Dear all,
Following the response to the poll sent to this mailing-list a few
weeks ago, we have decided to proceed with the organization of the 10th
Coronal Loops Workshop. We can now confirm its dates, from 28 June to 01
July 2022, and the venue, Cité Internationale Universitaire in Paris.
The meeting in 2022 will celebrate the 20th anniversary of the series,
which was inaugurated in Orsay in 2002.
Some information about the venue can already be found at
https://loops10.sciencesconf.org/
Further details will be released soon, and registration should be opened
in the coming month.
Regards,
Susanna Parenti
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Dr. Susanna PARENTI
Institut d'Astrophysique Spatiale (IAS)
Bat. 121, Université Paris Saclay,
91405 Orsay Cedex, France
susanna.parenti(a)ias.u-psud.fr
https://www.ias.u-psud.fr/
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Dear all,
We recently published a paper that modifies the Enthalpy-Based Thermal Evolution of Loop (EBTEL) model by relaxing the assumption of subsonic flows. We also discuss a simple criterion to predict if Mach numbers produced by 0D simulations are reliable.
The paper can be accessed at https://iopscience.iop.org/article/10.3847/1538-4357/ac3009 or alternatively on arxiv at https://arxiv.org/abs/2110.03204. The code is available at https://github.com/rice-solar-physics/EBTEL3
The abstract of the work is given below:
Plasma-filled loop structures are common in the solar corona. Because detailed modeling of the dynamical evolution of these structures is computationally costly, an efficient method for computing approximate but quick physics-based solutions is to rely on space-integrated 0D simulations. The enthalpy-based thermal evolution of loops (EBTEL) framework is a commonly used method to study the exchange of mass and energy between the corona and transition region. EBTEL solves for density, temperature, and pressure, averaged over the coronal part of the loop, the velocity at the coronal base, and the instantaneous differential emission measure distribution in the transition region. The current single-fluid version of the code, EBTEL2, assumes that at all stages the flows are subsonic. However, sometimes the solutions show the presence of supersonic flows during the impulsive phase of heat input. It is thus necessary to account for this effect. Here, we upgrade EBTEL2 to EBTEL3 by including the kinetic energy term in the Navier–Stokes equation. We compare the solutions from EBTEL3 with those obtained using EBTEL2, as well as the state-of-the-art field-aligned hydrodynamics code HYDRAD. We find that the match in pressure between EBTEL3 and HYDRAD is better than that between EBTEL2 and HYDRAD. Additionally, the velocities predicted by EBTEL3 are in close agreement with those obtained with HYDRAD when the flows are subsonic. However, EBTEL3 solutions deviate substantially from HYDRAD's when the latter predicts supersonic flows. Using the mismatches in the solution, we propose a criterion to determine the conditions under which EBTEL can be used to study flows in the system.
I will be happy to get your valuable feedback.
Kind Regards,
Abhishek Rajhans
Dear all,
As some of you may know, we are organizing the next Coronal Loops
Workshop in Paris. We have started last year, but we had to postpone it
to 2022 due to the pandemic situation.
Following the philosophy of these workshops, we want to have an on-site
meeting only. Even if it was not advertised yet, we have a slot planned
for 28 June – 1 July 2022. Although the pandemic situation is still
uncertain, these dates are approaching and we need to decide now whether
we keep these dates or postpone the workshop again (maybe October 2022
or June 2023).
In order to decide what the best option is, we need to have an idea
about how many of you are ready to travel to France for the occasion, if
the workshop is held at the end of June 2022. Could you then please fill
this one-question poll?
https://doodle.com/poll/uvfdcwv5vnte9qm5
This, of course, does not commit you to attend the meeting, we are aware
that the situation can still evolve. But it will certainly help us to
understand if this is feasible. You can use the “Comments” field of the
poll to tell us more.
