Magnetic helicity and prospects for its observation in the interstellar medium

Magnetic helicity is one of the integrals of nonviscous flows in magnetohydrodynamics that determines the number of linkages of magnetic field lines in a medium. It is among a number of helicities that characterize the degree of mirror asymmetry of velocity and magnetic fields. The helicities play 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 and are therefore very attractive in this sense for observing the helicity of its magnetic field. Theoretical advances and the first successful attempts at such observations are reviewed.