Solar Physics

, Volume 192, Issue 1–2, pp 119–139 | Cite as

The Solar Dynamo and Emerging Flux – (Invited Review)

  • G.H. Fisher
  • Y. Fan
  • D.W. Longcope
  • M.G. Linton
  • A.A. Pevtsov

Abstract

The largest concentrations of magnetic flux on the Sun occur in active regions. In this paper, the properties of active regions are investigated in terms of the dynamics of magnetic flux tubes which emerge from the base of the solar convection zone, where the solar cycle dynamo is believed to operate, to the photosphere. Flux tube dynamics are computed using the `thin flux tube' approximation, and by using MHD simulation. Simulations of active region emergence and evolution, when compared with the known observed properties of active regions, have yielded the following results: (1) The magnetic field at the base of the convection zone is confined to an approximately toroidal geometry with a field strength in the range (3–10)×104 G. The latitude distribution of the toroidal field at the base of the convection zone is more or less mirrored by the observed active latitudes; there is not a large poleward drift of active regions as they emerge. The time scale for emergence of an active region from the base of the convection zone to the surface is typically 2–4 months. The equatorial gap in the distribution of active regions has two possible origins; if the toroidal field strength is close to 105 G, it is due to the lack of equilibrium solutions at low latitude; if it is closer to 3×104 G, it may be due to modest poleward drift during emergence. (2) The tilt of active regions is due primarily to the Coriolis force acting to twist the diverging flows of the rising flux loops. The dispersion in tilts is caused primarily by the buffeting of flux tubes by convective motions as they rise through the interior. (3) The Coriolis force also bends the active region flux tube shape toward the following (i.e., anti-rotational) direction, resulting in a steeper leg on the following side as compared to the leading side of an active region. When the active region emerges through the photosphere, this results in a more rapid separation of the leading spots away from the magnetic neutral line as compared to the following spots. This bending motion also results in the neutral line being closer to the following magnetic polarity. (4) Active regions behave kinematically after they emerge because of `dynamic disconnection', which occurs because of the lack of a solution to the hydrostatic equilibrium equation once the flux loop has emerged. This could explain why active regions decay once they have emerged, and why the advection-diffusion description of active regions works well after emergence. Smaller flux tubes may undergo `flux tube explosion', a similar process, and provide a source for the constant emergence of small-scale magnetic fields. (5) The slight trend of most active regions to have a negative magnetic twist in the northern hemisphere and positive twist in the south can be accounted for by the action of Coriolis forces on convective eddies, which ultimately writhes active region flux tubes to produce a magnetic twist of the correct sign and amplitude to explain the observations. (6) The properties of the strongly sheared, flare productive δ-spot active regions can be accounted for by the dynamics of highly twisted Ω loops that succumb to the helical kink instability as they emerge through the solar interior.

