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53-481: Alfvén may refer to: People [ edit ] Hannes Alfvén (1908–1995), Swedish plasma physicist and Nobel Prize in Physics laureate Hugo Alfvén (1872–1960), Swedish composer, conductor, violinist, and painter Marie Triepcke Krøyer Alfvén (1867–1940), commonly known as Marie Krøyer, Danish painter, wife of Hugo Other [ edit ] Alfvén wave ,

106-399: A numerical resistivity . In many MHD systems most of the electric current is compressed into thin nearly-two-dimensional ribbons termed current sheets . These can divide the fluid into magnetic domains, inside of which the currents are relatively weak. Current sheets in the solar corona are thought to be between a few meters and a few kilometers in thickness, which is quite thin compared to

159-495: A cellular structure. His theoretical work on field-aligned electric currents in the aurora (based on earlier work by Kristian Birkeland ) was confirmed in 1967, these currents now being known as Birkeland currents . British scientist Sydney Chapman was a strong critic of Alfvén. Many physicists regarded Alfvén as espousing unorthodox opinions R. H. Stuewer noting that "... he remained an embittered outsider, winning little respect from other scientists even after he received

212-509: A given fluid, each species σ {\displaystyle \sigma } has a number density n σ {\displaystyle n_{\sigma }} , mass m σ {\displaystyle m_{\sigma }} , electric charge q σ {\displaystyle q_{\sigma }} , and a mean velocity u σ {\displaystyle \mathbf {u} _{\sigma }} . The fluid's total mass density

265-742: A kinetic system, a closure approximation must be applied to highest moment of the particle distribution equation. This is often accomplished with approximations to the heat flux through a condition of adiabaticity or isothermality . In the adiabatic limit, that is, the assumption of an isotropic pressure p {\displaystyle p} and isotropic temperature, a fluid with an adiabatic index γ {\displaystyle \gamma } , electrical resistivity η {\displaystyle \eta } , magnetic field B {\displaystyle \mathbf {B} } , and electric field E {\displaystyle \mathbf {E} } can be described by

318-475: A later paper he noted, "As the term 'electromagnetic–hydrodynamic waves' is somewhat complicated, it may be convenient to call this phenomenon 'magneto–hydrodynamic' waves." In MHD, motion in the fluid is described using linear combinations of the mean motions of the individual species : the current density J {\displaystyle \mathbf {J} } and the center of mass velocity v {\displaystyle \mathbf {v} } . In

371-510: A plasma for long periods of time can cause violent explosions and bursts of radiation. When the fluid cannot be considered as completely conductive, but the other conditions for ideal MHD are satisfied, it is possible to use an extended model called resistive MHD. This includes an extra term in Ohm's Law which models the collisional resistivity. Generally MHD computer simulations are at least somewhat resistive because their computational grid introduces

424-510: A region of a given size before diffusion becomes too important to ignore. One can estimate the diffusion time across a solar active region (from collisional resistivity) to be hundreds to thousands of years, much longer than the actual lifetime of a sunspot—so it would seem reasonable to ignore the resistivity. By contrast, a meter-sized volume of seawater has a magnetic diffusion time measured in milliseconds. Even in physical systems —which are large and conductive enough that simple estimates of

477-450: A single continuous medium . It is primarily concerned with the low-frequency, large-scale, magnetic behavior in plasmas and liquid metals and has applications in multiple fields including space physics , geophysics , astrophysics , and engineering . The word magneto­hydro­dynamics is derived from magneto- meaning magnetic field , hydro- meaning water, and dynamics meaning movement. The field of MHD

530-464: A skater pulling their arms in. The high speed of rotation predicted by early theories would have flung the proto-Sun apart before it could have formed. However, magnetohydrodynamic effects transfer the Sun's angular momentum into the outer solar system, slowing its rotation. Breakdown of ideal MHD (in the form of magnetic reconnection) is known to be the likely cause of solar flares . The magnetic field in

