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
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