Helicity transformation under the collision and merging of two magnetic flux ropes

Abstract

Magnetic helicity has become a useful tool in the analysis of astrophysical plasmas. Its conservation in the magnetohydrodynamic limit (and other fluid approaches) constrains the global behavior of large plasma structures. One such astrophysical structure is a magnetic flux rope: a tube-like, current-carrying plasma embedded in an external magnetic field. Bundles of these ropes are commonly observed in the near-earth environment and solar atmosphere. In this well-diagnosed experiment (three-dimensional measurements of n e , T e , V p , B, J, E, and u flow), two magnetic flux ropes are generated in the Large Plasma Device at UCLA. These ropes are driven kink-unstable to trigger complex motion. As they interact, helicity conservation is examined in regions of reconnection. We examine (1) the transport of helicity and (2) the dissipation of the helicity. As the ropes move and the topology of the field lines diverge, a quasi-separatrix layer (QSL) is formed. As the QSL forms, magnetic helicity is dissipated within this region. At the same time, there is an influx of canonical helicity into the region such that the temporal derivative of magnetic helicity is zero.

Quantifying reconnective activity in braided vector fields

“…This analysis illustrates how the quantity R(x 0 , t) provides a valuable additional tool for probing magnetic fields of non-trivial internal topology, adding significant information to previous studies using the total magnetic helicity [39,50].…”

“…Contemporary use of GMR often involves identifying sheets of significant current (Quasi separatrix layers/QSL's) at which reconnection will occur in magnetized plasmas (e.g. [20,[37][38][39]). More direct approaches for have been applied by Wilmot-Smith and De Moortel [40] and Mackay and van Ballegooijen [41], who track the changing connectivity between particular field line "types" (specific to each numerical experiment).…”

“…The group have conducted a number of such experiments [18,19,42,43]. DeHaas and Gekelman [39] were able to analyze helicity flows in and out of a dominant QSL established between the interacting ropes. In another study Gekelman [20] used the GMR theory to measure reconnective activity.…”

Measurements of the Canonical Helicity of a Gyrating Kink

“…Magnetic helicity has been indirectly measured in plasmas by reconstructing internal magnetic field profiles with equilibrium models [25] and field line twist numbers [26]. References [27,28] measured magnetic helicity and canonical helicity in reconnecting current sheets and merging flux ropes. References [16,27,28] were able to use the applied fields as reference fields since the measurements spanned the entire vacuum chamber.…”

“…References [27,28] measured magnetic helicity and canonical helicity in reconnecting current sheets and merging flux ropes. References [16,27,28] were able to use the applied fields as reference fields since the measurements spanned the entire vacuum chamber. Here, we report experimental measurements of canonical flux tubes and their helicity evolution and transport in a section of a gyrating kink quasiequilibrium.…”