Friends,
The EM loci technique is frequently used to access whether a plasma is isothermal. The attached paper addresses the uncertainty in this technique and proposes a way to determine the range of thermal distribution widths that are consistent with a set of emission line data.
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 Fax: 1-301-286-7194 E-mail: James.A.Klimchuk@nasa.govmailto:James.A.Klimchuk@nasa.gov Home page: http://science.gsfc.nasa.gov/671/staff/bios/cs/James_Klimchuk_ssi.html
No endorsement by NASA is implied for any correspondence related to my role as an officer of professional organizations (American Geophysical Union, American Astronomical Society, International Astronomical Union).
********************************************************************************
Friends,
I'm glad to see that the EM loci method is put on a more quantitative basis, especially in relationship to Gaussian DEM functions with variable temperature spread. While the two methods can now being used interchangably thanks to this recent nice study of Landi and Klimchuk, the biggest diagnostic problem for coronal loops is (in my view) still the proper background subtraction. Every contaminations from background fluxes introduces always a bias towards broader multi-thermal DEMs. This problem is even worse for AIA data, because more filters involve more confusion.
Looking forward to a patented background subtraction technique.
Cheers, Markus
On Sep 23, 2010, at 6:25 AM, Klimchuk, James A. (GSFC-6710) wrote:
Friends,
The EM loci technique is frequently used to access whether a plasma is isothermal. The attached paper addresses the uncertainty in this technique and proposes a way to determine the range of thermal distribution widths that are consistent with a set of emission line data.
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 Fax: 1-301-286-7194 E-mail: James.A.Klimchuk@nasa.gov Home page: http://science.gsfc.nasa.gov/671/staff/bios/cs/James_Klimchuk_ssi.html
No endorsement by NASA is implied for any correspondence related to my role as an officer of professional organizations (American Geophysical Union, American Astronomical Society, International Astronomical Union).
<em_loci.pdf>_______________________________________________ Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
____________________________________________ Dr. Markus J. Aschwanden Solar & Astrophysics Laboratory Lockheed Martin Advanced Techology Center Org. ADBS, Bldg. 252 3251 Hanover St., Palo Alto, CA 94304, USA Phone: 650-424-4001, FAX: 650-424-3994 URL: http://www.lmsal.com/~aschwand/ e-mail: aschwanden@lmsal.com _______________________________________ ____________________________________
Markus,
I thought that your papers on the early STEREO data indicated that background subtraction was reasonably well understood. You obtained similar intensities from the two different vantage points.
Also, you've stressed the importance of computing the cross-correlation of the emission at different temperatures. Considering only highly correlated data removes many spurious broad DEM results.
I think that the most significant problem is our bias toward bright loops that we can measure easily. They might not be representative.
Finally, a quick comment on the paper. I was expecting to see some Monte Carlo simulations and least-square fits to the resulting data. Isn't the real question how many lines and what level of uncertainty do we need to determine the emission measure.
Best wishes,
Harry
On 9/23/10 10:20 AM, "Markus J. Aschwanden" aschwanden@lmsal.com wrote:
Friends,
I'm glad to see that the EM loci method is put on a more quantitative basis, especially in relationship to Gaussian DEM functions with variable temperature spread. While the two methods can now being used interchangably thanks to this recent nice study of Landi and Klimchuk, the biggest diagnostic problem for coronal loops is (in my view) still the proper background subtraction. Every contaminations from background fluxes introduces always a bias towards broader multi-thermal DEMs. This problem is even worse for AIA data, because more filters involve more confusion.
Looking forward to a patented background subtraction technique.
Cheers, Markus
On Sep 23, 2010, at 8:43 AM, Harry Warren wrote:
Markus,
I thought that your papers on the early STEREO data indicated that background subtraction was reasonably well understood. You obtained similar intensities from the two different vantage points.
Dear Harry, Good point, with STEREO we have a double-check of the background, which gives us more confidence on background definition methods. However I find a high sensitivity of the DEM width on the background definition using 6 coronal AIA filters. With 3 filters from STEREO it is easier in a more restricted temperature range.
Also, you've stressed the importance of computing the cross-correlation of the emission at different temperatures. Considering only highly correlated data removes many spurious broad DEM results.
