Comptes Rendus
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.

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 divB 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 divB. 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 divB may mathematically depart from zero. Anisotropic shielding constitutes an alternative to the existence of monopoles for being responsible for non-zero divB. We evaluate that the solar corona is conversely not a strongly stratified plasma, so that the conditions are very different there.

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 divB 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 divB. Les vitesses aussi sont anisotropes, et font que le volume de Debye est une sphère aplatie. Nous montrons que, dans ce cas, divB peut mathématiquement être non nul. L'écrantage anisotrope constitue une alternative à l'existence de monopôles magnétiques pour expliquer la non-nullité de divB. Nous montrons qu'au contraire la couronne solaire n'est pas un plasma fortement stratifié et que les conditions y sont très différentes.

Published online:
DOI: 10.1016/j.crhy.2014.03.003
Keywords: Magnetic fields, Magnetohydrodynamics (MHD), Plasmas, Stratified media, Sun: surface magnetism, Sun: photosphere, Sunspots
Mot clés : Champs magnétiques, Magnétohydrodynamique (MHD), Plasmas, Milieux stratifiés, Soleil : magnétisme de surface, Soleil : photosphère, Taches solaires

Véronique Bommier 1

1 LESIA, Observatoire de Paris, CNRS–INSU–UMR8109, UPMC (Université Paris-6), Université Paris-Diderot – Paris-7, 5, place Jules-Janssen, 92190 Meudon, France
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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/

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