was named long before its correct nature was known, or because an earlier form of something has been replaced by a later form to which the name ...
. In ECME, the injection of nonthermal, semi-relativistic electrons into a plasma produces a population inversion analogous to that of a maser in the sense that a high-energy population was added to an equilibrium distribution. This is very similar to the beginning of the plasma emission process described in the previous section, but when the plasma density is low and/or the magnetic field strength is high such that
>
(Equations and ), energy from the nonthermal electrons cannot efficiently be converted into Langmuir waves.
This leads instead to direct emission at
through a plasma instability that is expressed analytically as a negative
absorption coefficient
The linear attenuation coefficient, attenuation coefficient, or narrow-beam attenuation coefficient characterizes how easily a volume of material can be penetrated by a beam of light, sound, particles, or other energy or matter. A coefficient valu ...
(i.e. positive growth rate) for a particular particle distribution, most famously the loss-cone distribution.
ECME is the accepted mechanism for microwave spike bursts from the chromosphere
and is sometimes invoked to explain features of coronal radio bursts that cannot be explained by plasma emission or gyrosynchrotron emission.
Magnetoionic theory and polarization
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
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
.
The corona is generally treated with the "cold plasma approach," which assumes that the characteristic velocities of the waves are much faster than the thermal velocities of the plasma particles.
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.
Under these approximations, the
dispersion equation
In the physical sciences and electrical engineering, dispersion relations describe the effect of dispersion on the properties of waves in a medium. A dispersion relation relates the wavelength or wavenumber of a wave to its frequency. Given the d ...
for electromagnetic waves includes two free-space modes that can escape the plasma as radiation (radio waves). These are called the ''ordinary'' (
) and ''extraordinary'' (
) modes.
The ordinary mode is "ordinary" in the sense that the plasma response is the same as if there were no magnetic field, while the ''
''-mode has a somewhat different refractive index. Importantly, each mode is polarized in opposite senses that depend on the angle with respect to the magnetic field. A quasi-circular approximation generally applies, in which case both modes are 100%
circularly polarized
In electrodynamics, circular polarization of an electromagnetic wave is a polarization state in which, at each point, the electromagnetic field of the wave has a constant magnitude and is rotating at a constant rate in a plane perpendicular to t ...
with opposite senses.
The
- and
-modes are produced at different rates depending on the emission mechanism and plasma parameters, which leads to a net circular polarization signal. For example, thermal bremsstrahlung slightly favors the
-mode, while plasma emission heavily favors the
-mode.
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. While circular polarization is most prevalent in solar radio observations, it is also possible to produce
linear polarization
In electrodynamics, linear polarization or plane polarization of electromagnetic radiation is a confinement of the electric field vector or magnetic field vector to a given plane along the direction of propagation. The term ''linear polarizati ...
s in certain circumstances. However, the presence of intense magnetic fields leads to
Faraday rotation
The Faraday effect or Faraday rotation, sometimes referred to as the magneto-optic Faraday effect (MOFE), is a physical magneto-optical phenomenon. The Faraday effect causes a polarization rotation which is proportional to the projection of the m ...
that distorts linearly-polarized signals, making them extremely difficult or impossible to detect. However, it is possible to detect linearly-polarized background astrophysical sources that are
occulted by the corona, in which case the impact of Faraday rotation can be used to measure the coronal magnetic field strength.
Propagation effects
The appearance of solar radio emission, particularly at low frequencies, is heavily influenced by propagation effects. A ''propagation effect'' is anything that impacts the path or state of an electromagnetic wave after it is produced. These effects therefore depend on whatever mediums the wave passed through before being observed. The most dramatic impacts to solar radio emission occur in the corona and in Earth's
ionosphere
The ionosphere () is the ionized part of the upper atmosphere of Earth, from about to above sea level, a region that includes the thermosphere and parts of the mesosphere and exosphere. The ionosphere is ionized by solar radiation. It plays an ...
. There are three primary effects: refraction, scattering, and mode coupling.
Refraction
In physics, refraction is the redirection of a wave as it passes from one medium to another. The redirection can be caused by the wave's change in speed or by a change in the medium. Refraction of light is the most commonly observed phenomeno ...
is the bending of light's path as it enters a new medium or passes through a material with varying density. The density of the corona generally decreases with distance from the Sun, which causes radio waves to refract toward the radial direction. When solar radio emission enters Earth's ionosphere, refraction may also severely distort the source's apparent location depending on the viewing angle and ionospheric conditions. The
- and
-modes discussed in the previous section also have slightly different
refractive indices
In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium.
