[Loops] Two new loops papers

[David Pontin]

Dear loops aficionados,

I hope this email finds you well in these challenging times. You may be interested in these two recently-published papers.

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Non-thermal line broadening due to braiding-induced turbulence in solar coronal loops (A&A, 639, A21, 2020. D. I. Pontin, H. Peter, and L. P. Chitta) https://arxiv.org/abs/2008.11915  https://www.aanda.org/articles/aa/abs/2020/07/aa37582-20/aa37582-20.html

ABSTRACT:
Aims: Emission line profiles from solar coronal loops exhibit properties that are unexplained by current models. We investigate the non-thermal broadening associated with plasma heating in coronal loops that is induced by magnetic field line braiding. Methods: We describe the coronal loop by a 3D magnetohydrodynamic model of the turbulent decay of an initially-braided magnetic field. From this we synthesize the Fe XII line at 193 Angstroms that forms around 1.5 MK. Results: Key features of current observations of extreme UV lines from the corona are reproduced in the synthesised spectra: (i) Typical non-thermal widths range from 15 to 20 km/s. (ii) The widths are approximately independent of the size of the field of view. (iii) There is a correlation between line intensity and the non-thermal broadening. (iv) Spectra are found to be non-Gaussian, with enhanced power in the wings of order 10-20%. Conclusions: Our model provides an explanation that self-consistently connects the heating process to the observed non-thermal line broadening. The non-Gaussian nature of the spectra is a consequence of the non-Gaussian nature of the underlying velocity fluctuations, interpreted as a signature of intermittency in the turbulence.
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The Parker problem: existence of smooth force-free fields and coronal heating (Living Reviews in Solar Physics, 17, 5, 2020. D. I Pontin and G. Hornig) https://link.springer.com/article/10.1007%2Fs41116-020-00026-5<https://link.springer.com/article/10.1007/s41116-020-00026-5>

ABSTRACT:
Parker (Astrophys J 174:499, 1972) put forward a hypothesis regarding the fundamental nature of equilibrium magnetic fields in astrophysical plasmas. He proposed that if an equilibrium magnetic field is subjected to an arbitrary, small perturbation, then—under ideal plasma dynamics—the resulting magnetic field will in general not relax towards a smooth equilibrium, but rather, towards a state containing tangential magnetic field discontinuities. Even at astrophysical plasma parameters, as the singular state is approached dissipation must eventually become important, leading to the onset of rapid magnetic reconnection and energy dissipation. This topological dissipation mechanism remains a matter of debate, and is a key ingredient in the nanoflare model for coronal heating. We review the various theoretical and computational approaches that have sought to prove or disprove Parker’s hypothesis. We describe the hypothesis in the context of coronal heating, and discuss different approaches that have been taken to investigating whether braiding of magnetic field lines is responsible for maintaining the observed coronal temperatures. We discuss the many advances that have been made, and highlight outstanding open questions.
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Best wishes,
David


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David I. Pontin | Assoc Prof Physics
School of Mathematical and Physics Sciences

T: +61 2 4055 3261
E: david.pontin at newcastle.edu.au<mailto:david.pontin at newcastle.edu.au>
W: newcastle.edu.au/profile/david-pontin<http://newcastle.edu.au/profile/david-pontin>

The University of Newcastle
University Drive, Callaghan NSW 2308 Australia


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