Wed 17 Jan 2018

Diocotron instability

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Diocotron instability vortices photographed on a fluorescent screen.
Auroral curls showing Kelvin Helmholtz instabilities, that are sometimes attributed to diocotron instability. Credit: Trond Trondsen / Institute for Space Research, Calgary, Canada.[1]

The diocotron instability (also called the slipping stream plasma instability), is "one of the most ubiquitous instabilities in low density nonneutral plasmas with shear in the flow velocity [.. that can ..] occur in propagating non-neutral electron beams and layers".[2] [3] It may give rise to electron vortices,[4], which resembles the Kelvin-Helmholtz fluid dynamical shear instability, and occurs when charge neutrality is not locally maintained.[5] The term diocotron derives from the Greek διωκειν, meaning "pursue."[5]


Although usually attributed to Kelvin Helmholtz instabilities,[6] the diocotron instability has been associated with the aurora (where they are known as auroral curls or auroral vortices), the mechanism having been proposed by Hannes Alfvén in 1950.[5][7]

See image, right.

Galactic arms

The arms of galaxies (eg. NGC 3646) are susceptible to the diocotron instability [8] In the simulation of galaxy formation, Peratt found that:

Since Ez is out of the plane of the page, the column electrons spiral downward in counter-clockwise rotation while the column ions spiral upward in clockwise rotation. A polarization induced charge separation also occurs in each arm, which, as it thins out, produces a radial electric field across the arm. Because of this field, the arm is susceptible to the diocotron instability. This instability appears as a wave motion in each arm.[9]


The Diocotron instability has also been associated with a pulsar's electrosphere.[10]

Solar coronal waves

Recent waves discovered in the Sun's corona,[11] are attributed to Kelvin-Helmholtz instabilities because the solar coronal plasma is generally considered to be neutral.[12]


  1. Trond Trondsen / Institute for Space Research, Calgary, Canada. See also.
  2. Ronald C. Davidson, "Physics of Nonneutral Plasmas", Published 2001, Imperial College Press, 750 pages, ISBN 1860943039 (page 289) ACADEMIC BOOK
  3. R. H. Levy, "Diocotron Instability in a Cylindrical Geometry" Phys. Fluids 8, 1288 (1965); DOI:10.1063/1.1761400 PEER REVIEWED
  4. Mikhail V. Nezlin, "Physics of Intense Beams in Plasmas", translated by E. W Laing and Vitaly I. Kisin, published 1993, CRC Press, ISBN 0750301864 (page 142) ACADEMIC BOOK
  5. 5.0 5.1 5.2 Anthony L. Peratt, Physics of the Plasma Universe Ch.1 Cosmic Plasma Fundamentals, 1.7.3 The Diocotron Instability, p.29 (1992) ACADEMIC BOOK
  6. T. S. Trondsen and L. L. Cogger "Fine-scale optical observations of Aurora", Physics and Chemistry of the Earth, Part C: Solar, Terrestrial & Planetary Science, Volume 26, Issues 1-3, January 2001, Pages 179-188. See also papers by T. Hallinan, Refs 36-39
  7. Hannes Alfvén, Cosmical Electrodynamics (1950) International Series of Monographs on Physics, Oxford: Clarendon Press, 1950 ACADEMIC BOOK
  8. Anthony L. Peratt, Physics of the Plasma Universe Ch.3 Biot-Savart Law in Cosmic Plasma (1992) ACADEMIC BOOK
  9. Charles M. Snell and Anthony L. Peratt, "Rotation Velocity and Neutral Hydrogen Distribution Dependency on Magnetic Field Strength in Spiral Galaxies" FULL TEXT, Astrophysics and Space Science 227 (May 1995)], Proceedings Second IEEE International Workshop on Plasma Astrophysics and Cosmology (10-12 May 1993). (1995) PEER REVIEWED
  10. J. Pétri, "Relativistic stabilisation of the diocotron instability in a pulsar "cylindrical" electrosphere", Astronomy & Astrophysics, 469, 843-855 (2007) DOI: 10.1051/0004-6361:20066985 PEER REVIEWED
  11. "NASA's Solar Dynamics Observatory Catches "Surfer" Waves on the Sun", 06.07.11
  12. Prof. Leon Ofman, private communication, 20 June 2011