During the last 5 years interest in magnetic helicity has grown dramatically in solar physics as a result of improved capabilities to measure and image magnetic fields. Magnetic helicity was introduced by K. Moffatt in the late 1950s as a topological invariant that describes the complexity of a magnetic field. The topological aspect of helicity is readily visualized in a Moebius strip, in which the system of interest is closed and helicity takes two forms, the writhing of the central axis of the strip and the twisting of the edges of the strip about that axis.
In many plasmas (but not in atmospheres like that of Earth, for example), helicity is conserved, just as the sum of twist and writhe is conserved in a Moebius strip. Mathematically, it is related to linking integrals, which K. F. Gauss employed to study asteroid paths on the sky. In the late 1970s the concept of magnetic helicity was introduced in laboratory plasma physics, turbulence theory, space physics, and statistical theory.