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References

  1. Babcock, H. W.: 1961, Astrophys. J. 133, 572.Google Scholar
  2. Batchelor, G. K.: 1967, An Introduction to Fluid Mechanics, Cambridge University Press, Cambridge, p. 341.Google Scholar
  3. Caligari, P., Moreno-Insertis, F., and Schüssler, M.: 1995, Astrophys. J. 441, 886.Google Scholar
  4. Caligari, P., Schüssler, M., and Moreno-Insertis F.: 1998, Astrophys. J. 502, 481.Google Scholar
  5. Canfield, R. C., Hudson, H. S., and McKenzie, D. E.: 1999, Geophys. Res. Letters 26(6), 627.Google Scholar
  6. Cauzzi, G., Canfield, R. C., and Fisher, G. H.: 1996, Astrophys. J. 456, 850.Google Scholar
  7. Cauzzi, G., Moreno-Insertis, F., and van Driel-Gesztelyi, L.: 1996, Astron. Soc. Pacific Conf. Ser. 109, 121.Google Scholar
  8. Chou, D.-Y. and Wang, H.: 1987, Solar Phys. 110, 81.Google Scholar
  9. Choudhuri, A. R.: 1989, Solar Phys. 123, 217.Google Scholar
  10. Choudhuri, A. R. and Gilman P. A.: 1987, Astrophys. J. 316, 788.Google Scholar
  11. Choudhuri, A. R., Schüssler, M., and Dikpati M.: 1995, Astron. Astrophys. 303, L29.Google Scholar
  12. Corbard, T., Blanc-Feraud, L., Berthomieu, G., and Provost, J.: 1999, Astron. Astrophys. 344, 696.Google Scholar
  13. DeLuca, E. E. and Gilman P. A.: 1991, 'The Solar Dynamo', in Solar Interior and Atmosphere, University of Arizona Press, Tucson, pp. 275–303.Google Scholar
  14. DeVore, C. R. and Sheeley, N. R.: 1987, Solar Phys. 108, 47.Google Scholar
  15. Dikpati, M. and Charbonneau P.: 1999, Astrophys. J. 518, 508.Google Scholar
  16. D'Silva, S. and Choudhuri, A. R.: 1993, Astron. Astrophys. 272, 621.Google Scholar
  17. D'silva, S. and Howard, R. F.: 1993, Solar Phys. 148, 1.Google Scholar
  18. Durney, B. R.: 1997, Astrophys. J. 486, 1065.Google Scholar
  19. Emonet, T. and Moreno-Insertis, F.: 1998, Astrophys. J. 492, 804.Google Scholar
  20. Fan, Y. and Fisher, G. H.: 1996, Solar Phys. 166, 17.Google Scholar
  21. Fan, Y. and Gong, D.: 1999, Solar Phys., 192, 141 (this issue).Google Scholar
  22. Fan, Y., Fisher, G. H., and DeLuca, E. E.: 1993, Astrophys. J. 405, 852.Google Scholar
  23. Fan, Y., Fisher, G. H., and McClymont A. N.: 1994, Astrophys. J. 436, 907.Google Scholar
  24. Fan, Y., Zweibel, E. G., and Lantz, S. R.: 1998, Astrophys. J. 493, 480.Google Scholar
  25. Fan, Y., Zweibel, E. G., Linton, M. G., and Fisher, G. H.: 1998, Astrophys. J. 505, L59.Google Scholar
  26. Fan, Y., Zweibel, E. G., Linton, M. G., and Fisher, G. H.: 1999, Astrophys. J. 521, 460.Google Scholar
  27. Fisher, G. H., Fan, Y., and Howard R. F.: 1995, Astrophys. J. 438, 463.Google Scholar
  28. Ferriz-Mas, A. and Schüssler M.: 1993, Geophys. Astrophys. Fluid Dyn. 72, 209.Google Scholar
  29. Ferriz-Mas, A. and Schüssler M.: 1995, Geophys. Astrophys. Fluid Dyn. 81, 233.Google Scholar
  30. Howard, R. F.: 1992, Solar Phys. 142, 233.Google Scholar
  31. Howard, R. F., Gilman, P. A., and Gilman, P. I.: 1984, Astrophys. J. 283, 373.Google Scholar
  32. Hughes, D. W. and Falle S. A. E. G.: 1998, Astrophys. J. 509L, 57.Google Scholar
  33. Kosovichev, A. G.: 1996, Astrophys. J. 469, L61.Google Scholar
  34. Kurokawa, H.: 1991, in Y. Uchida, R. C. Canfield, T. Watanabe, and E. Hiei (eds.), 'Flare Physics in Solar Activity Maximum 22', Lecture Notes in Physics 387, 39.Google Scholar
  35. Leighton, R. B.: 1969, Astrophys. J. 156, 1.Google Scholar
  36. Leka, K. D., Canfield, R. C., McClymont, A. N., and van Driel-Gesztelyi, L.: 1996, Astrophys. J. 462, 547.Google Scholar
  37. Linton, M. G., Longcope, D. W., and Fisher, G. H.: 1996, Astrophys. J. 469, 954.Google Scholar
