Sun and stars

From (The Plasma Universe Wikipedia-like Encyclopedia)

Jump to: navigation, search
 A solar flare takes a simlar shape to that of an Electric Birkeland current.
Enlarge
A solar flare takes a simlar shape to that of an Electric Birkeland current.
Enlarge
See also Star formation

A Star and hence our Sun, is an almost entirely ionized ball of plasma, consisting of electrons and ions, in which there is hardly any gas (neutral atoms). The movement of the plasma produces strong magnetic fields and corresponding electric currents.

The structure of our Sun is thought to include (from inside to out), the core, the radiative zone, the convection zone, the photosphere (visible surface), and its atmosphere comprising of five main zones: the temperature minimum, the chromosphere, the transition region, the corona, and the heliosphere.

Also associated with the Sun are sun spots, solar flares, coronal mass ejections.

Contents

Composition

The Sun and stars consist of very little actual hydrogen and helium gas. Because the temperatures are so high, the atoms are nearly completely ionized into hydrogen ions and helium ions, ie. a plasma. These ions are quite different to the gaseous atoms, and behave quite differently.

"Most of the atoms in the Sun are ionized. This is particularly true in the hot, dense interior, where essentially all the hydrogen and helium atoms are completely ionized. Such a highly ionized gas is called a plasma. So, although it is common to see the Sun referred to as a gaseous body, a more specific description is that it is made of plasma. In this case, the plasma consists of hydrogen and helium ions, together with the electrons that were liberated when those ions were produced."[1]

Magnetic field

The influence of the Sun's rotating magnetic field on the plasma in the interplanetary medium creates the heliospheric current sheet, which separates regions with magnetic fields pointing in different directions. The plasma in the interplanetary medium is also responsible for the strength of the Sun's magnetic field at the orbit of the Earth. If space were a vacuum, then the Sun's 10-4 tesla magnetic dipole field would reduce with the cube of the distance to about 10-11 tesla. But satellite observations show that it is about 100 times greater at around 10-9 tesla. Magnetohydrodynamic (MHD) theory predicts that the motion of a conducting fluid (e.g., the interplanetary medium) in a magnetic field, induces electric currents which in turn generates magnetic fields, and in this respect it behaves like an MHD dynamo.

Unipolar generator

Hannes Alfven argued that since a conductor rotating in a magnetic field produces an electric current, then the Sun behaves as a unipolar inductor:

The central body acts as a unipolar inductor and the e .m.f. is produced in region A (see Figure III.7). The mechanical force on the solar atmosphere dF = I ds x B tends to decelerate the rotation of the central body. The current transfers angular momentum from the central body to the surrounding plasma. Hence, we have a decelerating force applied to the solar atmosphere in the polar region. This should produce a non-uniform rotation of the Sun of the type which is observed (angular velocity decreasing with increasing latitude. Whether this interpretation is the correct quantitative explanation of the non-uniform rotation is an open question.[2]

Electric currents

The high density plasma in the Sun (and stars), is known to produce current loops in solar flares,[3] [4] [5] [6] current sheets in the Solar Corona,[7] and general twisting electric current helicity in the solar atmosphere.[8] The tenuous interplanetary medium (between the Sun and planets) is known to produce the heliospheric current sheet, whose currents are represented by the Heliospheric current circuit. The more tenuous interstellar medium (between solar systems) produces a current sheet,[9] and electric currents have also been described in interstellar molecular clouds.[10]

Hannes Alfven observed x-ray images of the sun from the skylab program, and attributed those million degree loops to electrical activity in the solar atmosphere[11], a similar process was described by Jacobsen and Carlqvist in 1964[12]. Alfven also considered the heliospheric current sheet to be part of a heliospheric current circuit , as he believed all cosmic plasmas to be part of a "plasma circuit". [13] [14]

Kristian Birkeland's Terella experiments gave further evidence for electrical activity on the sun by replicating many of the suns characteristics in tests on Earth (see extra links below). This involved a magnetic globe acting as an electrically charged cathode in a large vacuum-box. He was able to replicate many aspects of the sun, including the corona, plasma torus, sunspots, and the so called magnetic reconnection, which is better described as a filamental pinch in plasma physics. Later terella experiments were performed by various scientists, studying various aspects of the interaction of the Earth's magnetic field in space. Apparently such experiments are difficult to interpret, which is why such experiments have now been completely replaced by computer simulations.

There have been numerous other observations of strong electrical activity on the suns surface and in the solar wind[15].. Scientists using the Solar and Heliospheric Observatory (SOHO) spacecraft have discovered "jet streams" or "rivers" of hot electrically charged gas flowing beneath the surface of the Sun[16].

Stellar Winds and Coronal Heating

The solar wind is a stream of charged particles—a plasma—that are ejected from the upper atmosphere of the sun and stars. It consists mostly of electrons and protons with energies of about 1 keV. These particles are able to escape the sun's gravity, in part because of the high temperature of the corona, but also because of high kinetic energy that particles gain through a process that is not well-understood at this time. Thought to be related to the origin of the solar wind is one of the most persistently enigmatic observations of the near-solar environment; the temperature inversion from approximately 5000K on the surface of the sun, to over 2 million K in the lower corona.

