[Loops] 1. 3D MHD modeling of twisted coronal loops (Fabio Reale)

[Philippa Browning]

Subject: More papers on 3D modeling of twisted coronal loops.

Thanks to Fabio for circulating this very interesting paper. 

I would like to draw people's attention also to our two recent papers on the subject of energy release in twisted coronal loops. These may also be of interest.

There are some similarities with the Reale et al paper. We also perform 3D MHD simulations of a loop twisted by photospheric motions, in a gravitationally-stratified atmosphere incorporating upper chromosphere as well as corona. We also account for thermal conduction
and radiation in the energy balance. Synthetic emission is calculated.  We find similarly that heating is more intense near the footpoints due to the concentration of the fields (and currents there).
Some differences:
We consider also curved loops, as well as straightened out loops as in Reale et al.
In paper 1 we also incorporate non-thermal particles, and synthesise non-thermal as well as thermal emission.
We consider fine-scale structure and current sheets triggered by the ideal kink instability.
We use a critical current for onset of anomalous resistivity which is related to the onset of ion acoustic instability (hence density dependent, rather than constant).

1. Thermal and non-thermal emission from reconnecting twisted coronal loops
R. F. Pinto, M. Gordovskyy, P. K. Browning, and N. Vilmer
http://www.aanda.org/articles/aa/abs/2016/01/aa26633-15/aa26633-15.html
(Astronomy and Astrophysics January 2016)
Here we present 3D MHD simulations of a coronal loop in a gravitationally stratified atmosphere which is twisted by photospheric footpoint motions and becomes kink unstable, leading to reconnection
in many current sheets. 3 models with varying loop size and field strength are studied. We predict temperature and density distributions, and hence synthesise the spatial distributions and temporal evolution of 
thermal emission as well as Differential Emission Measure. By coupling the MHD simulations to a test particle code, we also predict the evolution of the non-thermal particles and synthesise non-thermal (HXR) emission. 


2. Energy release in driven twisted coronal loops
M. Bareford, M Gordovskyy, P.K, Browning and A.W,Hood
http://link.springer.com/article/10.1007/s11207-015-0824-7
(Solar Phys 291 January 2016)
In this paper we focus on the effects of loop geometry (curvature and field line expansion) on energy release and heating in twisted loops, as well analysing the effects of thermal conduction on the temperature distribution.
We consider again kink-unstable loops arising from photospheric motions with vorticity, comparing a straight cylindrical loop, an expanding straight loops, and curved loops with different degrees of expansion. In all geometries,
as in earlier idealised models of unstable cylindrical loops, fine scale current structures are created leading to distributed heating. In the case of curved loops, there is also an asymmetry between the upper and lower side of the loop,
and large-scale current structures are created as the loop reconnects with the ambient field. In expanding loops, currents - and hence heating - are more strongly concentrated near the footpoints.

Best regards
Philippa Browning
Prof Philippa Browning
Jodrell Bank Centre for Astrophysics
University of Manchester
Manchester
M13 9PL
Tel: (44) (0) 161 306 3912

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Sent: 20 July 2016 19:00
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Subject: Loops Digest, Vol 81, Issue 3

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Today's Topics:

   1. 3D MHD modeling of twisted coronal loops (Fabio Reale)


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Message: 1
Date: Wed, 20 Jul 2016 14:10:25 +0200
From: Fabio Reale <reale at astropa.unipa.it>
To: Loops at solar.physics.montana.edu
Subject: [Loops] 3D MHD modeling of twisted coronal loops
Message-ID: <AFC15169-0246-4EC2-A176-90F011F66F14 at astropa.unipa.it>
Content-Type: text/plain; charset=us-ascii

Hi all,
      at the link below please find a paper just accepted for publication on the ApJ. It describes the model of a coronal loop that is heated because of twisting driven by random footpoint rotation. The heating is produced by the dissipation of the stressed and reconnecting magnetic field triggered above a current threshold. It is therefore a switch-on heating. The model includes the chromosphere and transition region and the expansion of the magnetic field there. It was a computing challenge (millions of CPU hours).
Best regards
Fabio Reale

https://arxiv.org/abs/1607.05500

3D MHD modeling of twisted coronal loops

F. Reale, S. Orlando, M. Guarrasi, A. Mignone, G. Peres, A. W. Hood, E. R. Priest

Abstract
We perform MHD modeling of a single bright coronal loop to include the interaction with a non-uniform magnetic field. The field is stressed by random footpoint rotation in the central region and its energy is dissipated into heating by growing currents through anomalous magnetic diffusivity that switches on in the corona above a current density threshold. We model an entire single magnetic flux tube, in the solar atmosphere extending from the high-beta chromosphere to the low-beta corona through the steep transition region. The magnetic field expands from the chromosphere to the corona. The maximum resolution is ~30 km. We obtain an overall evolution typical of loop models and realistic loop emission in the EUV and X-ray bands. The plasma confined in the flux tube is heated to active region temperatures (~3 MK) after ~2/3 hr. Upflows from the chromosphere up to ~100 km/s fill the core of the flux tube to densities above 10^9 cm^-3. More heating is released in the low corona than the
 high corona and is finely structured both in space and time.



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