In principle, yes, but I there are more near-cospatial loops in 6 AIA filters than in 3 filter data from TRACE or STEREO.
I think that the most significant problem is our bias toward bright loops that we can measure easily. They might not be representative.
I fully agree with this bias. On the other side, bright loops have a higher probability of multi-strand structuring, so finding near-isothermal bright loops is even a stronger argument than near-isothermal week strands. Cheers, Markus
Finally, a quick comment on the paper. I was expecting to see some Monte Carlo simulations and least-square fits to the resulting data. Isn't the real question how many lines and what level of uncertainty do we need to determine the emission measure.
Best wishes,
Harry
On 9/23/10 10:20 AM, "Markus J. Aschwanden" aschwanden@lmsal.com wrote:
Friends,
I'm glad to see that the EM loci method is put on a more quantitative basis, especially in relationship to Gaussian DEM functions with variable temperature spread. While the two methods can now being used interchangably thanks to this recent nice study of Landi and Klimchuk, the biggest diagnostic problem for coronal loops is (in my view) still the proper background subtraction. Every contaminations from background fluxes introduces always a bias towards broader multi-thermal DEMs. This problem is even worse for AIA data, because more filters involve more confusion.
Looking forward to a patented background subtraction technique.
Cheers, Markus
-- // --------------------------------------------------------------------- // Harry P. Warren phone : 202-404-1453 // Naval Research Laboratory fax : 202-404-7997 // Code 7673HW email : hwarren@nrl.navy.mil // Washington, DC 20375 www : http://tcrb.nrl.navy.mil/~hwarren // ---------------------------------------------------------------------
Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
____________________________________________ Dr. Markus J. Aschwanden Solar & Astrophysics Laboratory Lockheed Martin Advanced Techology Center Org. ADBS, Bldg. 252 3251 Hanover St., Palo Alto, CA 94304, USA Phone: 650-424-4001, FAX: 650-424-3994 URL: http://www.lmsal.com/~aschwand/ e-mail: aschwanden@lmsal.com _______________________________________ ____________________________________
Can I point out a paper to the list:
http://adsabs.harvard.edu/abs/2010ApJ...708.1238J
This paper encapsulates the problem in a concise form. I could point out a couple more, but those are nearly 10 years old now, wow how time flies. I have drawn my line in the sand with emission measures as a practical diagnostic - once we have a DEM/EM, etc what can we do with it? As far as I can tell there are only a few papers in the literature that deal with that particular issue in a real sense and those are from further back in the 80s [look for papers by Carole Jordan].
I am all for learning something about the solar plasma from a zeroth order approach. However, no EM/DEM/Line Ratio estimation deals with the real time-varying corona (or outer solar atmosphere as we need to deal with a intrinsically coupled system). By definition, these diagnostic deal with equilibrium processes and thermal populations - I am not entirely sure that we have that in place and if we do is, is that the process doing the "heavy lifting" or just the after effect procing the apparent stationary state.....there is strong evidence of non-thermal processes affecting the chromospheric, TR and coronal plasmas in the very same spectra from which such diagnostics are derived [probably even worse for broadband EUV imagers].
Sorry, to be a bummer!
-S.
**---------------------------------------------------------------------------------------------** Dr. Scott W. McIntosh
National Center for Atmospheric Research Phone: 303 497 1544 (Room 3624) High Altitude Observatory Fax: 303 497 1589 3080 Center Green Drive - CG1 email: mscott@ucar.edu Boulder, CO 80301
http://people.hao.ucar.edu/mscott/
On Sep 23, 2010, at 7:25 AM, Klimchuk, James A. (GSFC-6710) wrote:
Friends,
The EM loci technique is frequently used to access whether a plasma is isothermal. The attached paper addresses the uncertainty in this technique and proposes a way to determine the range of thermal distribution widths that are consistent with a set of emission line data.
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 Fax: 1-301-286-7194 E-mail: James.A.Klimchuk@nasa.gov Home page: http://science.gsfc.nasa.gov/671/staff/bios/cs/James_Klimchuk_ssi.html
No endorsement by NASA is implied for any correspondence related to my role as an officer of professional organizations (American Geophysical Union, American Astronomical Society, International Astronomical Union).
