Electromagnetism in a strongly stratified plasma showing an unexpected effect of the Debye shielding

Véronique Bommier

doi:10.1016/j.crhy.2014.03.003

Electromagnetism in a strongly stratified plasma showing an unexpected effect of the Debye shielding
[Électromagnétisme dans les plasmas fortement stratifiés : mise en évidence d'un effet inattendu de l'écrantage de Debye]

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

Comptes Rendus. Physique, Volume 15 (2014) no. 5, pp. 430-440.

Résumé
Abstract

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 $div \vec{B}$ 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 $div \vec{B}$ . Les vitesses aussi sont anisotropes, et font que le volume de Debye est une sphère aplatie. Nous montrons que, dans ce cas, $div \vec{B}$ peut mathématiquement être non nul. L'écrantage anisotrope constitue une alternative à l'existence de monopôles magnétiques pour expliquer la non-nullité de $div \vec{B}$ . 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 $div \vec{B}$ 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 $div \vec{B}$ . 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 $div \vec{B}$ may mathematically depart from zero. Anisotropic shielding constitutes an alternative to the existence of monopoles for being responsible for non-zero $div \vec{B}$ . We evaluate that the solar corona is conversely not a strongly stratified plasma, so that the conditions are very different there.

Publié le : 2014-05-01

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

Affiliations des auteurs :

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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     title = {Electromagnetism in a strongly stratified plasma showing an unexpected effect of the {Debye} shielding},
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     doi = {10.1016/j.crhy.2014.03.003},
     language = {en},
}

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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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