583-438: A solar active region over a sunspot can store energy that is released suddenly as a burst of motion, X-rays , and radiation when the main current sheet collapses, reconnecting the field. MHD describes a wide range of physical phenomena occurring in fusion plasmas in devices such as tokamaks or stellarators . The Grad-Shafranov equation derived from ideal MHD describes the equilibrium of axisymmetric toroidal plasma in

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636-625: A type of magnetohydrodynamic wave, named after Hannes Alfvén 1778 Alfvén , an asteroid discovered in 1960, named after Hannes Alfvén Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title Alfvén . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=Alfvén&oldid=1040403664 " Categories : Disambiguation pages Disambiguation pages with surname-holder lists Hidden categories: Short description

689-507: Is crucial that such events are detected early. The Space Weather Prediction Center (SWPC) runs MHD models to predict the arrival and impacts of space weather events at Earth. MHD applies to astrophysics , including stars, the interplanetary medium (space between the planets), and possibly within the interstellar medium (space between the stars) and jets . Most astrophysical systems are not in local thermal equilibrium, and therefore require an additional kinematic treatment to describe all

742-807: Is different from Wikidata All article disambiguation pages All disambiguation pages Hannes Alfv%C3%A9n Alfvén received his PhD from the University of Uppsala in 1934. His thesis was titled "Investigations of High-frequency Electromagnetic Waves." In 1934, Alfvén taught physics at both the University of Uppsala and the Nobel Institute for Physics (later renamed the Manne Siegbahn Institute of Physics ) in Stockholm , Sweden. In 1940, he became professor of electromagnetic theory and electrical measurements at

795-524: Is named in his honour. Alfvén was one of the few scientists who was a foreign member of both the United States and Soviet Academies of Sciences. For full list of publications see. Magnetohydrodynamics In physics and engineering , magnetohydrodynamics ( MHD ; also called magneto-fluid dynamics or hydro­magnetics ) is a model of electrically conducting fluids that treats all interpenetrating particle species together as

848-429: Is the frozen-in flux theorem which states that the bulk fluid and embedded magnetic field are constrained to move together such that one can be said to be "tied" or "frozen" to the other. Therefore, any two points that move with the bulk fluid velocity and lie on the same magnetic field line will continue to lie on the same field line even as the points are advected by fluid flows in the system. The connection between

901-402: Is the first criterion listed above), so that the time scale of collisions is shorter than the other characteristic times in the system, and the particle distributions are Maxwellian . This is usually not the case in fusion, space and astrophysical plasmas. When this is not the case, or the interest is in smaller spatial scales, it may be necessary to use a kinetic model which properly accounts for

954-537: Is then ρ = ∑ σ m σ n σ {\textstyle \rho =\sum _{\sigma }m_{\sigma }n_{\sigma }} , and the motion of the fluid can be described by the current density expressed as and the center of mass velocity expressed as: MHD can be described by a set of equations consisting of a continuity equation , an equation of motion , an equation of state , Ampère's Law , Faraday's law , and Ohm's law . As with any fluid description to

1007-726: The 1989 Loma Prieta earthquake in California , although a subsequent study indicates that this was little more than a sensor malfunction. On December 9, 2010, geoscientists announced that the DEMETER satellite observed a dramatic increase in ULF radio waves over Haiti in the month before the magnitude 7.0 M w 2010 earthquake . Researchers are attempting to learn more about this correlation to find out whether this method can be used as part of an early warning system for earthquakes. The study of space plasmas near Earth and throughout

1060-460: The Lorentz force term J × B {\displaystyle \mathbf {J} \times \mathbf {B} } can be expanded using Ampère's law and a vector calculus identity to give where the first term on the right hand side is the magnetic tension force and the second term is the magnetic pressure force. In view of the infinite conductivity, every motion (perpendicular to

1113-456: The Lundquist number suggest that the resistivity can be ignored—resistivity may still be important: many instabilities exist that can increase the effective resistivity of the plasma by factors of more than 10 . The enhanced resistivity is usually the result of the formation of small scale structure like current sheets or fine scale magnetic turbulence , introducing small spatial scales into