<em_loci.pdf>_______________________________________________ Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
Scott,
I think that you're ultimately correct that the DEM is not the "final answer" to any questions in solar physics. If you think of what we're doing as an iterative process, however, it does provide a useful starting point.
For example, if I gave you the intensities from some hot loops in an active region:
Si VII 275.368 47.0 (erg/cm^2/s/sr) Ca XV 200.972 238.9
How else would start to think about them? With the DEM you quickly see that the amount of plasma at 4 MK appears to be much, much higher than it is at 0.6 MK, and that gets you thinking about physical models.
Of course at the end of the day, as you're saying, one needs to forward model the observations.
Best wishes,
Harry
On 9/23/10 8:53 PM, "Scott W. McIntosh" mscott@ucar.edu wrote:
Can I point out a paper to the list:
http://adsabs.harvard.edu/abs/2010ApJ...708.1238J
This paper encapsulates the problem in a concise form. I could point out a couple more, but those are nearly 10 years old now, wow how time flies. I have drawn my line in the sand with emission measures as a practical diagnostic - once we have a DEM/EM, etc what can we do with it? As far as I can tell there are only a few papers in the literature that deal with that particular issue in a real sense and those are from further back in the 80s [look for papers by Carole Jordan].
I am all for learning something about the solar plasma from a zeroth order approach. However, no EM/DEM/Line Ratio estimation deals with the real time-varying corona (or outer solar atmosphere as we need to deal with a intrinsically coupled system). By definition, these diagnostic deal with equilibrium processes and thermal populations - I am not entirely sure that we have that in place and if we do is, is that the process doing the "heavy lifting" or just the after effect procing the apparent stationary state.....there is strong evidence of non-thermal processes affecting the chromospheric, TR and coronal plasmas in the very same spectra from which such diagnostics are derived [probably even worse for broadband EUV imagers].
Sorry, to be a bummer!
-S.
Scott,
I fully agree with most of what you say. We need to be very cautious. The whole point of the paper (and Phil's) is to show that various uncertainties limit what you can infer about the thermal properties of an observed plasma. We propose a simple method to set approximate bounds on the range of valid interpretations. I disagree with what you seemed to say about not being able to do much with DEM/EM. We can learn a great deal about the physics of the corona and transition region if we know the thermal distribution of the plasma. The key question is whether we can determine DEM/EM reliably from observations. You make an excellent point that all of the "inversion" methods, including ours, assume ionization equilibrium. There are many situations where this is not valid (e.g., early in a nanoflare event). Expect to see more on this in the near future....
Thanks for your comments, Jim
From: loops-bounces@solar.physics.montana.edu [mailto:loops-bounces@solar.physics.montana.edu] On Behalf Of Scott W. McIntosh Sent: Thursday, September 23, 2010 8:54 PM To: A mailing list for scientists involved in the observation and modeling of solar loop structures Subject: Re: [Loops] EM loci
Can I point out a paper to the list:
http://adsabs.harvard.edu/abs/2010ApJ...708.1238J
This paper encapsulates the problem in a concise form. I could point out a couple more, but those are nearly 10 years old now, wow how time flies. I have drawn my line in the sand with emission measures as a practical diagnostic - once we have a DEM/EM, etc what can we do with it? As far as I can tell there are only a few papers in the literature that deal with that particular issue in a real sense and those are from further back in the 80s [look for papers by Carole Jordan].
I am all for learning something about the solar plasma from a zeroth order approach. However, no EM/DEM/Line Ratio estimation deals with the real time-varying corona (or outer solar atmosphere as we need to deal with a intrinsically coupled system). By definition, these diagnostic deal with equilibrium processes and thermal populations - I am not entirely sure that we have that in place and if we do is, is that the process doing the "heavy lifting" or just the after effect procing the apparent stationary state.....there is strong evidence of non-thermal processes affecting the chromospheric, TR and coronal plasmas in the very same spectra from which such diagnostics are derived [probably even worse for broadband EUV imagers].
Sorry, to be a bummer!
-S.
