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
Dear Loop Group,
We have just published a new AIA loop paper in ApJ:
Atmospheric Imaging Assembly Observations of Coronal Loops: Cross-Field Temperature Distributions
Schmelz, J.T., Jenkins, B.S., Pathak, S. (2013), ApJ, 770, 14
Abstract: We construct revised response functions for the Atmospheric Imaging Assembly (AIA) using the new atomic data, ionization equilibria, and coronal abundances available in CHIANTI 7.1. We then use these response functions in multithermal analysis of coronal loops, which allows us to determine a specific cross-field temperature distribution without ad hoc assumptions. Our method uses data from the six coronal filters and the Monte Carlo solutions available from our differential emission measure (DEM) analysis. The resulting temperature distributions are not consistent with isothermal plasma. Therefore, the observed loops cannot be modeled as single flux tubes and must be composed of a collection of magnetic strands. This result is now supported by observations from the High-resolution Coronal Imager, which show fine-scale braiding of coronal strands that are reconnecting and releasing energy. Multithermal analysis is one of the major scientific goals of AIA, and these results represent an important step toward the successful achievement of that goal. As AIA DEM analysis becomes more straightforward, the solar community will be able to take full advantage of the state-of-the-art spatial, temporal, and temperature resolution of the instrument.
You can download a copy here:
http://stacks.iop.org/0004-637X/770/14
I look forward to seeing some of you at Loops VI!
Regards,
Joan
Dear Loop Enthusiasts,
ApJ has just accepted our paper 'Modeling the Line-of-Sight Integrated Emission in the Corona: Implications for Coronal Heating'.
Here is the abstract:
One of the outstanding problems in all of space science is uncovering how the solar corona is heated to temperatures greater than 1 MK. Though studied for decades, one of the major difficulties in solving this problem has been unraveling the line-of-sight (LOS) effects in the observations. The corona is optically thin, so a single pixel measures counts from an indeterminate number (perhaps tens of thousands) of independently heated flux tubes, all along that pixel's LOS. In this paper we model the emission in individual pixels imaging the active region corona in the Extreme Ultraviolet. If LOS effects are not properly taken into account, erroneous conclusions regarding both coronal heating and coronal dynamics may be reached. We model the corona as a LOS integration of many thousands of completely independently heated flux tubes. We demonstrate that despite the superposition of randomly heated flux tubes, nanoflares leave distinct signatures in light curves observed with multi-wavelength and high time cadence data, such as those data taken with the Atmospheric Imaging Assembly on board the Solar Dynamics Observatory. These signatures are readily detected with the time-lag analysis technique of Viall & Klimchuk (2012). Steady coronal heating leaves a different and equally distinct signature that is also revealed by the technique.
You can download a copy at astro-ph until it is officially published by ApJ:
http://arxiv.org/abs/1304.5439v2
Cheers,
Nicki
**************
Nicholeen Viall
Code 671
NASA/Goddard Space Flight Center
Greenbelt, MD 20771
Email: Nicholeen.M.Viall(a)nasa.gov<mailto:Nicholeen.M.Viall@nasa.gov>
Phone: 301-286-4054
Dear all,
this is to inform you that the deadline for the abstract submission
to the Coronal Loops Workshop VI has been extended to May 12.
You can submit the abstract and register via the online form at
http://www.stce.be/coronalloops/
Regards
Susanna
Dear colleagues,
We remind you that the abstract submission deadline for the 6^th Coronal
Loops Workshop is April 30 2013.
We encourage people to submit their abstract via the on-line form at
http://www.stce.be/coronalloops/.
The workshop to be held in Floréal Club, La Roche-en-Ardenne,Belgium,
from the 25 to 27 of June 2013.
Considering the travel restrictions imposed to the US colleagues, we are
forced to look for a possible solution to allow them to attend the
meeting remotely. However, such a solution is not guaranteed yet.
Updated will be announced later on the workshop webpage.
Susanna Parenti on behalf of the SOC
Dear Colleagues,
please find here the second announcement for the CORONAL LOOPS WORKSHOP
VI to be held in Floréal Club, La Roche‐en‐Ardenne, Belgium, from the 25
to 27 of June 2013.
Visit also the web page : http://www.stce.be/coronalloops/
With my best regards,
Susanna
On behalf of the SOC