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1166-692: The Royal Institute of Technology in Stockholm. In 1945, he acquired the nonappointive position of Chair of Electronics. His title was changed to Chair of Plasma Physics in 1963. From 1954 to 1955, Alfvén was a Fulbright Scholar at the University of Maryland, College Park . In 1967, after leaving Sweden and spending time in the Soviet Union , he moved to the United States. Alfvén worked in the departments of electrical engineering at both

1219-631: The Solar System is known as space physics . Areas researched within space physics encompass a large number of topics, ranging from the ionosphere to auroras , Earth's magnetosphere , the Solar wind , and coronal mass ejections . MHD forms the framework for understanding how populations of plasma interact within the local geospace environment. Researchers have developed global models using MHD to simulate phenomena within Earth's magnetosphere, such as

1272-442: The University of California, San Diego and the University of Southern California . In 1991, Alfvén retired as professor of electrical engineering at the University of California, San Diego and professor of plasma physics at the Royal Institute of Technology in Stockholm. Alfvén spent his later adult life alternating between California and Sweden. He died at the age of 86. In 1937, Alfvén argued that if plasma pervaded

1325-446: The 1980s through external measurements of cometary and planetary magnetospheres. However, Alfvén himself noted that astrophysical textbooks poorly represented known plasma phenomena: A study of how a number of the most used textbooks in astrophysics treat important concepts such as double layers , critical velocity , pinch effects , and circuits is made. It is found that students using these textbooks remain essentially ignorant of even

1378-453: The Earth's magnetic fields into the atmosphere caused the aurora and polar magnetic disturbances. Areas of technology benefiting from Alfvén's contributions include: Contributions to astrophysics: Alfvén waves (low frequency hydromagnetic plasma oscillations ) are named in his honor, and propagate at the Alfvén speed. Many of his theories about the solar system were verified as late as

1431-503: The Nobel Prize..." and was often forced to publish his papers in obscure journals. Alfvén recalled: When I describe [plasma phenomena] according to this formalism most referees do not understand what I say and turn down my papers. With the referee system which rules US science today, this means that my papers are rarely accepted by the leading US journals. Alfvén played a central role in the development of: In 1939, Alfvén proposed

1484-431: The case of ultra-high intensity laser interactions, the incredibly short timescales of energy deposition mean that hydrodynamic codes fail to capture the essential physics. Beneath the Earth's mantle lies the core, which is made up of two parts: the solid inner core and liquid outer core. Both have significant quantities of iron . The liquid outer core moves in the presence of the magnetic field and eddies are set up into

1537-499: The conditions for ideal MHD break down, allowing magnetic reconnection that releases the stored energy from the magnetic field. In ideal MHD, the resistive term η J {\displaystyle \eta \mathbf {J} } vanishes in Ohm's law giving the ideal Ohm's law, Similarly, the magnetic diffusion term η ∇ 2 B / μ 0 {\displaystyle \eta \nabla ^{2}\mathbf {B} /\mu _{0}} in

1590-420: The continuous equation the equation of state the equation of motion the low-frequency Ampère's law Faraday's law and Ohm's law Taking the curl of this equation and using Ampère's law and Faraday's law results in the induction equation , where η / μ 0 {\displaystyle \eta /\mu _{0}} is the magnetic diffusivity . In the equation of motion,

1643-405: The dispersion relation where is the Alfvén speed. This branch corresponds to the shear Alfvén mode. Additionally the dispersion equation gives where is the ideal gas speed of sound. The plus branch corresponds to the fast-MHD wave mode and the minus branch corresponds to the slow-MHD wave mode. A summary of the properties of these waves is provided: The MHD oscillations will be damped if

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1696-404: The existence of these concepts, despite the fact that some of them have been well known for half a century (e.g, double layers, Langmuir, 1929; pinch effect, Bennet, 1934). Alfvén reported that of 17 of the most used textbooks on astrophysics, none mention the pinch effect, none mentioned critical ionization velocity, only two mentioned circuits, and three mentioned double layers. Alfvén believed