Many thanks,
Susanna Parenti and the Coronal Loops Workshop LOC
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°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°°
Dr. Susanna PARENTI
Institut d'Astrophysique Spatiale (IAS)
Bat. 121, Université Paris Saclay,
91405 Orsay Cedex, France
susanna.parenti(a)ias.u-psud.fr
https://www.ias.u-psud.fr/
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Dear Loops friends
We have recently updated the EBTEL loop model to include area
variations. This can be found at:
http://arxiv.org/abs/2111.09339
and at:
https://academic.oup.com/mnras/article-abstract/509/3/4420/6414539
with abstract:
The Enthalpy Based Thermal Evolution of Loops (EBTEL) approximate model
for static and dynamic coronal loops is developed to include the effect
of a loop cross-sectional area which increases from the base of the
transition region (TR) to the corona. The TR is defined as the part of a
loop between the top of the chromosphere and the location where thermal
conduction changes from an energy loss to an energy gain. There are
significant differences from constant area loops due to the manner in
which the reduced volume of the TR responds to conductive and enthalpy
fluxes from the corona. For static loops with modest area variation the
standard picture of loop energy balance is retained, with the corona and
TR being primarily a balance between heating and conductive losses in
the corona, and downward conduction and radiation to space in the TR. As
the area at the loop apex increases, the TR becomes thicker and the
density in TR and corona larger. For large apex areas, the coronal
energy balance changes to one primarily between heating and radiation,
with conduction playing an increasingly unimportant role, and the TR
thickness becoming a significant fraction of the loop length.
Approximate scaling laws are derived that give agreement with full
numerical solutions for the density, but not the temperature. For
non-uniform areas, dynamic loops have a higher peak temperature and are
denser in the radiative cooling phase by of order 50% than the constant
area case for the examples considered. They also show a final rapid
cooling and draining once the temperature approaches 1 MK. Although the
magnitude of the emission measure will be enhanced in the radiative
phase, there is little change in the important observational diagnostic
of its temperature dependence.
The EBTEL code is also updated and can be found at:
https://github.com/rice-solar-physics/EBTEL
Before using the new version of EBTEL, please read both the paper and
the user notes for limitations of the validity of the model. These are
NEW limitations introduced by the non-uniform area.
Peter
Dear loops enthusiasts,
I hope this email finds you well and that we are able to see each other in person in the near future. I wanted to bring your attention to a paper that was recently published in ApJ by myself, Steve Bradshaw, and Nicki Viall, “Understanding Heating in Active Region Cores through Machine Learning II. Classifying Observations.” This is a follow up to our first paper in this series from 2019<https://doi.org/10.3847/1538-4357/ab290c> which presented the modeling portion of this work. It builds upon the results of our first paper by showing how our modeled emission measure slopes and time lags can be used to systematically classify observations of active region cores in terms of heating frequency.
The title, abstract, and arXiv and ApJ links are included below. Those of you who attended the last loops workshop in St Andrews in 2019 may recognize some of the results!
Title: Understanding Heating in Active Region Cores through Machine Learning. II. Classifying Observations
ApJ: https://doi.org/10.3847/1538-4357/ac1514
arXiv: https://arxiv.org/abs/2107.07612
Abstract: To adequately constrain the frequency of energy deposition in active region cores in the solar corona, systematic comparisons between detailed models and observational data are needed. In this paper, we describe a pipeline for forward modeling active region emission using magnetic field extrapolations and field-aligned hydrodynamic models. We use this pipeline to predict time-dependent emission from active region NOAA 1158 for low-, intermediate-, and high-frequency nanoflares. In each pixel of our predicted multi-wavelength, time-dependent images, we compute two commonly used diagnostics: the emission measure slope and the time lag. We find that signatures of the heating frequency persist in both of these diagnostics. In particular, our results show that the distribution of emission measure slopes narrows and the mean decreases with decreasing heating frequency and that the range of emission measure slopes is consistent with past observational and modeling work. Furthermore, we find that the time lag becomes increasingly spatially coherent with decreasing heating frequency while the distribution of time lags across the whole active region becomes more broad with increasing heating frequency. In a follow-up paper, we train a random forest classifier on these predicted diagnostics and use this model to classify real observations of NOAA 1158 in terms of the underlying heating frequency.