**---------------------------------------------------------------------------------------------** Dr. Scott W. McIntosh
National Center for Atmospheric Research Phone: 303 497 1544 (Room 3624) High Altitude Observatory Fax: 303 497 1589 3080 Center Green Drive - CG1 email: mscott@ucar.edumailto:mscott@ucar.edu Boulder, CO 80301
http://people.hao.ucar.edu/mscott/
On Sep 23, 2010, at 7:25 AM, Klimchuk, James A. (GSFC-6710) wrote:
Friends,
The EM loci technique is frequently used to access whether a plasma is isothermal. The attached paper addresses the uncertainty in this technique and proposes a way to determine the range of thermal distribution widths that are consistent with a set of emission line data.
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 Fax: 1-301-286-7194 E-mail: James.A.Klimchuk@nasa.govmailto:James.A.Klimchuk@nasa.gov Home page: http://science.gsfc.nasa.gov/671/staff/bios/cs/James_Klimchuk_ssi.html
No endorsement by NASA is implied for any correspondence related to my role as an officer of professional organizations (American Geophysical Union, American Astronomical Society, International Astronomical Union).
********************************************************************************
<em_loci.pdf>_______________________________________________ Loops mailing list Loops@solar.physics.montana.edumailto:Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
Dear Jim et al.,
there seems to be a bit of confusion and some elephants in the room, so I list a few clarifications below.
- I believe it was my 'fault' to re-introduce in 2003 the old EM loci method to study AR loops for the first time, and to show that *most* 'warm' loops are nearly isothermal at each location. I have to agree that, despite the limitations of the method, this is an useful SOHO/CDS result, that so far has been confirmed by Hinode/EIS (in general, iso-thermality appears to be the norm in AR structures, rather than multi-thermality).
- I did not and would not 'trust' too much any EM method, however forward modelling the line intensities in the loops (see Del Zanna & Mason 2003) gave consistent results (though still static).
- Of course, it has been known for a very long time that any dynamics in the loops will change the shape of any DEM/EM distributions, but by not too much I believe. I've shown in 2007 how clear patterns of dopplershifts are present in coronal loops from Hinode/EIS, so we now have some extra observational constraints.
- One elehpant in the room is the problem that large (up to a factor of 10) discrepancies are present in any EM modelling, when the lines from the 'anomalous' ions are considered. This was obvious even in the Pottasch (1963) results but went un-noticed until 1971 and has since been considered by very few people (Phil Judge is one of them). There are a lot of erroneous statements in the literature by the way (interested readers should look at some of my papers).
- Even when considering lines from ions 'well behaved', the largest variations in results are coming from the use of different ion abundances. The latest ionization and recombination rates we have in CHIANTI are giving results for some ions that are quite different, for example.
- The EM loci plots were first introduced by K. Strong in 1978 I believe, and not by Pottasch who did something very different. The method is often not explained (I am afraid to say understood) properly.
- As any EM method, the EM loci will not give you an unique solution.
- To measure the electron temperature, there are better methods. In the last couple of years I showed that some diagnostics (athough very limited) for coronal loops are available from Fe VII, Fe VIII, Fe IX and Fe XI lines observed by Hinode/EIS. It is a start. When proposing for new instruments I always suggest that some temperature diagnostics should be included..
- Another elphant in the room is the unresolved AR emission. It was ignored until the first loop meeting when I and Peter Young showed how dominant it is. I am glad to see that it is now a comonly-accepted issue. All EUV imagers are inherently multi-thermal, so it is more difficult to use them to measure the unresolved AR emission, compared to a spectrometer. Ideally, we would need two spectrometers with high spatial resolution and very different viewing angles.
Giulio --
Thanks, Giulio. You make many excellent points. I have two quick comments. First, the DEM distributions for dynamic and static loops can be vastly different (theoretically). Second, I agree completely that we need to pay more attention to the diffuse emission, which is much stronger than the emission contained in discrete loop features. I hope people realize that the Loops Workshops are not restricted to discrete loops. Every closed magnetic flux tube can be thought of as a loop, even those that make up the diffuse corona.
