Magnetically confined plasma structures—on the sun and in the lab. (a) Magnetic arches in the solar corona. The structures are axially collimated over several layers in the Sun’s atmosphere, even though the densities of the layers vary greatly. (b) Stenson and Bellan’s experiment. Hydrogen gas is confined in a magnetic fluxtube shortly after the tube forms. The structure is axially collimated, as in actual coronal magnetic structures. (c) Schematic of the forces that cause collimation in a magnetic flux tube, as seen from above the arch. In a typical dipole field, the field lines spanning the two poles would bow out, but in the coronal structures, the existence of a magnetic field azimuthal to the main field lines generates a J×B force that pushes plasma flow towards the less constricted regions. These flows also transport azimuthal flux from the stronger field regions resulting in a dynamic collimation of the structure.