The solar dynamo is a physical process hypothesized to generate the Sun's magnetic field, as explained with a variant of the dynamo theory. A naturally occurring interior electric generator presumably produces electric currents and a magnetic field, following the Ampère, Faraday, Ohm, and fluid dynamics laws, which compose the laws of magnetohydrodynamics.

While the solar dynamo's detailed mechanism is unknown,[1] its very nature remains elusive despite our proximity to the Sun.[2] According to data, the Sun behaves dynamically as a magneto-alternator rather than a dynamo.[3]

Mechanism

A dynamo converts kinetic energy into electric-magnetic energy. An electrically conducting fluid with shear or more complicated motion, such as turbulence, can temporarily amplify a magnetic field through Lenz's law: fluid motion relative to a magnetic field induces electric currents in the fluid that distort the initial field. If the fluid motion is sufficiently complicated, it can sustain its own magnetic field, with advective fluid amplification essentially balancing diffusive or ohmic decay. Such systems are called self-sustaining dynamos. The Sun is a self-sustaining dynamo that converts convective motion and differential rotation within the Sun to electric-magnetic energy.

Currently, the geometry and width of the tachocline are hypothesized to play an important role in the solar dynamo models by winding up the weaker poloidal field to create a much stronger toroidal field. However, recent radio observations of relatively colder stars and brown dwarfs, which do not have a radiative core and only have a convection zone, have demonstrated that they maintain large-scale, solar-strength magnetic fields and display solar-like activity despite the absence of tachoclines. This finding suggests that the convection zone alone may be responsible for the function of the solar dynamo.[4]

Modeling

The most prominent time variation of the solar magnetic field is related to the quasi-periodic 11-year solar cycle, characterized by an increasing and decreasing number and size of sunspots.[5][6] Sunspots are visible as dark patches on the Sun's photosphere and correspond to concentrations of magnetic field. At a typical solar minimum, few or no sunspots are visible. Those that do appear are at high solar latitudes. As the solar cycle progresses towards its maximum, sunspots tend to form closer to the solar equator, following Spörer's law.

The 11-year sunspot cycle is one-half a 22-year solar dynamo cycle in Babcock–Leighton dynamo models, presumably corresponding to an oscillatory exchange of energy between toroidal and poloidal solar magnetic fields. At solar-cycle maximum, the external poloidal dipolar magnetic field is near its dynamo-cycle minimum strength, but an internal toroidal quadrupolar field, generated through differential rotation within the tachocline, is near its maximum strength. At this point in the dynamo cycle, buoyant upwelling within the convection zone forces the emergence of the toroidal magnetic field through the photosphere, giving rise to sunspot pairs roughly aligned east–west with opposite magnetic polarities. The magnetic polarity of sunspot pairs alternates every solar cycle, a phenomenon known as the Hale cycle.[7][8]

According to theory, during the solar cycle's declining phase, energy shifts from the internal toroidal magnetic field to the external poloidal field, and sunspots diminish in number. At a solar minimum, the toroidal field is, correspondingly, at minimum strength, sunspots relatively rare, and the poloidal field at maximum strength. During the next cycle, differential rotation converts magnetic energy back from the poloidal to the toroidal field, with a polarity opposite to the previous cycle. The process carries on continuously, and in an idealized, simplified scenario, each 11-year sunspot cycle corresponds to a change in the polarity of the Sun's large-scale magnetic field.[8][9][10] Long minima of solar activity can be associated with the interaction between double dynamo waves of the solar magnetic field caused by the beating effect of the wave interference.[11]

See also

References

  1. ↑ Tobias, S.M. (2002). "The Solar Dynamo". Philosophical Transactions of the Royal Society A. 360 (1801): 2741–2756. Bibcode:2002RSPTA.360.2741T. doi:10.1098/rsta.2002.1090. PMID 12626264. S2CID 6473234.
  2. ↑ Brown, B. (2011). "Dynamos in stellar convection zones: of wreaths and cycles" (PDF). Journal of Physics: Conference Series. 271: 012064. arXiv:1011.2992. Bibcode:2011JPhCS.271a2064B. OCLC 1363386340. S2CID 120541706. Retrieved 2024-01-13.{{cite journal}}: CS1 maint: url-status (link)
  3. ↑ Omerbashich, M. (2023). "The Sun as a revolving-field magnetic alternator with a wobbling-core rotator from real data" (PDF). Journal of Geophysics (published 2023-12-18). 65 (1): 48–77. arXiv:2301.07219. Bibcode:2023JGeop..65...48O. ISSN 2643-2986. OCLC 1098213652. Retrieved 2024-01-11.{{cite journal}}: CS1 maint: url-status (link)
  4. ↑ Route, Matthew (October 20, 2016). "The Discovery of Solar-like Activity Cycles Beyond the End of the Main Sequence?". The Astrophysical Journal Letters. 830 (2): 27. arXiv:1609.07761. Bibcode:2016ApJ...830L..27R. doi:10.3847/2041-8205/830/2/L27. S2CID 119111063.
  5. ↑ Charbonneau, P. (2014). "Solar Dynamo Theory". Annual Review of Astronomy and Astrophysics. 52: 251–290. Bibcode:2014ARA&A..52..251C. doi:10.1146/annurev-astro-081913-040012.
  6. ↑ Zirker, J. B. (2002). Journey from the Center of the Sun. Princeton University Press. pp. 119–120. ISBN 978-0-691-05781-1.
  7. ↑ Hale, G. E.; Ellerman, F.; Nicholson, S. B.; Joy, A. H. (1919). "The Magnetic Polarity of Sun-Spots". The Astrophysical Journal. 49: 153. Bibcode:1919ApJ....49..153H. doi:10.1086/142452.
  8. 1 2 "NASA Satellites Capture Start of New Solar Cycle". PhysOrg. 4 January 2008. Retrieved 10 July 2009.
  9. ↑ "Sun flips magnetic field". CNN. 16 February 2001. Retrieved 11 July 2009.
  10. ↑ Phillips, T. (15 February 2001). "The Sun Does a Flip". NASA. Retrieved 11 July 2009.
  11. ↑ Zharkova, V. V.; Shepherd, S. J.; Popova, E.; Zharkov, S. I. (29 October 2015). "Heartbeat of the Sun from Principal Component Analysis and prediction of solar activity on a millenium timescale". Scientific Reports. 5: 15689. Bibcode:2015NatSR...515689Z. doi:10.1038/srep15689. PMC 4625153. PMID 26511513.
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