How is helicity (and twist) partitioned in magnetohydrodynamic simulations of reconnecting magnetic flux tubes?

James Threlfall, Andrew N. Wright, Alan W. Hood

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Magnetic helicity conservation provides a convenient way to analyze specific properties (namely, the linkage and twist) of reconnecting flux tubes and yield additional insight into the pre- and post-reconnection states of magnetic structures in the solar atmosphere. A previous study considered two flux tubes with footpoints anchored in two parallel planes. They showed that reconnection would add self-helicity equivalent to a half turn of twist to each flux tube. We address a related and fundamental question here: if two flux tubes anchored in a single plane reconnect, what are the resulting twists imparted to each of the reconnected tubes? Are they equal and do they have a simple exact value independent of footpoint location? To do this, we employ a new (computationally efficient) method which subdivides each flux tube into distinct elements and calculates the mutual helicity of many elemental pairs, the sum of which determines the self-helicity of the overall flux tube. Having tested the method using a simple analytical model, we apply the technique to a magnetohydrodynamic simulation where initially untwisted magnetic flux tubes are sheared and allowed to reconnect (based on a previous reconnection model). We recover values of self-helicity and twist in the final end state of the simulations which show excellent agreement with theoretical predictions.
Original languageEnglish
Article number1
Number of pages9
JournalAstrophysical Journal
Issue number1
Early online date16 Jul 2020
Publication statusPublished - 20 Jul 2020


  • Magnetic fields
  • Solar magnetic reconnection
  • Magnetohydrodynamical simulations
  • Magnetohydrodynamics


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