Dear all,
the following paper has recently been accepted by ApJ Letters. You can
find a copy of the paper and relevant movies at
www.lmsal.com/~ptesta/hic_moss/
thanks,
Paola
title:
Observing coronal nanoflares in active region moss
authors:
P. Testa, B. De Pontieu, J. Martinez-Sykora, E. DeLuca, V. Hansteen, J.
Cirtain, A. Winebarger, L. Golub, K. Kobayashi, K. Korreck, S. Kuzin, R.
Walsh, C. DeForest, A. Title, M.Weber
abstract:
The High-resolution Coronal Imager (Hi-C) has provided Fe XII 193A
images of the upper transition region moss at an unprecedented spatial
(~0.3-0.4 arcsec) and temporal (5.5s) resolution.
The Hi-C observations show in some moss regions variability on
timescales down to ~15s, significantly shorter than the minute scale
variability typically found in previous observations of moss, therefore
challenging the conclusion of moss being heated in a mostly steady
manner. These rapid variability moss regions are located at the
footpoints of bright hot coronal loops observed by SDO/AIA in the 94A
channel, and by Hinode/XRT.
The configuration of these loops is highly dynamic, and suggestive of
slipping reconnection.
We interpret these events as signatures of heating events associated
with reconnection occurring in the overlying hot coronal loops, i.e.,
coronal nanoflares.
We estimate the order of magnitude of the energy in these events to be
of at least a few $10^{23}$~erg, also supporting the nanoflare scenario.
These Hi-C observations suggest that future observations at comparable
high spatial and temporal resolution, with more extensive temperature
coverage are required to determine the exact characteristics of the
heating mechanism(s).
This message was originally HTML formatted. View in a HTML capable client to see the original version.\r\n\r\nDear colleagues,
our paper about cross-sections of coronal loops has been accepted to ApJ and it is now available on arXiv: http://arxiv.org/abs/1307.3440 , the abstract is below.
Key points:
- coronal loops may be very non-circular in cross-section
- when we see bright thin overdense strands over dim underdense fuzz, we might be looking at the structures of similar size at the base and of the same scale height, which simply happened to expand in different directions with respect to our line of sight
- we need to be aware of this effect as it may pose selection bias towards loops which expand along our line of sight more than across it
It was a pleasure to meet many of you at the SPD and I was very happy to receive such a positive feedback on this work!
Anny
"On Anisotropy in Expansion of Magnetic Flux Tubes in the Solar Corona"
A. Malanushenko, C. J. Schrijver
Abstract: Most 1d hydrodynamic models of plasma confined to magnetic flux tubes assume circular cross-section of these tubes. We use potential field models to show that flux tubes in circumstances relevant to the solar corona do not in general maintain the same cross-sectional shape through their length and therefore the assumption of a circular cross-section is rarely true. We support our hypothesis with mathematical reasoning and numeric experiments. We demonstrate that lifting this assumption in realistic non-circular loops make apparent expansion of magnetic flux tubes consistent with that of observed coronal loops. We propose that in a bundle of ribbon-like loops those that are viewed along the wide direction would stand out against those that are viewed across the wide direction, due to the difference in their column depths. That would impose a bias towards selecting loops that appear not to be expanding seen projected in the plane of sky. An implication of this selection bias is that the preferentially selected non-circular loops would appear to have increased pressure scale height even if they are resolved by current instruments.
FYI - our latest paper is on the solar archives at
http://solar.physics.montana.edu/cgi-bin/eprint/index.pl?entry=18486
Abstract:
A Rapid, Manual Method to Map Coronal-Loop Structures of an Active Region Using Cubic Bezier Curves and Its Applications to Misalignment Angle Analysis
G. Allen Gary, Qiang Hu, and Jong Kwan Lee
A rapid and flexible manual method is described which maps individual coronal loops of a 2D EUV image as Bezier curves using only four points per loop. Using the coronal loops as surrogates of magnetic-field lines, the mapping results restrict the magnetic-field models derived from extrapolations of magnetograms to those admissible and inadmissible via a fitness parameter. We outline explicitly how the coronal loops can be employed in constraining competing magnetic-field models by transforming 2D coronal-loop images into 3D field lines. The magnetic-field extrapolations must satisfy not only the lower boundary conditions of the vector field, the vector magnetogram, but also must have a set of field lines that satisfies the mapped coronal loops in the volume, analogous to an upper boundary condition. This method uses the minimization of the misalignment angles between the magnetic-field model and the best set of 3D field lines that match a set of closed coronal loops. The presented method is an important tool in determining the fitness of magnetic-field models for the solar atmosphere. The magnetic-field structure is crucial in determining the overall dynamics of the solar atmosphere.
Dear all,
as you know last week we had the 6th Coronal Loops Workshop. If you
want to know more, we put online the talks, posters and pictures.
Follows the link: www.sidc.be/coronalloops/
Regards
Susanna
Hello everyone,
Here are two new papers by Peter Cargill and me that may be of interest to
you.
The first "The influence of numerical resolution on coronal density in
hydrodynamic models of impulsive heating" takes what we believe to be a
long-overdue look at the issues surrounding the question of grid resolution
in numerical models and some of the physical implications of under-resolving
particular regions of the solar atmosphere. The paper has already been
published by ApJ and can be obtained here:
http://iopscience.iop.org/0004-637X/770/1/12/pdf/0004-637X_770_1_12.pdf
The second "The Cooling of Coronal Plasmas. iv: Catastrophic Cooling of
Loops" is a continuation of our series on the relevant cooling processes in
the solar atmosphere. Here we provide a physical explanation of the onset of
catastrophic cooling, which may provide an explanation for the relatively
weak active region emission detected in wavelength channels sensitive to
lower temperature lines. The manuscript has been accepted for publication by
ApJ and is available on astro-ph:
http://arxiv.org/ftp/arxiv/papers/1305/1305.5484.pdf
Hope to see many of you in Montana!
Best,
Steve
Dr Stephen J. Bradshaw
(Assistant Professor of Physics and Astronomy)
Department of Physics and Astronomy, MS-108,
Rice University,
6100 Main Street,
Houston,
TX 77005,
USA.
Tel: +1 713 348 4045
Email: stephen.bradshaw(a)rice.edu