Best,
Will
Dr. Will Barnes
NRC Research Associate
Space Science Division
U.S. Naval Research Laboratory
Code 7680
Hi All,
Hope you have been well!
I wanted to share this new paper just accepted by ApJ. https://arxiv.org/abs/2109.03355
[2109.03355] Signatures of Type III Solar Radio Bursts from Nanoflares: Modeling<https://arxiv.org/abs/2109.03355>
There is a wide consensus that the ubiquitous presence of magnetic reconnection events and the associated impulsive heating (nanoflares) is a strong candidate for solving the solar coronal heating problem. Whether nanoflares accelerate particles to high energies like full-sized flares is unknown. We investigate this question by studying the type III radio bursts that the nanoflares may produce ...
arxiv.org
Regards
Sherry
Signatures of Type III Solar Radio Bursts from Nanoflares: Modeling
Sherry Chhabra
George Mason University
James A. Klimchuk
NASA Goddard Space Flight Center
Dale E. Gary
Center for Solar-Terrestrial Research, New Jersey Institute of Technology
Abstract
There is a wide consensus that the ubiquitous presence of magnetic reconnection events and the associated impulsive heating (nanoflares) is a strong candidate for solving the solar coronal heating problem. Whether nanoflares accelerate particles to high energies like full-sized flares is unknown. We investigate this question by studying the type III radio bursts that the nanoflares may produce on closed loops. The characteristic frequency-drifts that type III bursts exhibit can be detected using a novel application of the time-lag technique developed by Viall & Klimchuk (2012) even when there are multiple overlapping bursts. We present a simple numerical model that simulates the expected radio emission from nanoflares in an active region (AR), which we use to test and calibrate the technique. We find that in the case of closed loops the frequency spectrum of type III bursts is expected to be extremely steep such that significant emission is produced at a given frequency only for a rather narrow range of loop lengths. We also find that the signature of bursts in the time-lag signal diminishes as: (1)the variety of participating loops within that range increases; (2)the occurrence rate of bursts increases; (3) the duration of bursts increases; and (4) the brightness of the bursts decreases relative to noise. In addition, our model suggests a possible origin of type I bursts as a natural consequence of type III emission in a closed-loop geometry.
--
Sherry Chhabra
Postdoctoral Research Fellow
Naval Research Laboratory
George Mason University
Mobile: +1(201)-719-3935
Email: schhabr(a)gmu.edu
chhabra.sherry(a)gmail.com
Dear All,
I am Abhishek Rajhans, a graduate student at IUCAA, working under supervision of Durgesh Tripathi (IUCAA) and Vinay Kashyap (CfA). We recently got our paper "Hydrodynamics of small transient brightenings in Solar Corona" accepted in the Astrophysical Journal. In this work we have used EBTEL code, which is based on 0-D description of coronal loops, to study transient brightenings detected by Hi-C and studied using AIA by Subramanian et al 2018. We see that these brightenings can be modelled as loops of ~ 1 Mm, with energy deposition of log[E(ergs)] ~ 23 in ~ 50 seconds. We have used two approaches : [1] in which the background + transient is used for constraining input parameters for simulations and [2] in which only transient has been used. We see that the latter method works better, in terms of agreement of synthetic and observed intensities in the six AIA filters (9.4, 13.1, 17.1, 19.3, 21.1, and 33.5 nm).
It is well known that impulsive events like large flares, microflares and nanoflares, show an initial conduction dominated cooling phase followed by enthalpy flux into corona, and subsequent radiation. Their hydrodynamics can be explained well by physics that goes into EBTEL. The fact that these small transient brightenings can also be explained by same physics is suggestive of a common underlying mechanism.
Here is the arxiv link of the paper
https://arxiv.org/abs/2105.08800
I hope you enjoy reading it. I would be delighted to get your valuable feedbacks, comments, questions and suggestions.