Thanks again, Jim
-----Original Message----- From: loops-bounces@solar.physics.montana.edu [mailto:loops- bounces@solar.physics.montana.edu] On Behalf Of Giulio Del Zanna Sent: Tuesday, September 28, 2010 5:10 AM To: A mailing list for scientists involved in the observation and modeling of solar loop structures Subject: Re: [Loops] EM loci
Dear Jim et al.,
there seems to be a bit of confusion and some elephants in the room, so I list a few clarifications below.
I believe it was my 'fault' to re-introduce in 2003 the old EM loci method to study AR loops for the first time, and to show that *most* 'warm' loops are nearly isothermal at each location. I have to agree that, despite the limitations of the method, this is an useful SOHO/CDS result, that so far has been confirmed by Hinode/EIS (in general, iso-thermality appears to be the norm in AR structures, rather than multi-thermality).
I did not and would not 'trust' too much any EM method, however forward modelling the line intensities in the loops (see Del Zanna & Mason 2003) gave consistent results (though still static).
Of course, it has been known for a very long time that any dynamics in the loops will change the shape of any DEM/EM distributions, but by not too
much I believe. I've shown in 2007 how clear patterns of dopplershifts are present in coronal loops from Hinode/EIS, so we now have some extra observational constraints.
- One elehpant in the room is the problem that large (up to a factor of 10) discrepancies are present in any EM modelling, when the lines from the 'anomalous' ions are considered. This was obvious even in the Pottasch
(1963) results but went un-noticed until 1971 and has since been considered by very few people (Phil Judge is one of them). There are a lot of erroneous statements in the literature by the way (interested readers should look at some of my papers).
Even when considering lines from ions 'well behaved', the largest variations in results are coming from the use of different ion abundances. The latest ionization and recombination rates we have in CHIANTI are giving results for some ions that are quite different, for example.
The EM loci plots were first introduced by K. Strong in 1978 I believe, and not by Pottasch who did something very different. The method is often not
explained (I am afraid to say understood) properly.
As any EM method, the EM loci will not give you an unique solution.
To measure the electron temperature, there are better methods. In the last couple of years I showed that some diagnostics (athough very
limited) for coronal loops are available from Fe VII, Fe VIII, Fe IX and Fe XI lines observed by Hinode/EIS. It is a start. When proposing for new instruments I always suggest that some temperature diagnostics should be included..
- Another elphant in the room is the unresolved AR emission. It was ignored until the first loop meeting when I and Peter Young showed how dominant it
is. I am glad to see that it is now a comonly-accepted issue. All EUV imagers are inherently multi-thermal, so it is more difficult to use them to measure the unresolved AR emission, compared to a spectrometer. Ideally, we would need two spectrometers with high spatial resolution and very different viewing angles.
Giulio
Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
Thanks Gulio!
That is very useful information.
There are a lot of erroneous statements in the literature by the way (interested readers should look at some of my papers).
You mean, look there for the "erroneous statements"? :) I suppose not.
- Even when considering lines from ions 'well behaved', the largest
variations in results are coming from the use of different ion abundances.
Is there any clever way to deal with that? I've never given it much thought, but one cannot keep ignoring this...
Cheers,
Piet
Giulio,
Careful! Before you claim to have re-introduced the EM Loci method, here are just a couple counter-examples:
http://adsabs.harvard.edu/abs/1994ApJ...429..909W
and
http://adsabs.harvard.edu/abs/1996ApJ...473..519S
We've been using it for a long time. I think some good recent examples are
http://adsabs.harvard.edu/abs/2008ApJ...686L.131W
and
http://adsabs.harvard.edu/abs/2009ApJ...704..863S
Regards, Joan
-----Original Message----- From: loops-bounces@solar.physics.montana.edu [mailto:loops-bounces@solar.physics.montana.edu] On Behalf Of Giulio Del Zanna Sent: Tuesday, September 28, 2010 4:10 AM To: A mailing list for scientists involved in the observation and modeling of solar loop structures Subject: Re: [Loops] EM loci
Dear Jim et al.,
there seems to be a bit of confusion and some elephants in the room, so I list a few clarifications below.
