Magnetic Flux Tube Reconnection: Tunneling Versus Slingshot

The discrete nature of the solar magnetic field as it emerges into the coronathrough the photosphere indicates that it exists as isolated flux tubes in theconvection zone, and will remain as discrete flux tubes in the corona until itcollides and reconnects with other coronal fields. Collisions of these fluxtubes will in general be three dimensional, and will often lead toreconnection, both rearranging the magnetic field topology in fundamental ways,and releasing magnetic energy. With the goal of better understanding thesedynamics, we carry out a set of numerical experiments exploring fundamentalcharacteristics of three dimensional magnetic flux tube reconnection. We firstshow that reconnecting flux tubes at opposite extremes of twist behave verydifferently: in some configurations, low twist tubes slingshot while high twisttubes tunnel. We then discuss a theory explaining these differences: byassuming helicity conservation during the reconnection one can show that athigh twist, tunneled tubes reach a lower magnetic energy state than slingshottubes, whereas at low twist the opposite holds. We test three predictions madeby this theory. 1) We find that the level of twist at which the transition fromslingshot to tunnel occurs is about two to three times higher than predicted onthe basis of energetics and helicity conservation alone, probably because thedynamics of the reconnection play a large role as well. 2) We find that thetunnel occurs at all flux tube collision angles predicted by the theory. 3) Wefind that the amount of magnetic energy a slingshot or a tunnel reconnectionreleases agrees reasonably well with the theory, though at the highresistivities we have to use for numerical stability, a significant amount ofmagnetic energy is lost to diffusion, independent of reconnection.Comment: 21 pages, 15 figures, submitted to Ap