Regards,
Abhishek Rajhans
Hi Jim
Thanks for sharing this nice paper. The ideas you develop do seem very consistent with some of my earlier work, and indeed you do reference Hood et al (2009) (doi: 10.1051/0004-6361/200912285) - thank you! We showed here that even very smooth and simple motions in the photosphere can produce a very complex field in the corona with multiple fragmented current sheets. Whether one calls this "turbulence" or not is a matter of nomenclature, but it is certainly not classic Kolmogorov. Your statement "It nonetheless has a complex structure that bears no direct relationship to the pattern of driving" sums this up very nicely.
Prior to this, Browning and Van der Linden (2003) (doi 10.1051/0004-6361:20021887) proposed that a repeated series of energy release triggered by kink instability could produce a distribution of nanoflares - this was verified by Bareford et al (2010, 2011) (10.1051/0004-6361/201014067) where we show you can actually get a power law distribution from a random twistings. These were simple semi-analytical models, but I think there is scope to follow through on some of these ideas with simulations.
Of course the reality involves ongoing and more complex patterns of driving- and the possibility of avalanching, as we showed in Hood et al (2016) (10.3847/0004-637X/817/1/5) and pursued in the interesting recent work of Knizhnik et al of course. It seems one localised kink instability can generate a lot of current sheets and energy release!
Best wishes
Philippa
-----Original Message-----
From: Loops <loops-bounces(a)solar.physics.montana.edu> On Behalf Of loops-request(a)solar.physics.montana.edu
Sent: 18 May 2021 19:00
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Today's Topics:
1. Is the corona turbulent? (Klimchuk, James A. (GSFC-6710))
----------------------------------------------------------------------
Message: 1
Date: Tue, 18 May 2021 13:24:09 +0000
From: "Klimchuk, James A. (GSFC-6710)" <james.a.klimchuk(a)nasa.gov>
To: " (Loops(a)solar.physics.montana.edu)"
<Loops(a)solar.physics.montana.edu>
Subject: [Loops] Is the corona turbulent?
Message-ID:
<BY3PR09MB857890CB88D737802F4BB975B02C9(a)BY3PR09MB8578.namprd09.prod.outlook.com>
Content-Type: text/plain; charset="us-ascii"
Dear "loops" friends,
You might be interested in the paper that Spiro and I just published concerning the role of turbulence in forming the current sheets that spawn nanoflares in the magnetically closed corona:
http://journal.frontiersin.org/article/10.3389/fspas.2021.662861/full?&utm_…<https://gcc02.safelinks.protection.outlook.com/?url=http%3A%2F%2Flinks.emai…
g6QSXzWA2wXgA4DkQrYG8zw9-2BrhOoZY5D8wkA-3D-3D&data=04%7C01%7Cjames.a.klimchuk%40nasa.gov%7C5e1b386e3d2042eee27f08d919b402e2%7C7005d45845be48ae8140d43da96dd17b%7C0%7C0%7C637569083199946822%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C1000&sdata=hjrQ5rwifhz4O8XDZt6jIUTLTdxEi45UOl%2FQuFGWNOU%3D&reserved=0>
We argue that the sheets form primarily from other processes, unrelated to turbulence. Localized temporary bursts of turbulence may nonetheless be generated when the sheets reconnect.
Abstract:
We argue that the magnetically closed corona evolves primarily quasi-statically, punctuated by many localized bursts of activity associated with magnetic reconnection at a myriad of small current sheets. The sheets form by various processes that do not involve a traditional turbulent cascade whereby energy flows losslessly through a continuum of spatial scales starting from the large scale of the photospheric driving. If such an inertial range is a defining characteristic of turbulence, then the magnetically closed corona is not a turbulent system. It nonetheless has a complex structure that bears no direct relationship to the pattern of driving.
