Coronal Magnetic Structure: the Role of Ideal MHD Invariants
Magnetic helicity measures properties of field lines such as linking, knotting, and twisting. Severe bounds can be placed on its dissipation in a highly conducting plasma. Motions of field line endpoints at the photosphere inject helicity into the corona. An associated invariant is the net twist between two neighboring field lines. Random photospheric motions should twist neighboring lines through a few turns in one day. The current associated with this twist is small, suggesting that most coronal current resides in sheets. Other ideal invariants can be defined which measure the mutual braiding of three or more field lines. Coronal heating models which rely on the buildup of helicity and braiding will be reviewed.
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