Alfvén wave

In plasma physics, an Alfvén wave, named after Hannes Alfvén, is a type of magnetohydrodynamic wave in which ions oscillate in response to a restoring force provided by an effective tension on the magnetic field lines.[1]

Definition

An Alfvén wave in a plasma is a low-frequency (compared to the ion cyclotron frequency) travelling 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.

Alfvén velocity

The low-frequency relative permittivity \epsilon\, of a magnetized plasma is given by

\epsilon = 1 + \frac{1}{B^2}c^2 \mu_0 \rho

where B\, is the magnetic field strength, c\, is the speed of light, \mu_0\, is the permeability of the vacuum, and \rho = \Sigma n_s m_s\, is the total mass density of the charged plasma particles. Here, s\, goes over all plasma species, both electrons and (few types of) ions.

Therefore, the phase velocity of an electromagnetic wave in such a medium is

v = \frac{c}{\sqrt{\epsilon}} = \frac{c}{\sqrt{1 + \frac{1}{B^2}c^2 \mu_0 \rho}}

or

v = \frac{v_A}{\sqrt{1 + \frac{1}{c^2}v_A^2}}

where

v_A = \frac{B}{\sqrt{\mu_0 \rho}}

is the Alfvén velocity. If v_A \ll c, then v \approx v_A. On the other hand, when v_A \gg c, then v \approx c. That is, at high field or low density, the velocity of the Alfvén wave approaches the speed of light, and the Alfvén wave becomes an ordinary electromagnetic wave.

Neglecting the contribution of the electrons to the mass density and assuming that there is a single ion species, we get

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

where n_i\, is the ion number density and m_i\, is the ion mass.

Alfvén time

In plasma physics, the Alfvén time  \tau_A is an important timescale for wave phenomena. It is related to the Alfvén velocity by:

\tau_A = \frac{a}{v_A}

where  a denotes the characteristic scale of the system, for example  a is the minor radius of the torus in a tokamak.

Relativistic case

The general Alfvén wave velocity is defined by Gedalin (1993):[2]

v = \frac{c}{\sqrt{1 + \frac{e + P}{2 P_m}}}

where

e\, is the total energy density of plasma particles, P\, is the total plasma pressure, and P_m = \frac{1}{2\mu_0}B^2\, is the magnetic field pressure. In the non-relativistic limit P \ll e \approx \rho c^2, and we immediately get the expression from the previous section.

Heating the Corona

Cold plasma floating in the corona above the solar limb. Alfvén waves were observed for the first time, extrapolated from fluctuations of the plasma.

The coronal heating problem is a longstanding question in heliophysics. It is unknown why the sun's corona lives in a temperature range higher than one million degrees while the sun's surface (photosphere) is only a few thousand degrees in temperature. Natural intuition would predict a decrease in temperature while getting farther away from a heat source, but it is theorized that the photosphere, influenced by the sun's magnetic fields, emits certain waves which carry energy (i.e. heat) to the corona and solar wind. It is important to note that because the density of the corona is quite a bit smaller than the photosphere, the heat and energy level of the photosphere is much higher than the corona. Temperature depends only on the average speed of a species, and less energy is required to heat fewer particles to higher temperatures in the coronal atmosphere. Alfvén first proposed the existence of an electromagnetic-hydrodynamic wave in 1942 in Nature. He claimed the sun had all necessary criteria to support these waves and that they may in turn be responsible for sun spots. From his paper:

Magnetic waves, called Alfvén S-waves, flow from the base of black hole jets.
If a conducting liquid is placed in a constant magnetic field, every motion of the liquid gives rise to an E.M.F. which produces electric currents. Owing to the magnetic field, these currents give mechanical forces which change the state of motion of the liquid. Thus a kind of combined electromagnetic-hydrodynamic wave is produced.
Hannes Alfvén, Existence of Electromagnetic-Hydrodynamic Waves, [3]

Beneath the sun's photosphere lies the convection zone. The rotation of the sun, as well as varying pressure gradients beneath the surface, produces the periodic electromagnetism in the convection zone which can be observed on the sun's surface. This random motion of the surface gives rise to Alfvén waves. The waves travel through the chromosphere and transition zone and interact with much of the ionized plasma. The wave itself carries energy as well as some of the electrically charged plasma. De Pontieu[4] and Haerendel [5] suggested in the early 1990s that Alfven waves may also be associated with the plasma jets known as spicules. It was theorized these brief spurts of superheated gas were carried by the combined energy and momentum of their own upward velocity, as well as the oscillating transverse motion of the Alfven waves. In 2007, Alfven waves were reportedly observed for the first time traveling towards the corona by Tomcyzk et al., but their predictions could not conclude that the energy carried by the Alfven waves were sufficient to heat the corona to its enormous temperatures, for the observed amplitudes of the waves were not high enough.[6] However, in 2011, McIntosh et al. reported the observation of highly energetic Alfven waves combined with energetic spicules which could sustain heating the corona to its million Kelvin temperature. These observed amplitudes (20.0 km/s against 2007's observed 0.5 km/s) contained over one hundred times more energy than the ones observed in 2007.[7] The short period of the waves also allowed more energy transfer into the coronal atmosphere. The 50,000 km long spicules may also play a part in accelerating the solar wind past the corona.[8]

History

How this phenomenon became understood

See also

References

  1. Iwai, K; Shinya, K,; Takashi, K. and Moreau, R. (2003) "Pressure change accompanying Alfvén waves in a liquid metal" Magnetohydrodynamics 39(3): pp. 245-250, page 245
  2. Gedalin, M. (1993), "Linear waves in relativistic anisotropic magnetohydrodynamics", Physical Review E 47 (6): 4354–4357, Bibcode:1993PhRvE..47.4354G, doi:10.1103/PhysRevE.47.4354
  3. Hannes Alfvén (1942). "Existence of Electromagnetic-Hydrodynamic Waves". Nature 150 (3805): 405–406. Bibcode:1942Natur.150..405K. doi:10.1038/150405a0.
  4. Bart de Pontieu (18 December 1997). "Chromospheric Spicules driven by Alfvén waves". Max-Planck-Institut für extraterrestrische Physik. Retrieved 1 April 2012.
  5. Gerhard Haerendel (1992). "Weakly damped Alfven waves as drivers of solar chromospheric spicules". Nature 360: 241–243. Bibcode:1992Natur.360..241H. doi:10.1038/360241a0.
  6. Tomczyk, S.; McIntosh, S.W.; Keil, S.L.; Judge, P.G.; Schad, T.; Seeley, D.H.; Edmondson, J. (2007). "Alfven waves in the solar corona". Science 317 (5842): 1192–1196. Bibcode:2007Sci...317.1192T. doi:10.1126/science.1143304.
  7. McIntosh; et al. (2011). "Alfvenic waves with sufficient energy to power the quiet solar corona and fast solar wind.". Nature 475 (7357): 477–480. Bibcode:2011Natur.475..477M. doi:10.1038/nature10235.
  8. Karen Fox (27 July 2011). "SDO Spots Extra Energy in the Sun's Corona.". NASA. Retrieved 2 April 2012.
  9. http://adsabs.harvard.edu/abs/2009ApJ...703.1318K
  10. Thierry Alboussière; Philippe Cardin; François Debray; Patrick La Rizza; Jean-Paul Masson; Franck Plunian; Adolfo Ribeiro; Denys Schmitt (2011). "Experimental evidence of Alfvén wave propagation in a Gallium alloy". Phys. Fluids 23 (9): 096601. arXiv:1106.4727. Bibcode:2011PhFl...23i6601A. doi:10.1063/1.3633090.

Further reading

External links

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