Skip to main content

Thank you for visiting You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Evidence against turbulent and canopy-like magnetic fields in the solar chromosphere


Measurements of the degree of polarization of light in the spectral lines emitted by gas near the Sun's limb (its outer edge) can be used to investigate the electron densities and magnetic field strengths in the solar atmosphere; these quantities are important for determining the balance and transport of energy through the Sun's atmosphere. Recent measurements1,2 revealed a surprising degree of polarization in the sodium doublet; these observations have remained an enigma. Here I report a mechanism that may explain these observations, in which it is assumed that the populations of the magnetic sublevels of the electronic ground state of the sodium atom are not equal: this leads to ground-level atomic polarization. This mechanism explains very well the observed line shapes, and implies that depolarization does not occur in the solar chromosphere, which would seem to rule out the existence of turbulent magnetic fields and of horizontal, canopy-like fields stronger than0.01 G. This is difficult to understand, because there is substantial evidence from othertypes of observation for both types of field3,4,5,6,7,8,9,10. There are obviously aspects of the Sun's atmosphere that remain very poorly understood.

This is a preview of subscription content

Access options

Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Figure 1: Polarization profiles Q/I, with the positive Q-direction defined as the tangent to the solar limb.


  1. Stenflo, J. O. & Keller, C. U. New window for spectroscopy. Nature 382, 588 (1996).

    ADS  CAS  Article  Google Scholar 

  2. Stenflo, J. O. & Keller, C. U. The second solar spectrum. A new window for diagnostics of the Sun. Astron. Astrophys. 321, 927–934 (1997).

    ADS  CAS  Google Scholar 

  3. Stenflo, J. O. Small-scale magnetic structures on the Sun. Astron. Astrophys. Rev. 1, 3–48 (1989).

    ADS  Article  Google Scholar 

  4. Stenflo, J. O. Solar Magnetic Fields (Kluwer, Dordrecht, 1994).

    Book  Google Scholar 

  5. Faurobert-Scholl, M. Investigation of microturbulent magnetic fields in the solar photosphere by their Hanle effect in the SrI 4607 Å line. Astron. Astrophys. 268, 765–774 (1993).

    ADS  CAS  Google Scholar 

  6. Faurobert-Scholl, M., Feautrier, N., Machefert, F., Petrovay, K. & Spielfiedel, A. Turbulent magnetic fields in the solar photosphere: diagnostics and interpretation. Astron. Astrophys. 298, 289–302 (1995).

    ADS  CAS  Google Scholar 

  7. Stenflo, J. O., Keller, C. U. & Gandorfer, A. Differential Hanle effect and the spatial variation of turbulent magnetic fields on the Sun. Astron. Astrophys. 329, 319–328 (1998).

    ADS  Google Scholar 

  8. Bianda, M., Solanki, S. K. & Stenflo, J. O. Hanle depolarisation in the solar chromosphere: The CaI 4227 Å line. Astron. Astrophys.(in the press).

  9. Jones, H. P. & Giovanelli, R. G. Magnetic canopies in unipolar regions. Solar Phys. 87, 37–42 (1983).

    ADS  CAS  Article  Google Scholar 

  10. Solanki, S. K. & Steiner, O. How magnetic is the solar atmosphere? Astron. Astrophys. 234, 519–529 (1990).

    ADS  Google Scholar 

  11. Happer, W. Optical pumping. Rev. Mod. Phys. 44, 169–249 (1972).

    ADS  CAS  Article  Google Scholar 

  12. Landi Degl'Innocenti, E. The density matrix approach to polarized radiative transfer. Solar Phys. 164, 21–28 (1996).

    ADS  Article  Google Scholar 

  13. Trujillo Bueno, J. & Landi Degl'Innocenti, E. Linear polarization due to lower-level depopulation pumping in stellar atmospheres. Astrophys. J. 482, L183–L186 (1997).

    ADS  Article  Google Scholar 

  14. Landi Degl'Innocenti, E., Landi Degl'Innocenti, M. & Landolfi, M. in Forum Themis (eds Mein, N. & Sahal Bréchot, S.) 59–77 (Observatoire de Paris, 1996).

    Google Scholar 

  15. Shurcliff, W. A. Polarized Light (Harvard Univ. Press, Cambridge, Mass., 1966).

    Google Scholar 

  16. Petrovay, K. & Szakály, G. The origin of intranetwork fields: a small-scale solar dynamo. Astron. Astrophys. 274, 543–554 (1993).

    ADS  Google Scholar 

  17. Landolfi, M. & Landi Degl'Innocenti, E. Polarization of the sodium D lines in prominences. Solar Phys. 98, 53–66 (1985).

    ADS  CAS  Article  Google Scholar 

Download references


I thank J. O. Stenflo for helping in the presentation of the Letter and for providing the original data shown in Fig. 1.

Author information

Authors and Affiliations


Corresponding author

Correspondence to Egidio Landi Degl'Innocenti.

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Degl'Innocenti, E. Evidence against turbulent and canopy-like magnetic fields in the solar chromosphere. Nature 392, 256–258 (1998).

Download citation

  • Received:

  • Accepted:

  • Issue Date:

  • DOI:

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.


Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing