Magnetic helicity in primordial and dynamo scenarios of galaxies

Abstract

Abstract. Some common properties of helical magnetic fields in decaying and driven turbulence are discussed. These include mainly the inverse cascade that produces fields on progressively larger scales. Magnetic helicity also restricts the evolution of the large-scale field: the field decays less rapidly than a non-helical field, but it also saturates more slowly, i.e. on a resistive time scale if there are no magnetic helicity fluxes. The former effect is utilized in primordial field scenarios, while the latter is important for successfully explaining astrophysical dynamos that saturate faster than resistively. Dynamo action is argued to be important not only in the galactic dynamo, but also in accretion discs in active galactic nuclei and around protostars, both of which contribute to producing a strong enough seed magnetic field. Although primordial magnetic fields may be too weak to compete with these astrophysical mechanisms, such fields could perhaps still be important in producing polarization effects in the cosmic background radiation.

Lepton asymmetries and primordial hypermagnetic helicity evolution

“…It looks natural to base an understanding of cosmic MHD on balance equations for conserved quantities such as magnetic field energy and magnetic helicity. It has been noted that the magnetic helicity conservation is much more restrictive in astrophysical objects than the energy conservation [1,2]. The point is that usually there is a huge supply of kinetic energy in the form of a general rotation of a celestial body and it is quite easy to imagine a spectral energy flux which influences energy balance including magnetic field energy.…”

“…2 , omitting the initial helicity value H Y (t 0 ).Dividing the helicity density (3.6) by its value at T EW (3.2) we get the ratio valid at x ≥ 1:…”

Disentangling the Entangled Linkages of Relative Magnetic Helicity

“…Magnetic helicity is an important astrophysical quantity for understanding dynamos (Moffatt 1978), the emergence of largescale magnetic fields in the primordial universe (Field & Carroll 2000;Brandenburg 2006), galactic jets (Koenigl & Choudhuri 1985), the structure of stars (Schrijver & Zwaan 2000;Brandenburg 2020), stellar eruptive phenomena (Berger 1984), and coronal heating (Heyvaerts & Priest 1984). The concept of helicity has its mathematical origins in linkages with Gauß (1867), Călugăreanu (1959), and White (1969) and vortex motion with Thomson (1868).…”