Nous avons trouvé dans la littérature 15 références montrant sans exception que, dans la photosphère des taches solaires, le gradient vertical du champ magnétique est de 3–4 G/km, l'intensité du champ décroissant avec l'altitude, tandis que le gradient horizontal est neuf fois plus faible, de l'ordre de 0.4–0.5 G/km. Nos récentes observations THÉMIS confirment ces résultats, dont la conséquence est que l'annulation de n'est pas réalisée. Cet article est consacré à l'étude de ce problème. Nous faisons remarquer que la photosphère est un plasma fortement stratifié. Les longueurs caractéristiques horizontale et verticale sont différentes, dans le rapport d'aspect 1/9, de même que les différents termes qui contribuent à la valeur observée de . Les vitesses aussi sont anisotropes, et font que le volume de Debye est une sphère aplatie. Nous montrons que, dans ce cas, peut mathématiquement être non nul. L'écrantage anisotrope constitue une alternative à l'existence de monopôles magnétiques pour expliquer la non-nullité de . Nous montrons qu'au contraire la couronne solaire n'est pas un plasma fortement stratifié et que les conditions y sont très différentes.
In the literature, we found 15 references showing, without exception, that the sunspot photospheric magnetic field vertical gradient is on the order of 3–4 G/km, with field strength decreasing with height, whereas the horizontal gradient is nine times weaker, on the order of 0.4–0.5 G/km. This is confirmed by our recent THÉMIS observations. As a consequence, the vanishing of is not realized, and the present paper is devoted to the investigation of this problem. We point out that the photosphere is a strongly stratified plasma, having different horizontal and vertical characteristic lengths of aspect ratio 1/9 as the different terms contributing to the observed . The velocities are also anisotropic under the stratification effect. As a consequence, the Debye volume is a flattened sphere. We show that in this case may mathematically depart from zero. Anisotropic shielding constitutes an alternative to the existence of monopoles for being responsible for non-zero . We evaluate that the solar corona is conversely not a strongly stratified plasma, so that the conditions are very different there.
Mot clés : Champs magnétiques, Magnétohydrodynamique (MHD), Plasmas, Milieux stratifiés, Soleil : magnétisme de surface, Soleil : photosphère, Taches solaires
Véronique Bommier 1
@article{CRPHYS_2014__15_5_430_0, author = {V\'eronique Bommier}, title = {Electromagnetism in a strongly stratified plasma showing an unexpected effect of the {Debye} shielding}, journal = {Comptes Rendus. Physique}, pages = {430--440}, publisher = {Elsevier}, volume = {15}, number = {5}, year = {2014}, doi = {10.1016/j.crhy.2014.03.003}, language = {en}, }
Véronique Bommier. Electromagnetism in a strongly stratified plasma showing an unexpected effect of the Debye shielding. Comptes Rendus. Physique, Volume 15 (2014) no. 5, pp. 430-440. doi : 10.1016/j.crhy.2014.03.003. https://comptes-rendus.academie-sciences.fr/physique/articles/10.1016/j.crhy.2014.03.003/
[1] Sunspots: an overview, Astron. Astrophys. Rev., Volume 11 (2003), pp. 153-286 | DOI
[2] Astrophysical Quantities, The Athlone Press, University of London, 1973
[3] Optical tomography of a sunspot. II. Vector magnetic field and temperature stratification, Astrophys. J., Volume 547 (2001), pp. 1130-1147 | DOI
[4] Polarimetry and spectroscopy of a simple sunspot. 2: On the height and temperature dependence of the magnetic field, Astron. Astrophys., Volume 279 (1993), pp. 243-250
[5] Polarimetry and spectroscopy of a simple sunspot. 3: Velocity and magnetic field of sunspot umbral dots, Astron. Astrophys., Volume 283 (1994), pp. 241-246
[6] Magnetic field, relative Doppler shift and temperature for an inhomogeneous model of sunspot umbrae, Astron. Astrophys., Volume 228 (1990), pp. 246-252
[7] Observed differences between large and small sunspots, Astron. Astrophys., Volume 291 (1994), pp. 622-634
[8] Infrared lines as probes of solar magnetic features. VIII. MgI 12 μm diagnostics of sunspots, Astron. Astrophys., Volume 293 (1995), pp. 225-239