  38. Linton, M. G., Dahlburg, R. B., Fisher, G. H., and Longcope D. W.: 1998, Astrophys. J. 507, 417.Google Scholar
  39. Linton, M. G., Fisher, G. H., Dahlburg, R. B., and Fan, Y.: 1999, Astrophys. J. 522, 1190.Google Scholar
  40. Longcope, D. W. and Fisher, G. H.: 1996, Astrophys. J. 458, 380.Google Scholar
  41. Longcope, D. W. and Klapper, I.: 1997, Astrophys. J. 488, 443.Google Scholar
  42. Longcope, D. W., Fisher, G. H., and Arendt, S.: 1996, Astrophys. J. 464, 999.Google Scholar
  43. Longcope, D. W., Fisher, G. H., and Pevtsov, A. A.: 1998, Astrophys. J. 507, 417.Google Scholar
  44. MacGregor K. B. and Charbonneau P.: 1997a, Astrophys. J. 486, 484.Google Scholar
  45. MacGregor K. B. and Charbonneau P.: 1997b, Astrophys. J. 486, 502.Google Scholar
  46. Markiel, J. A. and Thomas J. H.: 1999, Astrophys. J. 523, 827.Google Scholar
  47. Mickey, D. L.: 1985, Sol. Phys. 97, 223.Google Scholar
  48. Moffatt, H. K. and Ricca, R L.: 1992, Proc. R. Soc. London A439, 411.Google Scholar
  49. Moreno-Insertis, F. and Emonet, T.: 1996, Astrophys. J. 472, L53.Google Scholar
  50. Moreno-Insertis, F., Schüssler, M., and Ferriz-Mas, A: 1992, Astron. Astrophys. 264, 686.Google Scholar
  51. Moreno-Insertis, F., Schüssler, M., and Caligari, P.: 1994, Solar Phys. 153, 449.Google Scholar
  52. Moreno-Insertis, F., Caligari, P., and Schüssler, M.: 1995, Astrophys. J. 452, 894.Google Scholar
  53. Parker, E. N.: 1979, Cosmical Magnetic Fields, Oxford University Press, Oxford, p. 147.Google Scholar
  54. Parker, E. N.: 1993, Astrophys. J. 408, 707.Google Scholar
  55. Pevtsov, A. A., Canfield, R. C., and Metcalf, T. R.: 1995, Astrophys. J. 440, L109.Google Scholar
  56. Schüssler, M.: 1979, Astron. Astrophys. 71, 79.Google Scholar
  57. Schüssler, M., Caligari, P., Ferriz-Mas, A., and Moreno-Insertis, F.: 1994, Astron. Astrophys. 281, L69.Google Scholar
  58. Sheeley, N. R., Nash, A. G., and Wang, Y.-M.: 1987, Astrophys. J. 319, 481.Google Scholar
  59. Spruit, H. C.: 1979, Solar Phys. 61, 363.Google Scholar
  60. Spruit, H. C: 1981, Astron. Astrophys. 98, 155.Google Scholar
  61. Tanaka, K.: 1991, Solar Phys. 136, 133.Google Scholar
  62. Title, A. M. and Schrijver C. J.: 1998, Astron. Soc. Pacific Conf. Ser. 154, 345.Google Scholar
  63. van Ballegooijen, A. A.: 1982, Astron. Astrophys. 113, 99.Google Scholar
  64. van Ballegooijen, A A. and Choudhuri, A.R.: 1988, Astrophys. J. 333, 965.Google Scholar
  65. van Driel-Gesztelyi, L. and Petrovay, K.: 1990, Solar Phys. 126, 285.Google Scholar
  66. Vainshtein S. I. and Cattaneo F.: 1992, Astrophys. J. 393, 165.Google Scholar
  67. Wang, Y.-M., Nash, A. G., and Sheeley, N. R.: 1989, Astrophys. J. 347, 529.Google Scholar
  68. Wang, Y.-M., Sheeley, N. R., and Nash, A. G.: 1991, Astrophys. J. 383, 431.Google Scholar
  69. Zhang, H. and Bao, S.: 1999, Astrophys. J. 519, 876.Google Scholar
  70. Zirin, H.: 1988, Astrophysics of the Sun, Cambridge University Press, Cambridge, p. 307.Google Scholar

Copyright information

© Kluwer Academic Publishers 2000

Authors and Affiliations

  • G.H. Fisher
    • 1
  • Y. Fan
    • 2
  • D.W. Longcope
    • 3
  • M.G. Linton
    • 4
  • A.A. Pevtsov
    • 5
  1. 1.Space Sciences LaboratoryUniversity of CalifornaBerkeleyU.S.A.
  2. 2.HAONational Center for Atmospheric ResearchBoulderU.S.A.
  3. 3.Physics DepartmentMontana State UniversityBozemanU.S.A.
  4. 4.Naval Research Laboratory, Code 7675WashingtonU.S.A.
  5. 5.Physics DepartmentMontana State UniversityBozemanU.S.A.

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