- I believe it was my 'fault' to re-introduce in 2003 the old EM loci method to study AR loops for the first time, and to show that *most* 'warm' loops are nearly isothermal at each location. I have to agree that, despite the limitations of the method, this is an useful SOHO/CDS result, that so far has been confirmed by Hinode/EIS (in general, iso-thermality appears to be the norm in AR structures, rather than multi-thermality).
- I did not and would not 'trust' too much any EM method, however forward modelling the line intensities in the loops (see Del Zanna & Mason 2003) gave consistent results (though still static).
- Of course, it has been known for a very long time that any dynamics in the loops will change the shape of any DEM/EM distributions, but by not too much I believe. I've shown in 2007 how clear patterns of dopplershifts are present in coronal loops from Hinode/EIS, so we now have some extra observational constraints.
- One elehpant in the room is the problem that large (up to a factor of 10) discrepancies are present in any EM modelling, when the lines from the 'anomalous' ions are considered. This was obvious even in the Pottasch (1963) results but went un-noticed until 1971 and has since been considered by very few people (Phil Judge is one of them). There are a lot of erroneous statements in the literature by the way (interested readers should look at some of my papers).
- Even when considering lines from ions 'well behaved', the largest variations in results are coming from the use of different ion abundances. The latest ionization and recombination rates we have in CHIANTI are giving results for some ions that are quite different, for example.
- The EM loci plots were first introduced by K. Strong in 1978 I believe, and not by Pottasch who did something very different. The method is often not explained (I am afraid to say understood) properly.
- As any EM method, the EM loci will not give you an unique solution.
- To measure the electron temperature, there are better methods. In the last couple of years I showed that some diagnostics (athough very limited) for coronal loops are available from Fe VII, Fe VIII, Fe IX and Fe XI lines observed by Hinode/EIS. It is a start. When proposing for new instruments I always suggest that some temperature diagnostics should be included..
- Another elphant in the room is the unresolved AR emission. It was ignored until the first loop meeting when I and Peter Young showed how dominant it is. I am glad to see that it is now a comonly-accepted issue. All EUV imagers are inherently multi-thermal, so it is more difficult to use them to measure the unresolved AR emission, compared to a spectrometer. Ideally, we would need two spectrometers with high spatial resolution and very different viewing angles.
Giulio
A quick reply to Piet and Joan.
Piet: Within CHIANTI, we keep a few tabulated ion abundances, so if you plot the I/G(T) for a few ions and different tables, you can see how ion abundances have changed over the years. That does not mean it gives you an indication of an uncertainty. The last ones should be accurate, however if you deal with high-Ne (middle, low-TR) you also have other effects.
Joan: I was careful in what I wrote. The EM Loci method has been used a lot, but I had not seen a paper using it to look at the warm (about 1 MK) loops. I might have missed it of course (see reply below). I was aware of the SMM FCS results but they concerned the hot core loops. The spatial resolution was very limited and the core loops are so 'unresolved' that I do not think that anyone could actually resolve a loop. With CDS and EIS at least some 'fat' warm loops are resolvable towards their footpoints against the unresolved background/foreground emission.
Giulio
*************************************************************
On Tue, 28 Sep 2010, Joan T Schmelz (jschmelz) wrote:
Giulio,
Careful! Before you claim to have re-introduced the EM Loci method, here are just a couple counter-examples:
The above I think just shows what was known from e.g. Rosner et al. 1978, i.e. that the hot core loops are 'nearly isothermal' at 3-4 MK.
Had similar results in terms of temperature of the hot loops, again from the excellent SMM/FCS spectra of active regions.
We've been using it for a long time. I think some good recent examples are
When I wrote that my CDS results so far have been confirmed by Hinode/EIS I had in mind this nice paper from Harry et al. and that one from Tripathi et al.
and
This again is on the hot loops.
Regards, Joan
Dear all,
we have found and propose a number of new features related to the evolution of the hot loops, the coronal blue-shifts and the radio noise storms which may be of interest to you.
Most of the work was done almost two years ago, but we only presented one poster at COSPAR, and I will bring one at Hinode-4. The relevant A&A paper is linked through:
http://solar.physics.montana.edu/cgi-bin/eprint/index.pl?entry=14382
Hopefully, we might be able to test our ideas with better observations and modelling.
best wishes,
Giulio, Guillaume, Ludwig and Tibor.
