[Loops] Comments and questions about new AIA Loop Paper

Tue Nov 23 12:10:54 MST 2010

Dear Markus,

I was expecting your reply. Here are answers to your questions:

(1) What motivated your selection of this particular loop? Judging from your
Figure 2 is appears that you analyzed about the dimmest segment of a loop,
where one would expect substantial confusion by the background due to the
low signal-to-noise ratio of the loop.

>>Actually there are plenty of counts. The signal-to-noise was just fine, and there was no confusion with background. As we say in the Letter, we were looking for a loop segment in 171 with a simple background. This one fit the bill nicely. It was not the only loop we selected. We are working on a larger paper using more loops from different active regions. Please stay tuned.

(2) What are the total fluxes F_tot and background fluxes F_bg that you
used in your DEM modeling of the loop fluxes F_loop=F_tot - F_bg
in the 6 coronal AIA filters? If the reader knows these 12 numbers,
one could verify or reproduce your DEM modeling.

>>We’ve already used two quite different DEM methods on these data and gotten very similar results, and personally, I do not want a repeat of what happened after your re-analysis of our CDS loop data. (If anyone is reading this and does not know what I’m talking about, that’s probably a good thing.) I think what might be of greater interest is an entirely independent analysis of your own.

Also, you don’t just need 12 numbers; you need the uncertainties as well. These are extremely important, especially for loops that are not visible in all the filters. We always want high- and low-temperature constraints, but we also need to know how strong those constraints are.

(3) What are the cross-sectional flux profiles F_tot(x), F_bg(x), F_loop(x)
you inferred at the analyzed loop segments. If you show these profiles to the
reader, one could inquire or judge whether the same loop or multiple loops
are seen at the analyzed location of the loop, and what the background
contamination in the loop flux could be.

>>Same as (2) above.

(4) How did you deal with the 94 A response function in the DEM modeling.
After some discussion with Jim Lemen and Harry Warren last week I learned
that the response of the cooler line misses a lot of contributions from Fe X transitions
and thus the observed flux at log(T)~6.0-6.1 is substantially higher than predicted
by the current online response function. I am currently try to come up with an
empirical reponse function of the 94 A filter and find that the response below
log(T)<6.2 needs to be boosted by a factor of approximately 5-10 to be consistent
with the other filters. I will let you know once I have more accurate estimates.

>>Excellent! I am glad others are now spreading the word on this problem. Nancy Brickhouse told me about these cool lines (see Lepson et al. 2002) several years ago and we alerted the CfA AIA team. An explanation and Nancy’s scaling were part of my Cospar AIA DEM talk. BTW, it's not just Fe X but also Fe IX and quite possibly some other cooler lines (according to what Peter Beiersdorfer told Nancy a few weeks ago). Fortunately, the LLNL experimental group is now funded to do further studies, but we'd of course be interested in your empirical calibration for comparison.

(5) Just as a minor comment: You are correct that some early papers used
filter ratios, like the one you quoted from (1991), but most later studies used
forward-fitting of DEMs, rather than filter ratios, so it is a kind of beating a
dead horse to emphasize that filter-ratio methods are inadequate if one
wants to investigate the multi-thermal temperature structure.

>>We still hear that horse breathing every so often . . . but no one would be happier than I if it were officially declared dead and buried.

(6) You quoted a pixel size of 0.5" for AIA, but it is actually 0.6",
while it was 0.5" for TRACE. The actual spatial resolution is about
2.0-2.5 pixels (Boerner). There is also a draft now on the initial
calibration of AIA by Boerner et al.

>>We are all looking forward to the publication of the AIA instrument papers. Are they publically available yet?

Regards,
Joan

From: loops-bounces at solar.physics.montana.edu [mailto:loops-bounces at solar.physics.montana.edu] On Behalf Of Markus J. Aschwanden
Sent: Monday, November 22, 2010 11:01 AM
To: A mailing list for scientists involved in the observation and modeling of solar loop structures
Subject: [Loops] Comments and questions about new AIA Loop Paper

Dear Joan,

You tackled loop modeling with AIA data, an ideal dataset on which we place
high hopes to make progress in the understanding of their thermal structure.
I have developed a DEM forward-fitting code to AIA data over that last weeks
and would very much like to test it on the same data set you analyzed. So, I have
a few questions (and also comments), after I read your paper:

(1) What motivated your selection of this particular loop? Judging from your
Figure 2 is appears that you analyzed about the dimmest segment of a loop,
where one would expect substantial confusion by the background due to the
low signal-to-noise ratio of the loop.

