Heliospheric current sheet
Heliospheric current sheet, the largest structure in the heliosphere. Credit: Werner Heil, NASA artists, developed by Prof. John Wilcox.

Heliospheric current sheet

The Heliospheric current sheet (HCS) is the surface within the Solar System where the polarity of the Sun’s magnetic field changes from north to south. This field extends from the Sun’s equatorial plane throughout the entire Solar System and is the largest structure in the heliosphere.[1] The shape of the current sheet results from the influence of the Sun’s rotating magnetic field on the plasma in the interplanetary medium (Solar Wind).[2] (see also Unipolar generator). A small electrical current flows within the sheet, about 10-10 amps/m2. The thickness of the current sheet is about 10,000km.

The underlying magnetic field is called the interplanetary magnetic field, which has an associated interplanetary electric field [3], and the resulting electric current forms part of the heliospheric current circuit.[4] The Heliospheric current sheet is also sometimes called the Interplanetary Current Sheet and Heliospheric neutral sheet. See also “Current sheet“.

Characteristics

Ballerina’s skirt shape

As the Sun rotates, its magnetic field twists into a Parker spiral,[5] a form of an Archimedean spiral, named after its discovery by Eugene Parker. As the spiraling magnetic sheets changes polarity, it warps into a wavy spiral shape that has been likened to a ballerina’s skirt.[6][7] Further dynamics have suggested that “The Sun with the heliosheet is like a bashful ballerina who is repeatedly trying to push her excessively high flaring skirt downward”.[8]

Magnetic field

The heliospheric current sheet rotates along with the Sun once every 27 days, during which time the peaks and troughs of the skirt pass through the Earth’s magnetosphere, interacting with it. Near the surface of the Sun, the magnetic field produced by the radial electric current in the sheet is of the order of 5×10-6T.[4]

The magnetic field at the surface of the Sun is about 10-4 tesla. If the form of the field were a magnetic dipole, the strength would decrease with the cube of the distance, resulting in about 10-11 tesla at the Earth’s orbit. The heliospheric current sheet results in higher order multipole components so that the actual magnetic field at the Earth due to the Sun is 100 times greater.

Electric current

The electric current in the heliospheric current sheet has a radially component, the circuit being closed by currents aligned with the Sun’s magnetic field in the solar polar regions. The total current in the circuit is on the order of 3×109 amperes.[4] As a comparison with other astrophysical electric currents, the Birkeland currents that supply the Earth’s aurora are about a thousand times weaker at a million amperes. The maximum current density in the sheet is on the order of 10-10 A/m2 (10-4 amps/km2).

It has been noted that:

“It is remarkable that the radial component of the spiral structure implies a current the continually flows towards the Sun. The charge accumulating from this process must be removed elsewhere. This occurs most simply via line currents that originate over the Sun’s poles”[9]

Interplanetary electric field

The interplanetary electric field (IEF) extends throughout the interplanetary current sheet, and is generally orientated north-south. The separation of the field is relatively small, but its extent is the same as the heliospheric current sheet which extends throughout the plasmasphere.

The interplanetary electric field is caused by ions leaving the Sun, initially flowing along and parallel to the Sun’s magnetic field. But as the ions move further outwards, the azimuthal component of the Sun’s magnetic field becomes more influential, and protons are deflected to the south and electrons to the north, resulting in an electric field that compensates the magnetic forces.[10]

Solar wind

“The Solar Wind consists of a hot plasma — an electrically neutral mixture of electrons and ions (principally protons with some heavier atomic nuclei) at roughly 100,000°K. Its source is the Sun’s atmosphere, or corona, and it is continually present in interplanetary space. The gas flows radially outwards at a typical speed of 450km per second to at least 70 AU and probably much further. The average speed of the flowing gas is remarkably independent of its distance from the Sun”.[11]

Solar wind acceleration

“The speed of the solar wind away from Sun increases as the distance from the Sun increases. The wind accelerates rapidly in the first few tens of Ro, and accelerates only slowly after this.”[12]

History

The heliospheric current sheet was discovered by John M. Wilcox and Norman F. Ness, who published their finding in 1965 [13].

