The review by Jack Zirker and Oddbjørn Engvold (Physics Today, August 2017, page 36) of recent coronal research makes extensive use of the concept of field lines—in particular, magnetic reconnection. But like the analogous streamlines in fluids, magnetic field lines are not physical entities. They are just a convenient way of visualizing the structure and behavior of magnetic fields. To convince yourself of this, think of the familiar pattern of field lines connecting the poles of a bar magnet. How many lines are there?

It follows that field-line reconnection itself is not a physical phenomenon and cannot cause either coronal heating or the propulsion of plasma in coronal mass ejection events. Such coronal phenomena result from dynamical processes involving electromagnetic fields, currents, and pressures, which are physical. Reconnection merely describes the way the magnetic field topology changes.

As mentioned in the article, reconnection events are associated with thin current sheets, which are related to the magnetic field by Ampere’s law. Their evolution is fundamental to understanding coronal phenomena. Eric Priest and Terry Forbes (Zirker and Engvold’s reference 3) note that James Dungey, the originator of the reconnection model, “found that small perturbations in the vicinity of the null point lead to the explosive formation of a current sheet. Near the null point, plasma motions induced by a small current perturbation cause the current to grow, and this current further enhances the plasma motions resulting in a positive feedback.”1 Focusing on the role that the currents play in reconnection would make for a clearer understanding.

1.
E.
Priest
,
T.
Forbes
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Magnetic Reconnection: MHD Theory and Applications
,
Cambridge U. Press
(
2000
), p.
8
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2.
Jack B.
Zirker
,
Oddbjørn
Engvold
,
Physics Today
,
70
(
8
),
36
(
2017
).