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Title: Observable Signatures of Energy Release in Braided Coronal Loops

Abstract

We examine the turbulent relaxation of solar coronal loops containing non-trivial field line braiding. Such field line tangling in the corona has long been postulated in the context of coronal heating models. We focus on the observational signatures of energy release in such braided magnetic structures using MHD simulations and forward modeling tools. The aim is to answer the following question: if energy release occurs in a coronal loop containing braided magnetic flux, should we expect a clearly observable signature in emissions? We demonstrate that the presence of braided magnetic field lines does not guarantee a braided appearance to the observed intensities. Observed intensities may—but need not necessarily—reveal the underlying braided nature of the magnetic field, depending on the degree and pattern of the field line tangling within the loop. However, in all cases considered, the evolution of the braided loop is accompanied by localized heating regions as the loop relaxes. Factors that may influence the observational signatures are discussed. Recent high-resolution observations from Hi-C have claimed the first direct evidence of braided magnetic fields in the corona. Here we show that both the Hi-C data and some of our simulations give the appearance of braiding at a range ofmore » scales.« less

Authors:
 [1];  [2]; ; ;  [3];  [4]
  1. University of Dundee, Nethergate, Dundee, DD1 4HN (United Kingdom)
  2. Institut d’Astrophysique Spatiale, CNRS, Univ. Paris-Sud, Université Paris-Saclay, Bât. 121, F-91405, Orsay Cedex (France)
  3. NASA Marshall Space Flight Center, ZP 13, Huntsville, AL 35812 (United States)
  4. Niels Bohr Institute, Geological Museum Østervoldgade 5-7, DK-1350, Copenhagen K (Denmark)
Publication Date:
OSTI Identifier:
22661301
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 837; Journal Issue: 2; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; EMISSION; MAGNETIC FIELDS; MAGNETIC FLUX; MAGNETOHYDRODYNAMICS; RELAXATION; RESOLUTION; SIMULATION; SOLAR CORONA; SUN

