Magnetosonic wave
Encyclopedia
A magnetosonic wave is a longitudinal wave
of ion
s (and electron
s) in a magnetized plasma
propagating perpendicular to the stationary magnetic field. The wave is dispersionless
with a phase velocity
ω/k given by
,
where vs is the speed of the ion acoustic wave
, vA is the speed of the Alfvén wave
, and c is the speed of light
in vacuum.
In the limit of low magnetic field (vA→0), the wave turns into an ordinary ion acoustic wave
. In the limit of low temperature
(vs→0), the wave becomes a modified Alfvén wave. Because the phase velocity
of the magnetosonic mode is almost always larger than vA, the magnetosonic wave is often called the "fast" hydromagnetic wave
.
Both fast and slow magnetoacoustic waves have been recently discovered in the solar corona, which created an observational foundation for the novel technique for the coronal plasma diagnostics, coronal seismology
.
Longitudinal wave
Longitudinal waves, as known as "l-waves", are waves that have the same direction of vibration as their direction of travel, which means that the movement of the medium is in the same direction as or the opposite direction to the motion of the wave. Mechanical longitudinal waves have been also...
of ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s (and electron
Electron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
s) in a magnetized plasma
Plasma (physics)
In physics and chemistry, plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Heating a gas may ionize its molecules or atoms , thus turning it into a plasma, which contains charged particles: positive ions and negative electrons or ions...
propagating perpendicular to the stationary magnetic field. The wave is dispersionless
Dispersion relation
In physics and electrical engineering, dispersion most often refers to frequency-dependent effects in wave propagation. Note, however, that there are several other uses of the word "dispersion" in the physical sciences....
with a phase velocity
Phase velocity
The phase velocity of a wave is the rate at which the phase of the wave propagates in space. This is the speed at which the phase of any one frequency component of the wave travels. For such a component, any given phase of the wave will appear to travel at the phase velocity...
ω/k given by
,where vs is the speed of the ion acoustic wave
Ion acoustic wave
An ion acoustic wave is one type of longitudinal oscillation of the ions and electrons in a plasma, much like acoustic waves traveling in neutral gas. However, because the waves propagate through positively charged ions, ion acoustic waves can interact with their electromagnetic fields, as well as...
, vA is the speed of the Alfvén wave
Alfvén wave
An Alfvén wave, named after Hannes Alfvén, is a type of magnetohydrodynamic wave.-Definition:An Alfvén wave in a plasma is a low-frequency travelling oscillation of the ions and the magnetic field...
, and c is the speed of light
Speed of light
The speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time...
in vacuum.
In the limit of low magnetic field (vA→0), the wave turns into an ordinary ion acoustic wave
Ion acoustic wave
An ion acoustic wave is one type of longitudinal oscillation of the ions and electrons in a plasma, much like acoustic waves traveling in neutral gas. However, because the waves propagate through positively charged ions, ion acoustic waves can interact with their electromagnetic fields, as well as...
. In the limit of low temperature
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
(vs→0), the wave becomes a modified Alfvén wave. Because the phase velocity
Velocity
In physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
of the magnetosonic mode is almost always larger than vA, the magnetosonic wave is often called the "fast" hydromagnetic wave
Wave
In physics, a wave is a disturbance that travels through space and time, accompanied by the transfer of energy.Waves travel and the wave motion transfers energy from one point to another, often with no permanent displacement of the particles of the medium—that is, with little or no associated mass...
.
Both fast and slow magnetoacoustic waves have been recently discovered in the solar corona, which created an observational foundation for the novel technique for the coronal plasma diagnostics, coronal seismology
Coronal seismology
Coronal seismology is a technique of studying the plasma of the Sun's corona with the use of magnetohydrodynamic waves and oscillations. Magnetohydrodynamics studies the dynamics of electrically conducting fluids - in this case the fluid is the coronal plasma. Observed properties of the waves...
.
See also
- Waves in plasmasWaves in plasmasWaves in plasmas are an interconnected set of particles and fields which propagates in a periodically repeating fashion. A plasma is a quasineutral, electrically conductive fluid. In the simplest case, it is composed of electrons and a single species of positive ions, but it may also contain...
- Alfvén waveAlfvén waveAn Alfvén wave, named after Hannes Alfvén, is a type of magnetohydrodynamic wave.-Definition:An Alfvén wave in a plasma is a low-frequency travelling oscillation of the ions and the magnetic field...
- Ion acoustic waveIon acoustic waveAn ion acoustic wave is one type of longitudinal oscillation of the ions and electrons in a plasma, much like acoustic waves traveling in neutral gas. However, because the waves propagate through positively charged ions, ion acoustic waves can interact with their electromagnetic fields, as well as...
- Coronal seismologyCoronal seismologyCoronal seismology is a technique of studying the plasma of the Sun's corona with the use of magnetohydrodynamic waves and oscillations. Magnetohydrodynamics studies the dynamics of electrically conducting fluids - in this case the fluid is the coronal plasma. Observed properties of the waves...