The refractive index determines how much the path of light is bent, or ...
, which can lead to separation of the two modes.
The counterpart to refraction is
reflection Reflection or reflexion may refer to:
Science and technology
* Reflection (physics), a common wave phenomenon
** Specular reflection, reflection from a smooth surface
*** Mirror image, a reflection in a mirror or in water
** Signal reflection, in ...
. A radio wave can be reflected in the solar atmosphere when it encounters a region of particularly high density compared to where it was produced, and such reflections can occur many times before a radio wave escapes the atmosphere. This process of many successive reflections is called ''
scattering
Scattering is a term used in physics to describe a wide range of physical processes where moving particles or radiation of some form, such as light or sound, are forced to deviate from a straight trajectory by localized non-uniformities (including ...
'', and it has many important consequences. Scattering increases the apparent size of the entire Sun and compact sources within it, which is called ''angular broadening''. Scattering increases the cone-angle over which directed emission can be observed, which can even allow for the observation of low-frequency radio bursts that occurred on the far-side of the Sun. Because the high-density fibers that are primarily responsible for scattering are not randomly aligned and are generally radial, random scattering against them may also systematically shift the observed location of a radio burst to a larger height than where it was actually produced.
Finally, scattering tends to depolarize emission and is likely why radio bursts often exhibit much lower circular polarization fractions than standard theories predict.
''Mode coupling'' refers to polarization state changes of the
- and
-modes in response to different plasma conditions. If a radio wave passes through a region where the magnetic field orientation is nearly perpendicular to the direction of travel, which is called a quasi-transverse region, the polarization sign (i.e. left or right; positive or negative) may flip depending on the radio frequency and plasma parameters. This concept is crucial to interpreting polarization observations of solar microwave radiation and may also be important for certain low-frequency radio bursts.
Solar radio bursts
Solar radio bursts are brief periods during which the Sun's radio emission is elevated above the background level.
They are signatures of the same underlying physical processes that lead to the more widely-known forms of solar activity such as
sunspot
Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...
s,
solar flare
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s, and
coronal mass ejection
A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted ...
s.
Radio bursts can exceed the background radiation level only slightly or by several
orders of magnitude
An order of magnitude is an approximation of the logarithm of a value relative to some contextually understood reference value, usually 10, interpreted as the base of the logarithm and the representative of values of magnitude one. Logarithmic dis ...
(e.g. by 10 to 10,000 times) depending on a variety of factors that include the amount of energy released, the plasma parameters of the source region, the viewing geometry, and the mediums through which the radiation propagated before being observed. Most types of solar radio bursts are produced by the plasma emission mechanism operating in different contexts, although some are caused by (gyro)synchrotron and/or electron-cyclotron maser emission.
![Solar Radio Bursts Types I, II, and III](https://upload.wikimedia.org/wikipedia/commons/1/1e/Solar_Radio_Bursts_Types_I%2C_II%2C_and_III.jpg)
Solar radio bursts are classified largely based on how they appear in dynamic spectrum observations from radiospectrographs. 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.
This classification scheme is based primarily on how a burst's frequency drifts over time. Types IV and V were added within a few years of the initial three, and a number of other types and sub-types have since been identified.
Type I
Type I bursts are radiation spikes that last around one second and occur over a relatively narrow frequency range (
) with little-to-no discernible drift in frequency. They tend to occur in groups called ''noise storms'' that are often superimposed on enhanced
continuum
Continuum may refer to:
* Continuum (measurement), theories or models that explain gradual transitions from one condition to another without abrupt changes
Mathematics
* Continuum (set theory), the real line or the corresponding cardinal number ...
(broad-spectrum) emission with the same frequency range. While each individual Type I burst does not drift in frequency, a chain of Type I bursts in a noise storm may slowly drift from higher to lower frequencies over a few minutes. Noise storms can last from hours to weeks, and they are generally observed at relatively low frequencies between around 50 and 500 MHz.
Noise storms are associated with ''active regions''. Active regions are regions in the solar atmosphere with high concentrations of magnetic fields, and they include a
sunspot
Sunspots are phenomena on the Sun's photosphere that appear as temporary spots that are darker than the surrounding areas. They are regions of reduced surface temperature caused by concentrations of magnetic flux that inhibit convection. Sun ...
at their base in the
photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
except in cases where the magnetic fields are fairly weak. The association with active regions has been known for decades, but the conditions required to produce noise storms are still mysterious. Not all active regions that produce other forms of activity such as flares generate noise storms, and unlike other types of solar radio bursts, it is often difficult to identify non-radio signatures of Type I bursts.
