Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae


The structure of a sunspot is determined by the local interaction between magnetic fields and convection near the Sun's surface1,2. The dark central umbra is surrounded by a filamentary penumbra, whose complicated fine structure has only recently been revealed by high-resolution observations3,4,5,6,7,8,9,10,11,12,13,14. The penumbral magnetic field has an intricate and unexpected interlocking-comb structure and some field lines, with associated outflows of gas15, dive back down below the solar surface at the outer edge of the spot. These field lines might be expected to float quickly back to the surface because of magnetic buoyancy, but they remain submerged. Here we show that the field lines are kept submerged outside the spot by turbulent, compressible convection, which is dominated by strong, coherent, descending plumes16,17. Moreover, this downward pumping of magnetic flux explains the origin of the interlocking-comb structure of the penumbral magnetic field, and the behaviour of other magnetic features near the sunspot.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Sketch showing the interlocking-comb structure of the magnetic field in the filamentary penumbra of a sunspot.
Figure 2: Downward pumping of magnetic flux by turbulent granular convection.
Figure 3: Flux pumping by vigorous sinking plumes.
Figure 4: Downward pumping of magnetic flux in the numerical simulation.
Figure 5: Moving magnetic features (MMFs) in the moat around a sunspot.


  1. 1

    Thomas, J. H. & Weiss, N. O. (eds) Sunspots: Theory and Observations NATO ASI C 375 (Kluwer, Dordrecht, 1992)

  2. 2

    Schmieder, B., del Toro Iniesta, J. C. & Vázquez, M. (eds) Advances in the Physics of Sunspots Astronomical Society of the Pacific Conference Series 118, (San Francisco, 1997)

  3. 3

    Degenhardt, D. & Wiehr, E. Spatial variation of the magnetic field inclination in a sunspot penumbra. Astron. Astrophys. 252, 821–826 (1991)

    ADS  CAS  Google Scholar 

  4. 4

    Solanki, S. K. & Montavon, C. A. P. Uncombed fields as the source of broad-band circular polarization of sunspots. Astron. Astrophys. 275, 283–292 (1993)

    ADS  Google Scholar 

  5. 5

    Title, A. M. et al. On the magnetic and velocity field geometry of simple sunspots. Astrophys. J. 403, 780–796 (1993)

    ADS  Article  Google Scholar 

  6. 6

    Lites, B. W., Elmore, D. F., Seagraves, P. & Skumanich, A. Stokes profile analysis and vector magnetic fields. VI. Fine-scale structure of a sunspot. Astrophys. J. 418, 928–942 (1993)

    ADS  Article  Google Scholar 

  7. 7

    Solanki, S. K., Montavon, C. A. P. & Livingston, W. Infrared lines as a probe of solar magnetic features. VII. On the nature of the Evershed effect in sunspots. Astron. Astrophys. 283, 221–231 (1994)

    ADS  Google Scholar 

  8. 8

    Rimmele, T. R. Sun center observations of the Evershed effect. Astrophys. J. 445, 511–516 (1995)

    ADS  CAS  Article  Google Scholar 

  9. 9

    Stanchfield, D. C. H. II, Thomas, J. H. & Lites, B. W. The vector magnetic field, Evershed flow, and intensity in a sunspot. Astrophys. J. 477, 485–494 (1997)

    ADS  CAS  Article  Google Scholar 

  10. 10

    Westendorp Plaza, C. et al. Evidence for a downward mass flux in the penumbral region of a sunspot. Nature 389, 47–49 (1997)

    ADS  CAS  Article  Google Scholar 

  11. 11

    Rüedi, I., Solanki, S. K. & Keller, C. U. Infrared lines as probes of solar magnetic features. XV. Evershed flow in cool, weak penumbral fields. Astron. Astrophys. 348, L37–L40 (1999)

    ADS  Google Scholar 

  12. 12

    Schlichenmaier, R. & Schmidt, W. Flow geometry in a sunspot penumbra. Astron. Astrophys. 358, 1122–1132 (2000)

    ADS  Google Scholar 

  13. 13

    Martínez Pillet, V. Spectral signature of uncombed magnetic fields. Astron. Astrophys. 361, 734–742 (2000)

