Recent theoretical and observational developments in magnetic helicity studies
Pascal Demoulin Submitted: 2006-12-15 09:29
Magnetic helicity quantifies how the magnetic field is sheared
and twisted compared to its lowest energy state (potential
field). Such stressed magnetic fields are usually observed
in association with flares, eruptive filaments, and coronal
mass ejections (CMEs). Magnetic helicity plays a key role in
magnetohydrodynamics because it is almost preserved on a
timescale less than the global diffusion time scale.
Its conservation defines a constraint to the magnetic field evolution.
Only relatively recently, scientists have realized that magnetic
helicity can be computed from observations, and methods
have been derived to bridge the gap between theory and observations.
At the photospheric level, the rate (or flux) of magnetic helicity can be
computed from the evolution of longitudinal magnetograms.
The coronal helicity is estimated from magnetic extrapolation, while
the helicity ejected in magnetic clouds (interplanetary counter-part
of CMEs) is derived through modelling of in-situ magnetic
field measurements. Using its conserved property, a quantitative
link between phenomena observed in the corona and then in the
interplanetary medium has been achieved.
Authors: Démoulin, Pascal
Publication Status: Advances in Space Research, in press
Last Modified: 2006-12-15 10:37