The emission mechanism for Type I bursts is generally agreed to be fundamental plasma emission due to the high circular polarization fractions that are frequently observed. However, there is no consensus yet on what process accelerates the electrons needed to stimulate plasma emission. The leading ideas are minor
magnetic reconnection
Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnectio ...
events or
shock wave
In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
s driven by upward-propagating waves. Since the year 2000, different magnetic reconnection scenarios have generally been favored. One scenario involves reconnection between the open and closed magnetic fields at the boundaries of active regions, and another involves moving magnetic features in the photosphere.
Type II
Type II bursts exhibit a relatively slow drift from high to low frequencies of around 1 MHz per second, typically over the course of a few minutes. They often exhibit two distinct bands of emission that correspond to fundamental and harmonic plasma emission emanating from the same region. Type II bursts are associated with
coronal mass ejection
A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted ...
s (CMEs) and are produced at the leading edge of a CME, where a
shock wave
In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a med ...
accelerates the electrons responsible for stimulating plasma emission. The frequency drifts from higher to lower values because it depends on the
electron density
In quantum chemistry, electron density or electronic density is the measure of the probability of an electron being present at an infinitesimal element of space surrounding any given point. It is a scalar quantity depending upon three spatial va ...
, and the shock propagates outward away from the Sun through lower and lower densities. By using a model for the Sun's atmospheric density, the frequency drift rate can then be used to estimate the speed of the shock wave. This exercise typically results in speeds of around 1000 km/s, which matches that of CME shocks determined from other methods.
While plasma emission is the accepted mechanism, Type II bursts do not exhibit significant amounts of circular polarization as would be expected by standard plasma emission theory. 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
Magnetohydrodynamics (MHD; also called magneto-fluid dynamics or hydromagnetics) is the study of the magnetic properties and behaviour of electrically conducting fluids. Examples of such magnetofluids include plasmas, liquid metals, ...
shock.
Type II bursts sometimes exhibit fine structures called herringbone bursts that emanate from the main burst, as it appears in a dynamic spectrum, and extend to lower frequencies. Herringbone structures are believed to result from shock-accelerated electrons that were able to escape far beyond the shock region to excite Langmuir waves in plasma of lower density than the primary burst region.
Type III
Like Type II bursts, Type IIIs also drift from high to low frequencies and are widely attributed to the plasma emission mechanism.
However, Type III bursts drift much more rapidly, around 100 MHz per second, and must therefore be related to disturbances that move more quickly than the shock waves responsible for Type IIs. Type III bursts are associated with electrons beams that are accelerated to small fractions of light speed (
0.1 to 0.3 c) by
magnetic reconnection
Magnetic reconnection is a physical process occurring in highly conducting plasmas in which the magnetic topology is rearranged and magnetic energy is converted to kinetic energy, thermal energy, and particle acceleration. Magnetic reconnectio ...
, the process responsible for
solar flare
A solar flare is an intense localized eruption of electromagnetic radiation in the Sun's atmosphere. Flares occur in active regions and are often, but not always, accompanied by coronal mass ejections, solar particle events, and other solar phe ...
s. In the image below, the chain of color contours show the locations of three Type III bursts at different frequencies. The progression from violet to red corresponds to the trajectories of electron beams moving away from the Sun and exciting lower and lower frequency plasma emission as they encounter lower and lower densities. Given that they are ultimately caused by magnetic reconnection, Type IIIs are strongly associated with
X-ray
An X-ray, or, much less commonly, X-radiation, is a penetrating form of high-energy electromagnetic radiation. Most X-rays have a wavelength ranging from 10 picometers to 10 nanometers, corresponding to frequencies in the range 30&nb ...
flares and are indeed observed during nearly all large flares. 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.
![Images of Type III Solar Radio Bursts](https://upload.wikimedia.org/wikipedia/commons/d/d3/Images_of_Type_III_Solar_Radio_Bursts.png)
Type III bursts can occur alone, in small groups, or in chains referred to as Type III storms that may last many minutes. They are often subdivided into two types, ''coronal'' and ''interplanetary'' Type III bursts.
Coronal refers to the case for which an electron beam is traveling in the corona within a few solar radii of the
photosphere
The photosphere is a star's outer shell from which light is radiated.
The term itself is derived from Ancient Greek roots, φῶς, φωτός/''phos, photos'' meaning "light" and σφαῖρα/''sphaira'' meaning "sphere", in reference to it ...