    ADS  Google Scholar 

  14. 14

    Westendorp Plaza, C., del Toro Iniesta, J. C., Ruiz Cobo, B. & Martínez Pillet, V. Optical tomography of a sunspot. III. Velocity stratification and the Evershed effect. Astrophys. J. 547, 1148–1158 (2001)

    ADS  Article  Google Scholar 

  15. 15

    Montesinos, B. & Thomas, J. H. The Evershed effect in sunspots as a siphon flow along a magnetic flux tube. Nature 390, 485–487 (1997)

    ADS  Article  Google Scholar 

  16. 16

    Stein, R. F. & Nordlund, Å. Simulations of solar granulation. I. General properties. Astrophys. J. 499, 914–933 (1998)

    ADS  Article  Google Scholar 

  17. 17

    Brummell, N. H., Clune, T. L. & Toomre, J. Penetration and overshooting in turbulent compressible convection. Astrophys. J. 570, 825–854 (2002)

    ADS  Article  Google Scholar 

  18. 18

    Tobias, S. M., Brummell, N. H., Clune, T. L. & Toomre, J. Transport and storage of magnetic field by overshooting turbulent compressible convection. Astrophys. J. 549, 1183–1203 (2001)

    ADS  Article  Google Scholar 

  19. 19

    Nordlund, Å. et al. Dynamo action in stratified convection with overshoot. Astrophys. J. 392, 647–652 (1992)

    ADS  Article  Google Scholar 

  20. 20

    Brandenburg, A. et al. Magnetic structures in a dynamo simulation. J. Fluid Mech. 306, 325–352 (1996)

    ADS  MathSciNet  CAS  Article  Google Scholar 

  21. 21

    Tobias, S. M., Brummell, N. H., Clune, T. L. & Toomre, J. Pumping of magnetic fields by turbulent penetrative convection. Astrophys. J. 502, L177–L180 (1998)

    ADS  Article  Google Scholar 

  22. 22

    Dorch, S. B. F. & Nordlund, Å. On the transport of magnetic fields by solar-like stratified convection. Astron. Astrophys. 365, 562–570 (2001)

    ADS  Article  Google Scholar 

  23. 23

    Rucklidge, A. M., Schmidt, H. U. & Weiss, N. O. The abrupt development of penumbrae in sunspots. Mon. Not. R. Astron. Soc. 273, 491–498 (1995)

    ADS  Article  Google Scholar 

  24. 24

    Schlichenmaier, R., Jahn, K. & Schmidt, H. U. A dynamical model for the penumbral fine structure and the Evershed effect in sunspots. Astrophys. J. 493, L121–L124 (1998)

    ADS  Article  Google Scholar 

  25. 25

    Tildesley, M. J. On the origin of filamentary structure in sunspot penumbrae: Linear instabilities. Mon. Not. R. Astron. Soc. (submitted)

  26. 26

    Shine, R. A. & Title, A. M. in Encyclopedia of Astronomy and Astrophysics 3209–3212 (Nature Publishing Group, London, and Institute of Physics Publishing, Bristol, 2001)

    Google Scholar 

  27. 27

    Harvey, J. & Harvey, K. Observations of moving magnetic features near sunspots. Sol. Phys. 28, 61–71 (1973)

    ADS  Article  Google Scholar 

Download references


We thank J. Kuwabara and Y. Uchida for assistance in producing Fig. 3. This work was supported by the UK Particle Physics and Astrophysics Research Council (J.H.T. and N.O.W.), the Sun-Earth Connection Theory programme of the US National Aeronautics and Space Administration (N.H.B. and S.M.T.), and the Nuffield Foundation (S.M.T.).

Author information



Corresponding author

Correspondence to John H. Thomas.

Ethics declarations

Competing interests

The authors declare that they have no competing financial interests.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Thomas, J., Weiss, N., Tobias, S. et al. Downward pumping of magnetic flux as the cause of filamentary structures in sunspot penumbrae. Nature 420, 390–393 (2002).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Sign up for the Nature Briefing newsletter for a daily update on COVID-19 science.
Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing