Magnetic helicity and prospects for its observation in the interstellar medium
a Institute of Continuous Media Mechanics, Ural Branch of the Russian Academy of Sciences, ul. akad. Koroleva 1, Perm, 614013, Russian Federation
b Perm National Research Polytechnic University, Komsomol'skii prosp. 29, Perm, 614990, Russian Federation
c Lomonosov Moscow State University, Department of Physics, Leninskie Gory 1 build. 2, Moscow, 119991, Russian Federation
d Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radio Wave Propagation, Russian Academy of Sciences, Kaluzhskoe shosse 4, Troitsk, Москва, 108840, Russian Federation
Magnetic helicity is one of the integrals of non-viscous flows in magnetohydrodynamics that determines the number of linkage of magnetic field lines in a medium. It belongs to a number of helicities that characterize the degree of mirror asymmetry of velocity and magnetic fields. The helicities plays a crucial role in driving the generation of large-scale magnetic fields in stars and spiral galaxies. Until recently, measurements of various helicities were based on astronomical observations of the Sun's active regions, but not in the Sun's deep layers where the solar dynamo is operative. Galaxies are transparent to some extent, therefore, are very attractive in this sense to observe the helicity of its magnetic field. Theoretical advances and first successful attempts of such observations are reviewed.