1749-457: The field) of the liquid in relation to the lines of force is forbidden because it would give infinite eddy currents . Thus the matter of the liquid is "fastened" to the lines of force... Hannes Alfvén , 1943 The simplest form of MHD, ideal MHD , assumes that the resistive term η J {\displaystyle \eta \mathbf {J} } in Ohm's law is small relative to

1802-410: The fluid and magnetic field fixes the topology of the magnetic field in the fluid—for example, if a set of magnetic field lines are tied into a knot, then they will remain so as long as the fluid has negligible resistivity. This difficulty in reconnecting magnetic field lines makes it possible to store energy by moving the fluid or the source of the magnetic field. The energy can then become available if

1855-424: The fluid is not perfectly conducting but has a finite conductivity, or if viscous effects are present. MHD waves and oscillations are a popular tool for the remote diagnostics of laboratory and astrophysical plasmas, for example, the corona of the Sun ( Coronal seismology ). Another limitation of MHD (and fluid theories in general) is that they depend on the assumption that the plasma is strongly collisional (this

1908-424: The formation of the Sun and planets could not explain how the Sun has 99.87% of the mass, yet only 0.54% of the angular momentum in the Solar System . In a closed system such as the cloud of gas and dust from which the Sun was formed, mass and angular momentum are both conserved . That conservation would imply that as the mass concentrated in the center of the cloud to form the Sun, it would spin faster, much like

1961-403: The induction equation vanishes giving the ideal induction equation, Ideal MHD is only strictly applicable when: In an imperfectly conducting fluid the magnetic field can generally move through the fluid following a diffusion law with the resistivity of the plasma serving as a diffusion constant . This means that solutions to the ideal MHD equations are only applicable for a limited time for

2014-443: The linearized ideal-MHD equations for a fluid with a uniform and constant magnetic field: These modes have phase velocities that are independent of the magnitude of the wavevector, so they experience no dispersion. The phase velocity depends on the angle between the wave vector k and the magnetic field B . An MHD wave propagating at an arbitrary angle θ with respect to the time independent or bulk field B 0 will satisfy

2067-399: The location of Earth's magnetopause (the boundary between the Earth's magnetic field and the solar wind), the formation of the ring current , auroral electrojets , and geomagnetically induced currents . One prominent use of global MHD models is in space weather forecasting. Intense solar storms have the potential to cause extensive damage to satellites and infrastructure, thus it

2120-478: The magnetic domains (which are thousands to hundreds of thousands of kilometers across). Another example is in the Earth's magnetosphere , where current sheets separate topologically distinct domains, isolating most of the Earth's ionosphere from the solar wind . The wave modes derived using the MHD equations are called magnetohydrodynamic waves or MHD waves . There are three MHD wave modes that can be derived from

2173-452: The non-Maxwellian shape of the distribution function. However, because MHD is relatively simple and captures many of the important properties of plasma dynamics it is often qualitatively accurate and is therefore often the first model tried. Effects which are essentially kinetic and not captured by fluid models include double layers , Landau damping , a wide range of instabilities, chemical separation in space plasmas and electron runaway. In

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2226-405: The observations as the simulations have correctly predicted that the Earth's magnetic field flips every few hundred thousand years. During the flips, the magnetic field does not vanish altogether—it just gets more complex. Some monitoring stations have reported that earthquakes are sometimes preceded by a spike in ultra low frequency (ULF) activity. A remarkable example of this occurred before

2279-492: The occasion of Treder's 70th birthday. The relationships between Hans-Jürgen Treder, Hannes Alfvén and Wilfried Schröder were discussed in detail by Schröder in his publications. Alfvén died on 2 April, 1995 at Djursholm aged 86. The Hannes Alfvén Prize , awarded annually by the European Physical Society for outstanding contributions in plasma physics, is named after him. The asteroid 1778 Alfvén