Many phenomena are directly related to the solar wind, including geomagnetic storms that can knock out power grids on Earth, the Aurora, and the plasma tails of comets, that always point away from the sun. While early models of the solar wind used primarily thermal energy to accelerate the material, by the 1960s it was clear that thermal acceleration alone cannot account for the acceleration of the solar wind. An additional unknown acceleration mechanism is required. It is thought by most that the problems of the origin of solar wind acceleration and the coronal heating problem are interlinked issues, and most potential solutions proposed to date attempt to solve both problems with one theory[17][18]

One such mechanism that has been proposed as a solution for both issues is the formation of a plasma double layer on the suns surface, that acts in a similar way to a pnp transistor.[19] This suggests that ions near the suns surface are accelerated by the E-field produced by the double layer, and that in this transition region the movement of these ions becomes highly organized (parallel) which reduces the browninan motion of the ions and thus produces the temperature minimum. This method could also explain the fluctuations in the solar wind that have been observed, and could potentially explain the enigmatic period of two days when the solar wind was shown to nearly completely stop (a depletion of 97%)[20]. One of the other more feasable solutions that has been proposed for both problems are Alfvén waves, which have been shown to be able to produce a significant fraction of the acceleration of particles in the solar wind, and could also explain the photospheric temparature minimum.[21]. There have yet to be any definitive models proposed in the literature for these issues.


Footnotes

  1. ^ Simon F. Green, Mark H. Jones, S. Jocelyn Burnell, An Introduction to the Sun and Stars 2004, Cambridge University Press, 2004, ISBN 0521546222, 9780521546225, 373 pages. Page 46
  2. ^ Hannes Alfvén, "Cosmic Plasma", Ch.III.4.3. "Properties of the Heliospheric Current Circuit (page 56)
  3. ^ Hannes Alfvén, per Carlqvist "Currents in the Solar Atmosphere and a Theory of Solar Flares" FULL TEXT (1967) Solar Physics, Vol. 1, p.220 PEER REVIEWED
  4. ^ Carlqvist, P., "Current Limitation and Solar Flares" FULL TEXT (1969) Solar Physics, Vol. 7, p.377 PEER REVIEWED
  5. ^ Karlicky, M., "Evolution of force-free electric currents in the solar atmosphere" FULL TEXT (1997) Astronomy and Astrophysics, v.318, p.289-292 PEER REVIEWED
  6. ^ Zaitsev, V. V.; Stepanov, A. V., "Towards the circuit theory of solar flares" FULL TEXT (1992) Solar Physics (ISSN 0038-0938), vol. 139, no. 2, June 1992, p. 343-356 PEER REVIEWED
  7. ^ Strauss, H. R.; Otani, N. F., "Current sheets in the solar corona" FULL TEXT (1988) Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 326, March 1, 1988, p. 418-424 PEER REVIEWED
  8. ^ Seehafer, N., "Electric current helicity in the solar atmosphere" FULL TEXT (1990) Solar Physics (ISSN 0038-0938), vol. 125, Feb. 1990, p. 219-232. PEER REVIEWED
  9. ^ Zweibel, Ellen G.; Brandenburg, Axel, "Current Sheet Formation in the Interstellar Medium" (1997) Astrophysical Journal v.478, p.563 PEER REVIEWED. See also Erratum.
  10. ^ Carlqvist, Per; Gahm, Gosta F., "Manifestations of electric currents in interstellar molecular clouds" (1992) IEEE Transactions on Plasma Science vol. 20, no. 6, p. 867-873. PEER REVIEWED
  11. ^ Alfven, Hannes Double layers and circuits in astrophysics (1986) Space Technology Plasma Issues in 2001 p 409-463 PEER REVIEWED
  12. ^ Jacobsen, C; Carlqvist, P Solar Flares Caused by Circuit Interruptions (1964) Icarus, vol. 3, p.270. PEER REVIEWED
  13. ^ Alfven, H., "Double radio sources and the new approach to cosmical plasma physics" FULL TEXT (1978) Astrophysics and Space Science, vol. 54, no. 2, Apr. 1978, p. 279-292. PEER REVIEWED
  14. ^ Hannes Alfvén, "The Heliospheric Current System (sec III:4)" in Cosmic Plasma, Astrophysics and Space Science Library, Vol. 82 (1981) Springer Verlag. ISBN 90-277-1151-8 ACADEMIC BOOK
  15. ^ Coleman, P. J. Electric currents in the solar wind (1975) Journal of Geophysical Research. PEER REVIEWED
  16. ^ NASA Jet Stream Runs Swiftly Inside the Sun (1997) NASA Headquarters Washington DC. N97-60.
  17. ^ Marsch, E. & Tu, C.-Y The effects of high-frequency Alfven waves on coronal heating and solar wind acceleration. 03/1997 Astronomy and Astrophysics, v.319, p.L17-L20 PEER REVIEWED
  18. ^ McComas, D. J. Velli, M. Lewis, W. S. Acton, L. W. Balat-Pichelin, M. Bothmer, V. Dirling, R. B. Feldman, W. C. Gloeckler, G. Habbal, S. R. Understanding Coronal Heating And Solar Wind Acceleration: Case For In Situ Near-Sun Measurements (2007) Reviews Of Geophysics, VOL 45; NUMB 1, pages RG1004 PEER REVIEWED
  19. ^ Scott, D.E. A Solar Junction Transistor Mechanism 17-22 June 2007, IEEE Pulsed Power Plasma Science, 10.1109/PPPS.2007.4346305PEER REVIEWEDFULL TEXT
  20. ^ NASA The Day the Solar Wind Disappeared (1999) NASA/GSFC Press release. Dec. 13,
  21. ^ S. Tomczyk, S. W. McIntosh, S. L. Keil, P. G. Judge, T. Schad, D. H. Seeley and J. Edmondson, "Waves in the Solar Corona" (2007) Science Magazine, Vol. 317. no. 5842, pp. 1192-1196,PEER REVIEWEDFULL TEXT

Extra Links

Personal tools