[Loops] On the importance of background subtraction in the analysis of coronal loops observed with TRACE

[Markus J. Aschwanden]

Sergio and Fabio,

I like to encourage you to use STEREO EUVI data, which would provide
you the ultimate test to subtract the correct background, because you
see a loop from two different directions and have two independent
backgrounds. You can test the self-consistence of background subtraction,
if you obtain the same EM of the loop from two spacecraft. 
If you look into my recent STEREO paper, you see that one can
subtract the background with an accuracy of about 10%.
See Fig.8 therein.

Cheers,
Markus

 Aschwanden,M.J., Nitta,N.V., Wuelser,J.P., and Lemen,J.R. 2008, The Astrophysical Journal, 680, 1477-1495
URL1="http://www.lmsal.com/~aschwand/eprints/2008_stereo2.pdf" 
First 3D reconstructions of coronal loops with the STEREO A and B spacecraft: II. Electron Density and Temperature Measurements



On Feb 11, 2010, at 3:02 AM, Fabio Reale wrote:

> Dear friends
>    let me inform you about the following preprint on Astro-PH, 
> 
> http://arxiv.org/abs/1002.2121
> 
> regarding:
> 
> the importance of background subtraction in the analysis of coronal loops observed with TRACE
> 
> 
> by Sergio Terzo and Fabio Reale, accepted for publication on A&A
> 
> Abstract
> In the framework of TRACE coronal observations, we compare the analysis and diagnostics of a loop after subtracting the background with two different and independent methods. The dataset includes sequences of images in the 171 A, 195 A filter bands of TRACE. One background subtraction method consists in taking as background values those obtained from interpolation between concentric strips around the analyzed loop. The other method is a pixel-to-pixel subtraction of the final image when the loop had completely faded out, already used by Reale & Ciaravella 2006. We compare the emission distributions along the loop obtained with the two methods and find that they are considerably different. We find differences as well in the related derive filter ratio and temperature profiles. In particular, the pixel-to-pixel subtraction leads to coherent diagnostics of a cooling loop. With the other subtraction the diagnostics are much less clear. The background subtraction is a delicate issue in the analysis of a loop. The pixel-to-pixel subtraction appears to be more reliable, but its application is not always possible. Subtraction from interpolation between surrounding regions can produce higher systematic errors, because of intersecting structures and of the large amount of subtracted emission in TRACE observations.
> 
> 
> 
> <reale.vcf>_______________________________________________
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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
_______________________________________
____________________________________



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