Magnetic Helicity is a global quantity defined as in some volume, where A
is the
magnetic vector potential and B is the magnetic
field.
Note:
It can be thought of as representing how much the magnetic field
curls
around itself, and describes the extent to which the magnetic
field lines are linked, knotted, and twisted.
Helicity seems to play a large role in the process of reconnection,
affecting parameters such as: