Magnetic helicity

Résumé

In plasma physics, magnetic helicity is a measure of the linkage, twist, and writhe of a magnetic field. In ideal magnetohydrodynamics, magnetic helicity is conserved. When a magnetic field contains magnetic helicity, it tends to form large-scale structures from small-scale ones. This process can be referred to as an inverse transfer in Fourier space. This second property makes magnetic helicity special: three-dimensional turbulent flows tend to "destroy" structure, in the sense that large-scale vortices break up into smaller and smaller ones (a process called "direct energy cascade", described by Lewis Fry Richardson and Andrey Nikolaevich Kolmogorov). At the smallest scales, the vortices are dissipated in heat through viscous effects. Through a sort of "inverse cascade of magnetic helicity", the opposite happens: small helical structures (with a non-zero magnetic helicity) lead to the formation of large-scale magnetic fields. This is for example vi

Source officielle

https://en.wikipedia.org/wiki/Magnetic_helicity

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The dynamics of ordinary matter in the Universe follows the laws of (magneto)hydrodynamics. In this course, the system of equations that describes astrophysical fluids will be discussed on the basis of selected astrophysical examples, from the physics of stars, to galaxies and the early Universe.

The course first develops the basic laws of electricity and magnetism and illustrates the use in understanding various electromagnetic phenomena.

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Hypermagnetic fields and baryon asymmetry from pseudoscalar inflation

Eray Sabancilar

We show that maximally helical hypermagnetic fields produced during pseudoscalar inflation can generate the observed baryon asymmetry of the Universe via the B + L anomaly in the Standard Model. We find that most of the parameter space of pseudoscalar inflation that explains the cosmological data leads to baryon overproduction; hence, the models of natural inflation are severely constrained. We also point out a connection between the baryon number and topology of the relic magnetic fields. Both the magnitude and sign of magnetic helicity can be detected in future diffuse gamma ray data. This could hint to a link between inflation and the baryon asymmetry of the Universe.

Amer Physical Soc2015

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We consider a relativistic plasma of fermions coupled to an Abelian gauge field and carrying a chiral charge asymmetry, which might arise in the early Universe through baryogenesis. It is known that on large length scales, lambda greater than or similar to 1/(alpha mu(5)), the chiral anomaly opens an instability toward the erasure of chiral charge and growth of magnetic helicity. Here the chemical potential mu(5) parametrizes the chiral asymmetry and alpha is the fine-structure constant. We study the process of chiral charge erasure through the thermal fluctuations of magnetic helicity and contrast with the well-studied phenomenon of Chern-Simons number diffusion. Through the fluctuation-dissipation theorem we estimate the amplitude and time scale of helicity fluctuations on the length scale lambda, finding delta H similar to lambda T and tau similar to alpha lambda T-3(2) for a relativistic plasma at temperature T. We argue that the presence of a chiral asymmetry allows the helicity to grow diffusively for a time t similar to T-3/(alpha(5)mu(4)(5)) until it reaches an equilibrium value H similar to mu T-5(2)/alpha, and the chiral asymmetry is partially erased. If the chiral asymmetry is small, mu(5) < T/alpha, this avenue for chiral charge erasure is found to be slower than the chiral magnetic effect for which t similar to T/(alpha(3)mu(2)(5)). This mechanism for chiral charge erasure can be important for the hypercharge sector of the Standard Model as well as extensions including U(1) gauge interactions, such as asymmetric dark matter models.

Iop Publishing Ltd2016

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On the modulational stability of magnetic structures in electron drift turbulence

Martin Jucker

The generation of large-scale magnetic ﬁelds in magnetic electron drift mode turbulence is investigated. In particular, the mechanism of modulational instability caused by three-wave interactions is elucidated and the explicit increment is calculated. Also, a stability criterion similar to the known Lighthill criterion is found.

2007

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