. They typically start at frequencies in the 100s of MHz and drift down to 10s of MHz over a few seconds. The electron beams that excite radiation travel along specific magnetic field lines that may be closed or open to interplanetary space. Electron beams that escape into interplanetary space may excite Langmuir waves in the
solar wind
The solar wind is a stream of charged particles released from the upper atmosphere of the Sun, called the corona. This plasma mostly consists of electrons, protons and alpha particles with kinetic energy between . The composition of the sola ...
plasma to produce interplanetary Type III bursts that can extend down to 20 kHz and below for beams that reach 1
Astronomical Unit
The astronomical unit (symbol: au, or or AU) is a unit of length, roughly the distance from Earth to the Sun and approximately equal to or 8.3 light-minutes. The actual distance from Earth to the Sun varies by about 3% as Earth orbits t ...
and beyond.
The very low frequencies of interplanetary bursts are below the ionospheric cutoff (
10 MHz), meaning they are blocked by Earth's ionosphere and are observable only from space.
Direct,
in situ
''In situ'' (; often not italicized in English) is a Latin phrase that translates literally to "on site" or "in position." It can mean "locally", "on site", "on the premises", or "in place" to describe where an event takes place and is used in ...
observations of the electrons and
Langmuir waves Plasma oscillations, also known as Langmuir waves (after Irving Langmuir), are rapid oscillations of the electron density in conducting media such as plasmas or metals in the ultraviolet region. The oscillations can be described as an instability i ...
(plasma oscillations) associated with interplanetary Type III bursts are among the most important pieces of evidence for the plasma emission theory of solar radio bursts. Type III bursts exhibit moderate levels of circular polarization, typically less than 50%. This is lower than expected from plasma emission and is likely due to depolarization from scattering by density inhomogeneities and other propagation effects.
Type IV
Type IV bursts are spikes of broad-band
continuum
Continuum may refer to:
* Continuum (measurement), theories or models that explain gradual transitions from one condition to another without abrupt changes
Mathematics
* Continuum (set theory), the real line or the corresponding cardinal number ...
emission that include a few distinct sub-types associated with different phenomena and different emission mechanisms. The first type to be defined was the ''moving'' Type IV burst, which requires imaging observations (i.e. interferometry) to detect. 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
A coronal mass ejection (CME) is a significant release of plasma and accompanying magnetic field from the Sun's corona into the heliosphere. CMEs are often associated with solar flares and other forms of solar activity, but a broadly accepted ...
(CME).
The emission mechanism for Type IV bursts is generally attributed to gyrosynchrotron emission, plasma emission, or some combination of both that results from fast-moving electrons trapped within the magnetic fields of an erupting CME.
Stationary Type IV bursts are more common and are not associated with CMEs.
They are broad-band continuum emissions associated either with solar flares or Type I bursts.
Flare-associated Type IV bursts are also called ''flare continuum'' bursts, and they typically begin at or shortly after a flare's impulsive phase. Larger flares often include a ''storm continuum'' phase that follows after the flare continuum. The storm continuum can last from hours to days and may transition into an ordinary Type I noise storm in long-duration events.
Both flare and storm continuum Type IV bursts are attributed to plasma emission, but the storm continuum exhibits much larger degrees of circular polarization for reasons that are not fully known.
Type V
Type V bursts are the least common of the standard 5 types.
They are continuum emissions that last from one to a few minutes immediately after a group of Type III bursts, generally occurring below around 120 MHz.
Type Vs are generally thought to be caused by harmonic plasma emission associated with same streams of electrons responsible for the associated Type III bursts. 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. Type V bursts persist for much longer than Type IIIs because they are driven by a slower and less-
collimated
A collimated beam of light or other electromagnetic radiation has parallel rays, and therefore will spread minimally as it propagates. A perfectly collimated light beam, with no divergence, would not disperse with distance. However, diffraction pr ...
electron population, which produces broader-band emission and also leads to a reversal in the circular polarization sign from that of the associated Type III bursts due to the different Langmuir wave distribution. While plasma emission is the commonly-accepted mechanism, electron-cyclotron maser emission has also been proposed.
Other types
In addition to the classic five types, there are a number of additional types of solar radio bursts. These include variations of the standard types, fine structure within another type, and entirely distinct phenomena. Variant examples include Types J and U bursts, which are Type III bursts for which the frequency drift reverses to go from lower to higher frequencies, suggesting that an electron beam first traveled away and then back toward the Sun along a closed magnetic field trajectory.