Dear Loop Lovers,
A new paper on XRT-EIS loop analysis has been accepted for publication in ApJ. I've attached a copy. Comments welcome!
Regards, Joan
Hello Scott et al.,
I have drawn my line in the sand with emission measures as a practical diagnostic - once we have a DEM/EM, etc what can we do with it?
Well, I respectfully disagree. Once you have reliable DEMs for every pixel -- as AIA hopefully will deliver some time -- and you have dealt with the (complicated) background subtraction issues, you have completely characterized the hydrostatics for the loop plasma. (I am thinking about studying coronal loops here. Now that leaves out the dynamics, I agree on that.)
However, one can do all sorts of interesting things with DEMs. For example, Markus and Nariaki had a very insightful paper several years ago that demonstrated that a loop consisting of multiple strands, each isothermal along the axis, could still appear as a loop with a T increase along its axis, because of the differences in scale heights. I am sure most of you remember. However, that result in itself does not demonstrate that that is what is the case inside real loops. In order to decide that one needs to have DEMs at several locations along the loop. If it is so that the DEMs are broad enough, and the hot part of the DEM does not change much along the loop, but the cool edge decreases as one go up, that would be consistent with the Ansatz of Markus and Nariaki. But if there is hot material near the loop top that is not present in the DEMs along the legs, the assumption breaks down. Very simple to decide once you have the DEMs.
More generally, it can be demonstrated that different heating mechanisms result in different DEMs along the loop, even if the temperature structure does not change that much between one heating mechanism and another. So DEMs are a diagnostic, perhaps the best one, for the coronal heating mechanism.
As for line or passband ratios and EM-loci, these methods can easily lead to completely incorrect results and should be avoided in my opinion. Enrico and Jim just demonstrated very convincingly that even if a pixel were perfectly isothermal, the EM-loci method probably still would not bear that out because of the uncertainties in all the parameters involved. But the only point of the EM-loci method -- as well as the line/passband ratio method -- is to determine a single temperature!
Well, with a little bit of luck I let my opinion be known without insulting anyone this time.
Cheers,
Piet
As far as I can tell there are only a few papers in the literature
that deal with that particular issue in a real sense and those are from further back in the 80s [look for papers by Carole Jordan].
I am all for learning something about the solar plasma from a zeroth order approach. However, no EM/DEM/Line Ratio estimation deals with the real time-varying corona (or outer solar atmosphere as we need to deal with a intrinsically coupled system). By definition, these diagnostic deal with equilibrium processes and thermal populations - I am not entirely sure that we have that in place and if we do is, is that the process doing the "heavy lifting" or just the after effect procing the apparent stationary state.....there is strong evidence of non-thermal processes affecting the chromospheric, TR and coronal plasmas in the very same spectra from which such diagnostics are derived [probably even worse for broadband EUV imagers].
Sorry, to be a bummer!
-S.
**---------------------------------------------------------------------------------------------** Dr. Scott W. McIntosh
National Center for Atmospheric Research Phone: 303 497 1544 (Room 3624) High Altitude Observatory Fax: 303 497 1589 3080 Center Green Drive - CG1 email: mscott@ucar.edu mailto:mscott@ucar.edu Boulder, CO 80301 http://people.hao.ucar.edu/mscott/
On Sep 23, 2010, at 7:25 AM, Klimchuk, James A. (GSFC-6710) wrote:
Friends, The EM loci technique is frequently used to access whether a plasma is isothermal. The attached paper addresses the uncertainty in this technique and proposes a way to determine the range of thermal distribution widths that are consistent with a set of emission line data. 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 Fax: 1-301-286-7194 E-mail: James.A.Klimchuk@nasa.gov mailto:James.A.Klimchuk@nasa.gov Home page: http://science.gsfc.nasa.gov/671/staff/bios/cs/James_Klimchuk_ssi.html No endorsement by NASA is implied for any correspondence related to my role as an officer of professional organizations (American Geophysical Union, American Astronomical Society, International Astronomical Union).
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