[9] Three dimensional structure of a regular sunspot from the inversion of IR Stokes profiles, Astron. Astrophys., Volume 410 (2003), pp. 695-710 | DOI
[10] THEMIS/MSDP magnetic field measurements, Astron. Astrophys., Volume 445 (2006), pp. 1127-1132 | DOI
[11] Vertical current densities and magnetic gradients in sunspots, Astron. Astrophys., Volume 449 (2006), pp. 1169-1176 | DOI
[12] Vertical gradients of sunspot magnetic fields, Sol. Phys., Volume 84 (1983), pp. 13-31 | DOI
[13] Gradients of the line-of-sight magnetic fields in active region NOAA 6659, Sol. Phys., Volume 169 (1996), pp. 79-89 | DOI
[14] Some properties of an isolated sunspot, Astron. Astrophys., Volume 429 (2005), pp. 705-711 | DOI
[15] On the diagnostic of magnetic fields in sunspots through the interpretation of Stokes parameters profiles, Sol. Phys., Volume 93 (1984), pp. 269-287 | DOI
[16] UNNOFIT inversion of spectro-polarimetric maps observed with THEMIS, Astron. Astrophys., Volume 464 (2007), pp. 323-339 | DOI
[17] Line formation of a normal Zeeman triplet, Publ. Astron. Soc. Jpn., Volume 8 (1956), p. 108
[18] Izv. Ordena Trudovogo Krasnogo Znameni Krym. Astrofiz. Obs., 27 (1962), p. 148
[19] Izv. Ordena Trudovogo Krasnogo Znameni Krym. Astrofiz. Obs., 37 (1967), p. 56
[20] Direct measurement of the formation height difference of the 630 nm Fe I solar lines, Astron. Astrophys., Volume 507 (2009), p. L29-L32 | DOI
[21] Measuring line formation depths by cross-spectral analysis. Numerical simulations for the 630 nm Fe I line pair, Astron. Astrophys., Volume 514 (2010), p. A91 | DOI
[22] On the Stokes V amplitude ratio as an indicator of the field strength in the solar internetwork, Astrophys. J., Volume 659 (2007), pp. 1726-1735 | DOI
[23] Vertical structure of sunspots from THEMIS observations, Astron. Astrophys., Volume 381 (2002), pp. 290-299 | DOI
[24] A computer program for solving multi-level non-LTE radiative transfer problems in moving or static atmospheres, Uppsala Astronomical Observatory, 1986 (Tech. Rep. Report No 33)
[25] Quasi-separatrix layers in solar flares. II. Observed magnetic configurations, Astron. Astrophys., Volume 325 (1997), pp. 305-317
[26] Numerical solution of the magnetostatic equations for thick flux tubes, with application to sunspots, pores, and related structures, Astrophys. J., Volume 302 (1986), pp. 785-808 | DOI
[27] A geometrical height scale for sunspot penumbrae, Astrophys. J., Volume 720 (2010), pp. 1417-1431 | arXiv | DOI
[28] Inversion of Stokes profiles, Astrophys. J., Volume 398 (1992), pp. 375-385 | DOI
[29] Optically thin irregularities in the penumbrae of sunspots, Astrophys. J., Volume 497 (1998), p. 967 | DOI
[30] Eddy viscosity of parity-invariant flow, Phys. Rev. A, Volume 43 (1991), pp. 5355-5364 | DOI
[31] Non-LTE formation heights of Stokes profiles of Fe I lines, 27–31 August 1990 (L.J. November, ed.) (1991), pp. 444-456
[32] Aspects of Debye shielding, Am. J. Phys., Volume 61 (1993), pp. 249-257 | DOI
[33] Scaling analysis and simulation of strongly stratified turbulent flows, J. Fluid Mech., Volume 585 (2007), p. 343 | DOI
[34] Turbulence en milieu stratifié, Étude expérimentale et rôle de l'instabilité zigzag, École Normale Supérieure de Lyon, 2008 www.ens-lyon.fr/DSM/SDMsite/M2/stages_M2/Augier.pdf (Master's Thesis)
[35] Structure of the solar chromosphere. III – Models of the EUV brightness components of the quiet-sun, Astrophys. J. Suppl. Ser., Volume 45 (1981), pp. 635-725 | DOI
[36] Interpretation of second solar spectrum observations of the Sr I 4607 Å line in a quiet region: turbulent magnetic field strength determination, Astron. Astrophys., Volume 432 (2005), pp. 295-305 | DOI
[37] Basics of the Solar Wind, Cambridge Atmospheric and Space Science Series, Cambridge University Press, 2007
[38] Vertical length scale selection for pancake vortices in strongly stratified viscous fluids, J. Fluid Mech., Volume 504 (2004), pp. 229-238 | DOI
[39] The quiet Sun magnetic field: statistical description from THEMIS observations, Astron. Astrophys., Volume 530 (2011), p. A51 | DOI
Cité par Sources :
Commentaires - Politique