Citation Formats

Pontin, D. I., Janvier, M., Tiwari, S. K., Winebarger, A. R., Cirtain, J. W., and Galsgaard, K. Observable Signatures of Energy Release in Braided Coronal Loops. United States: N. p., 2017. Web. doi:10.3847/1538-4357/AA5FF9.
Pontin, D. I., Janvier, M., Tiwari, S. K., Winebarger, A. R., Cirtain, J. W., & Galsgaard, K. Observable Signatures of Energy Release in Braided Coronal Loops. United States. doi:10.3847/1538-4357/AA5FF9.
Pontin, D. I., Janvier, M., Tiwari, S. K., Winebarger, A. R., Cirtain, J. W., and Galsgaard, K. Fri . "Observable Signatures of Energy Release in Braided Coronal Loops". United States. doi:10.3847/1538-4357/AA5FF9.
@article{osti_22661301,
title = {Observable Signatures of Energy Release in Braided Coronal Loops},
author = {Pontin, D. I. and Janvier, M. and Tiwari, S. K. and Winebarger, A. R. and Cirtain, J. W. and Galsgaard, K.},
abstractNote = {We examine the turbulent relaxation of solar coronal loops containing non-trivial field line braiding. Such field line tangling in the corona has long been postulated in the context of coronal heating models. We focus on the observational signatures of energy release in such braided magnetic structures using MHD simulations and forward modeling tools. The aim is to answer the following question: if energy release occurs in a coronal loop containing braided magnetic flux, should we expect a clearly observable signature in emissions? We demonstrate that the presence of braided magnetic field lines does not guarantee a braided appearance to the observed intensities. Observed intensities may—but need not necessarily—reveal the underlying braided nature of the magnetic field, depending on the degree and pattern of the field line tangling within the loop. However, in all cases considered, the evolution of the braided loop is accompanied by localized heating regions as the loop relaxes. Factors that may influence the observational signatures are discussed. Recent high-resolution observations from Hi-C have claimed the first direct evidence of braided magnetic fields in the corona. Here we show that both the Hi-C data and some of our simulations give the appearance of braiding at a range of scales.},
doi = {10.3847/1538-4357/AA5FF9},
journal = {Astrophysical Journal},
number = 2,
volume = 837,
place = {United States},
year = {Fri Mar 10 00:00:00 EST 2017},
month = {Fri Mar 10 00:00:00 EST 2017}
}
  • We study the signatures of coronal heating on the differential emission measure (DEM) by means of hydrodynamic simulations capable of resolving the chromospheric-corona transition region sections of multi-stranded coronal loops and following their evolution. We consider heating either uniformly distributed along the loop or localized close to the chromospheric footpoints, in both steady and impulsive regimes. Our simulations show that condensation at the top of the loop forms when the impulsive heating, with a pulse cadence lower than the plasma cooling time, is localized at the loop footpoints, and the pulse energy is below a threshold above which the heatingmore » balances the radiative losses, thus preventing the catastrophic cooling which triggers the condensation. A condensation does not produce observable signatures in the DEM because it does not redistribute the plasma over a sufficiently large temperature range. On the other hand, the DEM coronal peak is found sensitive to the pulse cadence time when this is longer or comparable to the plasma cooling time. In this case, the heating pulses produce large oscillations in temperature in the bulk of the coronal plasma, which effectively smears out the coronal DEM structure. The pronounced DEM peak observed in active regions would indicate a predominance of conditions in which the cadence time is shorter or of the order of the plasma cooling time, whilst the structure of the quiet-Sun DEM suggests a cadence time longer than the plasma cooling time. Our simulations give an explanation of the warm overdense and hot underdense loops observed by TRACE, SOHO, and Yohkoh. However, they are unable to reproduce both the transition region and the coronal DEM structure with a unique set of parameters, which outlines the need for a more realistic description of the transition region.« less
  • In closed, magnetic field regions of the solar corona the plasma is largely confined in loop or archlike structures of enhanced density; such features are easily seen on X-ray or EUV images of the corona. During a large part of their lifetimes these loops undergo no striking changes in either brightness or structure; hence a quasi-static (steady-state) model, balancing energy input to each volume element with radiative losses, and losses or gains by conduction, is appropriate. Quasistatic models have been investigated analytically by several authors. Our treatment is numerical so that we can include: gravity; an energy input function thatmore » is dependent on density, temperature, etc.; an accurate form for the radiative losses; and a variable cross-sectional area for the loop geometry. In this paper we consider a uniform energy input epsilon (per unit volume) and a variety of loop geometries presenting results for the run of electron density (n) and temperature (T) along the loop and the variation of differential emission measure with T, xi (T). X-ray images are presented showing significant variations of loop cross-sectional area along the loop length. Using a line dipole magnetic field model, we parametrize such area variations by GAMMA, the ratio between the cross-sectional area at the loop apex and at the base. Numerical modeling of coronal loops with ..lambda..>1 shows that loop temperature and density structure are relatively weak functions of GAMMA. However, these variations in n and T act in a cumulative fashion, making xi (T) a strong function of GAMMA near the loop apex. Thus, for a given loop, increases in GAMMA cause substantial increases in the amount of material at coronal temperatures (> or approx. =5 x 10/sup 5/ K). For the examples studied, changes in energy input cause roughly proportional changes in radiated power (loop brightness as observed in EUV or X-rays), but only small variations in maximum temperature.« less
  • Previous studies of the magnetohydrodynamic (MHD) stability of solar coronal loops have not taken into account the effects of radiative or conductive energy loss in the energy equation. However, since coronal loops continuously lose energy by radiation and heat conduction, it is important to understand how these energy loss mechanisms affect MHD stability. We investigate the problem assuming that a magnetic loop has cylindrical geometry. As a first step, stability is studied for a localized mode, and the result is applied to a specific equilibrium. We find that the radiative energy loss effect not only changes the growth rate ofmore » ideally unstable modes, but also alters the stability boundary predicted by ideal MHD theory.« less
  • We have studied the effect of radiative energy loss on the stability of compressible plasma in coronal loops. By taking the limit as poloidal wavenumber m..-->..infinity, we derive stability conditions for local modes. We have found that the radiation effect can trigger MHD instabilities of coronal loops which are in ideally marginally stable stars. Compressibility is a stabilizing effect for ideal MHD local modes because the compression of magnetic field lines exerts a restoring force by increasing magnetic pressure. Compression of plasma induces two modes in a radiatively unstable plasma, magnetosonic and condensation modes. Compressibility affects the stability of ideallymore » stable (or unstable) coronal plasmas through magnetosonic modes, which are a stabilizing (destabilizing) effect for ideally stable (unstable) plasmas. For coronal plasmas in ideally marginally stable states, condensation as well as magnetosonic modes can trigger MHD instability. Because of these two modes, the effect of radiation on compressible coronal plasmas is more destabilizing than it is on incompressible plasmas when the plasmas are in ideal MHD unstable or marginally stable stars.« less
  • In this work, the well established two-fluid relaxation model based on the minimum energy principle is extended to include open systems like the solar corona. The Euler-Lagrange equations obtained are of double curl in nature and support non-zero plasma-{beta} along with mass flow of the magnetofluid. These equations are solved in Cartesian coordinates utilizing a geometry relevant to the solar atmosphere, and a basic comparative study of the non force-free, force-free, and potential magnetic field obtained as solutions of the same Euler-Lagrange equations is presented.