Apex Dips of Experimental Flux Ropes: Helix or Cusp?
We present a new theory for the presence of apex dips in certain experimental flux ropes. Previously such dips were thought to be projections of a helical loop axis generated by the kink instability. However, new evidence from experiments and simulations suggest that the feature is a 2D cusp rather than a 3D helix. The proposed mechanism for cusp formation is a density pileup region generated by nonlinear interaction of neutral gas cones emitted from fast-gas nozzles. The results indicate that density perturbations can result in large distortions of an erupting flux rope, even in the absence of significant pressure or gravitational forces. The density pileup at the apex also suppresses the m = 1 kink mode by acting as a stationary node. Consequently, more accurate density profiles should be considered when attempting to model the stability and shape of solar and astrophysical flux ropes.
© 2017 American Astronomical Society. Received 2017 June 8; revised 2017 August 28; accepted 2017 August 29; published 2017 October 17. This work was supported by NSF under award 1348393, AFOSR under award FA9550-11-1-0184, and DOE under awards DE-FG02-04ER54755 and DE-SC0010471. H.L. and S.L. acknowledge the support from DoE/OFES and LANL/LDRD programs.
Published - Wongwaitayakornkul_2017_ApJ_848_89.pdf