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The surface of the Sun is magnetized

7 February 2020

A recent compilation of several spectroscopic observations of the surface of the Sun shows that the magnetic field gradient in the vertical direction is 3 Gauss per kilometer, while in the horizontal direction, it is only 0.3 Gauss per km. This indicates a surprising departure from Maxwell’s equations! Véronique Bommier, director of research at the Paris Observatory, offers a solution to the problem, assuming an accumulation of electrons in the photosphere. The protons, much heavier, do not follow, and this results in an electric field inside the Sun. This phenomenon explains the apparent contradiction of the observations.

Why is solar meteorology much more difficult and complex than terrestrial meteorology, made from maps and numerical forecasting models? As the atmosphere of the Sun is a plasma, a charged matter, in theory we should be able to use the models of magneto-hydrodynamics associated with the maps of magnetic field and velocity field on the surface of the star. But it seems that it does not work very well. Perhaps it is due to the fact that the magnetic field which is measured is not the field that we believe to measure.

The article published this February 6, 2020 in the Astronomy & Astrophysics journal by Véronique Bommier, of the Observatoire de Paris, highlights a non-conservation of the magnetic flux, which suggests that indeed the field is not equal to what we think. To solve the problem, it is necessary to suppose the presence of a magnetization much more important than expected by the current models of the solar surface plasma. Only a much stronger magnetic field would be compatible with Maxwell’s equations. The accumulation of free electrons coming from inside the star, where the high temperature, coupled with the low mass of electrons, makes them escape gravity and protons, could explain this magnetization. A result which makes it possible to consider a modernization of the forecast in solar matter ejection!

Example of a magnetic field reconstructed above the surface of the sun. The charged material will follow the lines of the magnetic field. What we observe is in fact the magnetic field H, and not the magnetic induction B which has a conserved flux according to the Maxwell equations. The difference between these two quantities is contained in the magnetization M, according to the law B = µ0 (H + M). However, the interactions between the field and the matter are the fact of the magnetic field H. It is thus H which is measured by the Zeeman effect of the magnetic field on the emitting atoms. The magnetization M can be considered as magnetic flux stored and hidden inside the material itself. And it is the induction B which governs the effects induced by the field on the matter’s motions (magneto-hydrodynamics). Until now, we thought it was B that was measured.

The author of the article, Véronique Bommier, Directeur de Recherche CNRS, member of LESIA laboratory (Observatoire de Paris), has carried out numerous observations with the french solar telescope THEMIS built by the CNRS on the european site of Izaña (Tenerife island, Canaries, Spain). The interpretation of all these observations has revelaed the phenomenon.

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