(2) What are the total fluxes F_tot and background fluxes F_bg that you
used in your DEM modeling of the loop fluxes F_loop=F_tot - F_bg
in the 6 coronal AIA filters? If the reader knows these 12 numbers,
one could verify or reproduce your DEM modeling.

(3) What are the cross-sectional flux profiles F_tot(x), F_bg(x), F_loop(x)
you inferred at the analyzed loop segments. If you show these profiles to the
reader, one could inquire or judge whether the same loop or multiple loops
are seen at the analyzed location of the loop, and what the background
contamination in the loop flux could be.

(4) How did you deal with the 94 A response function in the DEM modeling.
After some discussion with Jim Lemen and Harry Warren last week I learned
that the response of the cooler line misses a lot of contributions from Fe X transitions
and thus the observed flux at log(T)~6.0-6.1 is substantially higher than predicted
by the current online response function. I am currently try to come up with an
empirical reponse function of the 94 A filter and find that the response below
log(T)<6.2 needs to be boosted by a factor of approximately 5-10 to be consistent
with the other filters. I will let you know once I have more accurate estimates.

(5) Just as a minor comment: You are correct that some early papers used
filter ratios, like the one you quoted from (1991), but most later studies used
forward-fitting of DEMs, rather than filter ratios, so it is a kind of beating a
dead horse to emphasize that filter-ratio methods are inadequate if one
wants to investigate the multi-thermal temperature structure.

(6) You quoted a pixel size of 0.5" for AIA, but it is actually 0.6",
while it was 0.5" for TRACE. The actual spatial resolution is about
2.0-2.5 pixels (Boerner). There is also a draft now on the initial
calibration of AIA by Boerner et al.

Looking forward to your complimentary information of your analysis.
It would be useful to show the parameters of item (2) and (3) in every
loop modeling paper. Thanks!

Cheers,
Markus

On Nov 17, 2010, at 1:56 PM, Joan T Schmelz (jschmelz) wrote:

Dear Loop Lovers,

The attached paper on AIA multithermal loop analysis has just been published in ApJ Letters. Comments welcome!

ATMOSPHERIC IMAGING ASSEMBLY MULTITHERMAL LOOP ANALYSIS: FIRST RESULTS
by
J. T. Schmelz, J. A. Kimble, B. S. Jenkins, B. T. Worley, D. J. Anderson, S. Pathak, and S. H. Saar

ABSTRACT
The Atmospheric Imaging Assembly (AIA) on board the Solar Dynamics Observatory has state-of-the-art spatial
resolution and shows the most detailed images of coronal loops ever observed. The series of coronal filters
peak at different temperatures, which span the range of active regions. These features represent a significant
improvement over earlier coronal imagers and make AIA ideal for multithermal analysis. Here, we targeted
a 171 Å coronal loop in AR 11092 observed by AIA on 2010 August 3. Isothermal analysis using the
171-to-193 ratio gave a temperature of log T ≈ 6.1, similar to the results of Extreme ultraviolet Imaging
Spectrograph (EIT) and TRACE. Differential emission measure analysis, however, showed that the plasma was
multithermal, not isothermal, with the bulk of the emission measure at log T > 6.1. The result from the isothermal
analysis, which is the average of the true plasma distribution weighted by the instrument response functions,
appears to be deceptively low. These results have potentially serious implications: EIT and TRACE results,
which use the same isothermal method, show substantially smaller temperature gradients than predicted by
standard models for loops in hydrodynamic equilibrium and have been used as strong evidence in support of
footpoint heating models. These implications may have to be re-examined in the wake of new results from AIA.

Regards,
Joan
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____________________________________________
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 at lmsal.com<mailto:aschwanden at lmsal.com>
_______________________________________
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