The image above is a painting by NASA artist, Werner Heil. It was developed by Prof. John Wilcox as a tool for visualizing the surface that separates the two magnetic polarity regions produced by the Sun in the solar system. His concept was that a “baseball seam” shape located near the Sun separates the two magnetic hemispheres of the interplanetary medium; the shape was determined by the large-scale magnetic field at the Sun. That geometrical shape is carried radially outward by the solar wind. As the Sun, and the magnetic field configuration it generates, continue to rotate underneath the structure, the resulting surface becomes the one you see in the painting.[14]

Hannes Alfvén and Per Carlqvist speculate[15] on the existence of a galactic current sheet, a counterpart of the heliospheric current sheet, with an estimated galactic current of 1017 – 1019 Amps, that might flow in the plane of symmetry of the galaxy.

References

  1. ↑ Dr. Tony Phillips, A Star with two North Poles April 22, 2003, Science@NASA via Archive.org
  2. ↑ Artist’s Conception of the Heliospheric Current Sheet, Wilcox Solar Observatory
  3. ↑ Duncan Alan Bryant “Electron Acceleration in the Aurora and Beyond”, Published 1999, CRC Press, 311 pages, ISBN 0750305339 (page 176) ACADEMIC BOOK
  4. ↑ 4.0 4.1 4.2 Israelevich, P. L., et al, “MHD simulation of the three-dimensional structure of the heliospheric current sheet” (2001) Astronomy and Astrophysics, v.376, p.288-291 FULL TEXT PEER REVIEWED
  5. ↑ Parker, E. N., “Dynamics of the Interplanetary Gas and Magnetic Fields“, (1958) Astrophysical Journal, vol. 128, p.664 FULL TEXT PEER REVIEWED
  6. ↑ Rosenberg, R. L. and P. J. Coleman, Jr., Heliographic latitude dependence of the dominant polarity of the interplanetary magnetic field, J. Geophys. Res., 74 (24), 5611-5622, 1969. PEER REVIEWED
  7. ↑ Wilcox, J. M.; Scherrer, P. H.; Hoeksema, J. T., “The origin of the warped heliospheric current sheet” (1980) PEER REVIEWED
  8. ↑ Mursula, K.; Hiltula, T., “Bashful ballerina: Southward shifted heliospheric current sheet]” (2003), Geophysical Research Letters, Volume 30, Issue 22, pp. SSC 2-1 PEER REVIEWED
  9. ↑ Gerd W. Prölss, “Physics of the Earth’s Space Environment: An Introduction” Translated by M. K. Bird, Published 2004, Springer, 514 pages ISBN 3540214267 (page 309) ACADEMIC BOOK
  10. ↑ Gerd W. Prölss, Physics of the Earth’s Space Environment: An Introduction, (2004) Translated by M. K. Bird, Springer, 514 pages, ISBN 3540214267 (pages 312-313)
  11. ↑ J. Kelly Beatty, Carolyn Collins Petersen, Andrew Chaikin, The New Solar System, Edition: 4, illustrated, revised, Published by Cambridge University Press, 1999, ISBN 0521645875, ISBN 9780521645874, 421 pages (page 40)
  12. ↑ Simon F. Green, Mark H. Jones, S. Jocelyn Burnell, An Introduction to the Sun and Stars, Published by Cambridge University Press, 2004, ISBN 0521546222, ISBN 9780521546225, 373 pages (page 75)
  13. ↑ John M. Wilcox and Norman F. Ness, “Quasi-Stationary Corotating Structure in the Interplanetary Medium” (1965) Journal of Geophysical Research, 70, 5793. PEER REVIEWED
  14. ↑ Personal correspondence, Todd Hoeksema, Wilcox Solar Observatory
  15. ↑ Hannes Alfvén and Per Carlqvist, “Interstellar clouds and the formation of stars” (1978) in Astrophysics and Space Science, vol. 55, no. 2, May 1978, p. 487-509. FULL TEXT PEER REVIEWED

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