Apex Dips of Experimental Flux Ropes: Helix or Cusp?

Abstract

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.

Magnetic Rayleigh–Taylor Instability in an Experiment Simulating a Solar Loop

“…For a sufficiently large magnetic field, the wavelength is larger than the loop dimensions and the MRTI cannot occur. This is most likely the reason for the absence of the MRTI in previous solar loop simulation experiments where a much larger axial magnetic field was used (e.g., Stenson & Bellan 2012;Wongwaitayakornkul et al 2017). In Equation (5), besides B 0 , the other two parameters, ρ, g, also influence the fastestgrowing wavelength.…”

“…In the Caltech single-loop experiment, a magnetized plasma loop is generated to simulate solar flux ropes. Mechanisms for flux rope expansion (Hansen & Bellan 2001;Ha & Bellan 2016), magnetically driven flows (Stenson & Bellan 2012), apex dips (Wongwaitayakornkul et al 2017), and coronal mass ejection cavity formation (Haw et al 2018) have been studied. We show here the existence of a hoop force driven MRTI in this experiment.…”

Reverse Current Model for Coronal Mass Ejection Cavity Formation

“…The experiment generates an expanding flux rope (argon) that collides with a background plasma (hydrogen). The apparatus consists of a magnetized plasma gun mounted at the end of a 1.6 m long, 0.92 m diameter vacuum chamber (Hansen & Bellan 2001;Stenson & Bellan 2012;Ha & Bellan 2016;Wongwaitayakornkul et al 2017). Figure 2(a) shows the apparatus and Cartesian coordinate system.…”

“…The background magnetic field is constructed from a set of 10 current loops arranged in a half-circle below the footpoints, each with a current of I=631 kA (see Figure 9 of Wongwaitayakornkul et al 2017). This arrangement produces a horseshoe-magnet field topology with a magnitude ranging from 0.2 T at the footpoints to 10 −3 T at the upper edge of the simulation.…”

“…This incremental addition of flux does not significantly perturb the system at a given time step but slowly increases the poloidal flux, corresponding to a rising current. A more detailed description of this injection scheme can be found in Wongwaitayakornkul et al (2017).…”