Alfvén wave

Alfvén wave
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An Alfvén wave, named after Hannes Alfvén, is a type of magnetohydrodynamic wave.

content

1 Mathematical definition
2 History
3 Further reading
4 External links

Mathematical definition

An Alfvén wave in a plasma is a traveling oscillation of the ions and the magnetic field. The ion mass density provides the inertia and the magnetic field line tension provides the restoring force.

The wave propagates in the direction of the magnetic field, although waves exist at oblique incidence and smoothly change into the magnetosonic wave when the propagation is perpendicular to the magnetic field.

The motion of the ions and the perturbation of the magnetic field are in the same direction and transverse to the direction of propagation.

The wave is dispersionless with a speed of

$v_A = \frac{B}{\sqrt{\mu_0 n_i m_i}}\,$ in SI

$v_A = \frac{B}{\sqrt{4 \pi n_i m_i}}\,$ in cgs

$\qquad \ = (2.18\times10^{11}\,\mbox{cm/s})\,(m_i/m_p)^{-1/2}\,(n_i/{\rm cm}^{-3})^{-1/2}\,(B/{\rm gauss})$

where $v_A\,$ is the velocity of the Alfvén wave, B is the magnetic field strength, $\mu_0\,$ is the permeability of the plasma, $n_i\,$ is the ion number density, and $m_i\,$ is the ion mass.

At high field or low density, the Alfvén speed approaches the speed of light, and the Alfvén wave becomes an ordinary electromagnetic wave.

History

How this phenomenon became understood

1942: Alfvén suggests the existence of electromagnetic-hydromagnetic waves in a paper published in Nature.
1949: Laboratory experiments by S. Lundquist produce such waves in magnetized mercury, with a velocity that approximated Alfvén's formula.
1949: Enrico Fermi uses Alfvén waves in his theory of cosmic rays. According to Alex Dessler in a 1970 Science journal article, Fermi had heard a lecture at the University of Chicago, Fermi nodded his head exclaiming "of course" and the next day, the physics world said "of course".
1950: Alfvén publishes the first edition of his book, Cosmical Electrodynamics, detailing hydromagnetic waves, and discussing their application to both laboratory and space plasmas.
1952: Additional confirmation appears in experiments by Winston Bostick and Morton Levine with ionized helium
1954: Bo Lehnert produces Alfvén waves in liquid sodium
1958: Eugene Parker suggests hydromagnetic waves in the interstellar medium
1958: Berthold, Harris, and Hope detect Alfvén waves in the ionosphere after the Argus nuclear test, generated by the explosion, and traveling at speeds predicted by Alfvén formula.
1958: Eugene Parker suggests hydromagnetic waves in the Solar corona extending into the Solar wind.
1959: D. F. Jephcott produces Alfvén waves in a gas discharge
1960: Coleman, et al, report the measurement of Alfvén waves by the magnetometer aboard the Pioneer and Explorer satellites
1960: Sugiura suggests evidence of hydromagnetic waves in the Earth's magnetic field
1966: R.O.Motz generates and observes Alfven waves in mercury
1970 Hannes Alfvén wins the 1970 Nobel Prize in physics for "for fundamental work and discoveries in magneto-hydrodynamics with fruitful applications in different parts of plasma physics"
1973: Eugene Parker suggests hydromagnetic waves in the intergalactic medium
1974: Hollweg suggests the existence of hydromagnetic waves in interplanetary space
1974: Ip and Mendis suggests the existence of hydromagnetic waves in the coma of Comet Kohoutek.
2007: Tomczyk, et al, report the detection of Alfvén waves in images of the solar corona with the Coronal Multi-Channel Polarimeter (CoMP) instrument at the National Solar Observatory, New Mexico.
2007: Alfvén wave discoveries appear in articles by Jonathan Cirtain and colleagues, Takenori J. Okamoto and colleagues, and Bart De Pontieu and colleagues. De Pontieu’s team also shows that the energy associated with the waves is sufficient to heat the corona and accelerate the solar wind. These results appear in a special collection of 10 articles, by scientists in Japan, Europe and the United States, in the 7 December issue of the journal Science.

External links

Related websites

Mysterious Solar Ripples Detected Dave Mosher 02 September 2007 Space.com
EurekAlert! notification of 7th December 2007 Science special issue

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Contents

Mathematical definition

History

Further reading

External links