Fine structure bursts include zebra patterns and fibre bursts that may be observed within Type IV bursts, along with the herringbone bursts
that sometimes accompany Type IIs. Type S bursts, which last only milliseconds, are an example of a distinct class. There are also a variety of high-frequency microwave burst types, such as microwave Type IV bursts, impulsive bursts, postbursts, and spike bursts.
Radio emission from other stars
Due to its proximity to Earth, the Sun is the brightest source of astronomical radio emission. But of course, other stars also produce radio emission and may produce much more intense radiation in absolute terms than is observed from the Sun. For "normal"
main sequence
In astronomy, the main sequence is a continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...
stars, the mechanisms that produce stellar radio emission are the same as those that produce solar radio emission.
However, emission from "
radio star
Stellar radio sources, radio source stars or radio stars are stellar objects that produce copious emissions of various radio frequencies, whether constant or pulsed. Radio emissions from stars can be produced in many varied ways.
Neutron stars
Pu ...
s" may exhibit significantly different properties compared to the Sun, and the relative importance of the different mechanisms may change depending on the properties of the star, particularly with respect to size and
rotation rate
Rotational frequency (also known as rotational speed or rate of rotation) of an object rotating around an axis is the frequency of rotation of the object. Its unit is revolution per minute (rpm), cycle per second (cps), etc.
The symbol for ...
, the latter of which largely determines the strength of a
star's magnetic field. Notable examples of stellar radio emission include quiescent steady emission from stellar
chromosphere
A chromosphere ("sphere of color") is the second layer of a star's atmosphere, located above the photosphere and below the solar transition region and corona. The term usually refers to the Sun's chromosphere, but not exclusively.
In the Su ...
s and
coronae, radio bursts from
flare star
A flare star is a variable star that can undergo unpredictable dramatic increases in brightness for a few minutes. It is believed that the flares on flare stars are analogous to solar flares in that they are due to the magnetic energy stored in th ...
s, radio emission from massive
stellar wind
A stellar wind is a flow of gas ejected from the upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spherically symmetric.
D ...
s, and radio emission associated with close
binary star
A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...
s.
Pre-main-sequence star
A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of ...
s such as
T Tauri
T Tauri is a variable star in the constellation Taurus, the prototype of the T Tauri stars. It was discovered in October 1852 by John Russell Hind. T Tauri appears from Earth amongst the Hyades cluster, not far from ε Tauri, but it i ...
stars also exhibit radio emission through reasonably well-understood processes, namely gyrosynchrotron and electron cyclotron maser emission.
Different radio emission processes also exist for certain
pre-main-sequence star
A pre-main-sequence star (also known as a PMS star and PMS object) is a star in the stage when it has not yet reached the main sequence. Earlier in its life, the object is a protostar that grows by acquiring mass from its surrounding envelope of ...
s, along with post-main sequence stars such as
neutron star
A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses, possibly more if the star was especially metal-rich. Except for black holes and some hypothetical objects (e.g. white ...
s.
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. Of particular interest is the fact that there is no consensus yet on the coherent radio emission mechanism responsible for
pulsar
A pulsar (from ''pulsating radio source'') is a highly magnetized rotating neutron star that emits beams of electromagnetic radiation out of its magnetic poles. This radiation can be observed only when a beam of emission is pointing toward Ea ...
s, which cannot be explained by the two well-established coherent mechanisms discussed here, plasma emission and electron cyclotron maser emission.
Proposed mechanisms for pulsar radio emission include coherent curvature emission, relativistic plasma emission, anomalous Doppler emission, and linear acceleration emission or free-electron maser emission.
All of these processes still involve the transfer of energy from moving electrons into radiation. However, in this case the electrons are moving at nearly the speed of light, and the debate revolves around what process accelerates these electrons and how their energy is converted into radiation.
References
Further reading
*
Radio Emission from the Sun and Stars' by Dulk, G. A. (1985)
*
Solar radiophysics: studies of emission from the sun at metre wavelengths' edited by McLean, D. J. and Labrum, N. R. (1985)
*
Solar and Space Weather Radiophysics - Current Status and Future Developments' edited by Gary, D. E. and Keller, C. U. (2004)
*
Sixty-five years of solar radioastronomy: flares, coronal mass ejections and Sun-Earth connection' by Pick, M. and Vilmer, N. (2008)
*
Coherent Emission' by Melrose, D. B. (2009)
{{Portal bar, Astronomy, Spaceflight, Outer space, Solar System, Science
Astrophysics
Solar phenomena
Sun
Astronomy
Radio astronomy
Space physics