2332-464: The other terms such that it can be taken to be equal to zero. This occurs in the limit of large magnetic Reynolds numbers during which magnetic induction dominates over magnetic diffusion at the velocity and length scales under consideration. Consequently, processes in ideal MHD that convert magnetic energy into kinetic energy, referred to as ideal processes , cannot generate heat and raise entropy . A fundamental concept underlying ideal MHD

2385-489: The phenomena within the system (see Astrophysical plasma ). Sunspots are caused by the Sun's magnetic fields, as Joseph Larmor theorized in 1919. The solar wind is also governed by MHD. The differential solar rotation may be the long-term effect of magnetic drag at the poles of the Sun, an MHD phenomenon due to the Parker spiral shape assumed by the extended magnetic field of the Sun. Previously, theories describing

2438-625: The problem with the Big Bang was that astrophysicists tried to extrapolate the origin of the universe from mathematical theories developed on the blackboard, rather than starting from known observable phenomena. He also considered the Big Bang to be a myth devised to explain creation. Alfvén and colleagues proposed the Alfvén–Klein model as an alternative cosmological theory to both the Big Bang and steady state theory cosmologies. Alfvén

2491-540: The same due to the Coriolis effect . These eddies develop a magnetic field which boosts Earth's original magnetic field—a process which is self-sustaining and is called the geomagnetic dynamo. Based on the MHD equations, Glatzmaier and Paul Roberts have made a supercomputer model of the Earth's interior. After running the simulations for thousands of years in virtual time, the changes in Earth's magnetic field can be studied. The simulation results are in good agreement with

2544-418: The system over which ideal MHD is broken and magnetic diffusion can occur quickly. When this happens, magnetic reconnection may occur in the plasma to release stored magnetic energy as waves, bulk mechanical acceleration of material, particle acceleration , and heat. Magnetic reconnection in highly conductive systems is important because it concentrates energy in time and space, so that gentle forces applied to

2597-549: The theory of magnetic storms and auroras and the theory of plasma dynamics in the Earth 's magnetosphere . This was the paper rejected by the U.S. journal Terrestrial Magnetism and Atmospheric Electricity . Applications of Alfvén's research in space science include: Alfvén's views followed those of the founder of magnetospheric physics, Kristian Birkeland . At the end of the nineteenth century, Birkeland proposed (backed by extensive data) that electric currents flowing down along

2650-617: The universe, it could then carry electric currents capable of generating a galactic magnetic field. After winning the Nobel Prize for his works in magnetohydrodynamics , he emphasized that: In order to understand the phenomena in a certain plasma region, it is necessary to map not only the magnetic but also the electric field and the electric currents. Space is filled with a network of currents which transfer energy and momentum over large or very large distances. The currents often pinch to filamentary or surface currents. The latter are likely to give space, as also interstellar and intergalactic space,

2703-465: Was irreligious and critical of religion. He spoke Swedish, English, German, French, and Russian, and some Spanish and Chinese. He expressed great concern about the difficulties of permanent high-level radioactive waste management ." Alfvén was also interested in problems in cosmology and all aspects of auroral physics, and used Schröder's well known book on aurora, Das Phänomen des Polarlichts . Letters of Alfvén, Treder, and Schröder were published on

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2756-569: Was initiated by Hannes Alfvén , for which he received the Nobel Prize in Physics in 1970. The MHD description of electrically conducting fluids was first developed by Hannes Alfvén in a 1942 paper published in Nature titled "Existence of Electromagnetic–Hydrodynamic Waves" which outlined his discovery of what are now referred to as Alfvén waves . Alfvén initially referred to these waves as "electromagnetic–hydrodynamic waves"; however, in

2809-497: Was married for 67 years to his wife Kerstin (1910–1992). They raised five children, one boy and four girls. Their son became a physician, while one daughter became a writer and another a lawyer in Sweden. The writer was Inger Alfvén and is well known for her work in Sweden. The composer Hugo Alfvén was Hannes Alfvén's uncle. Alfvén studied the history of science , oriental philosophy , and religion . On his religious views, Alfven

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