Cheers,
Jim
********************************************************************************
James A. Klimchuk
NASA Goddard Space Flight Center
Solar Physics Lab, Code 671
Bldg. 21, Rm. 158
Greenbelt, MD 20771
USA
Phone: 1-301-286-9060<tel:%28301%29%20286-9060>
Fax: 1-301-286-7194<tel:%28301%29%20286-7194>
E-mail: James.A.Klimchuk(a)nasa.gov<mailto:James.A.Klimchuk@nasa.gov>
Homepage: http://science.gsfc.nasa.gov/sed/bio/james.a.klimchuk<http://science.gsfc.nasa.gov/sed/index.cfm?fuseAction=people.jumpBio&&iPhon…>
No endorsement by NASA is implied for any correspondence related to my official role in professional organizations.
********************************************************************************
Dear "loops" friends,
You might be interested in the paper that Spiro and I just published concerning the role of turbulence in forming the current sheets that spawn nanoflares in the magnetically closed corona:
http://journal.frontiersin.org/article/10.3389/fspas.2021.662861/full?&utm_…<https://gcc02.safelinks.protection.outlook.com/?url=http%3A%2F%2Flinks.emai…>
We argue that the sheets form primarily from other processes, unrelated to turbulence. Localized temporary bursts of turbulence may nonetheless be generated when the sheets reconnect.
Abstract:
We argue that the magnetically closed corona evolves primarily quasi-statically, punctuated by many localized bursts of activity associated with magnetic reconnection at a myriad of small current sheets. The sheets form by various processes that do not involve a traditional turbulent cascade whereby energy flows losslessly through a continuum of spatial scales starting from the large scale of the photospheric driving. If such an inertial range is a defining characteristic of turbulence, then the magnetically closed corona is not a turbulent system. It nonetheless has a complex structure that bears no direct relationship to the pattern of driving.
Cheers,
Jim
********************************************************************************
James A. Klimchuk
NASA Goddard Space Flight Center
Solar Physics Lab, Code 671
Bldg. 21, Rm. 158
Greenbelt, MD 20771
USA
Phone: 1-301-286-9060<tel:%28301%29%20286-9060>
Fax: 1-301-286-7194<tel:%28301%29%20286-7194>
E-mail: James.A.Klimchuk(a)nasa.gov<mailto:James.A.Klimchuk@nasa.gov>
Homepage: http://science.gsfc.nasa.gov/sed/bio/james.a.klimchuk<http://science.gsfc.nasa.gov/sed/index.cfm?fuseAction=people.jumpBio&&iPhon…>
No endorsement by NASA is implied for any correspondence related to my official role in professional organizations.
********************************************************************************
Dear all,
Hope this mail finds you all in good health and spirits. I am Vishal
Upendran, graduate student working with Prof. Durgesh Tripathi at IUCAA,
Pune, India on coronal heating and solar wind emergence.
Our work on the impulsive heating of Quiet Corona was recently accepted for
publication in ApJ. Essentially, we study pixel-wise light curves of Quiet
Sun regions in the 171 Å, 193 Å and 211 Å from AIA by combining the
empirical, statistical impulsive heating forward model of Pauluhn & Solanki
(2007) with a machine-learning inversion model that allows uncertainty
quantification.
On performing inversions across approx. 300,000 light curves, we find that
there are approximately 2--3 impulsive events per min, with a lifetime of
about 10--20 min. The power law slope distribution peaks above 2 for all
passbands. We then explore correlations among the frequency of impulsive
events, their timescales and peak energy. The correlations suggest that
conduction losses dominate over radiative cooling losses, and there might
be a reservoir of energy either depleted by frequent, small events or
infrequent, large events.
I am excited to obtain feedback from the community on this work, and hope
it would be an enjoyable read to you.
The arxiv link: https://arxiv.org/abs/2103.16824.
Best regards,
Vishal
--
Vishal Upendran,
Senior Research Fellow, Inter-University Centre for Astronomy and
Astrophysics (IUCAA),
Pune, India - 411007