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

[Fabio Reale]

Thank you very much, Markus
      we are aware of your very interesting work on the analysis of 
STEREO data and your suggestion is very timely and appropriate. Sergio 
is busy with the analysis of Hinode/XRT data at the moment, but we hope 
to address STEREO data as next step.
Thank you again
Best regards
Fabio


On 02/16/2010 12:02 AM, Markus J. Aschwanden wrote:
> 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" 
> <http://www.lmsal.com/%7Easchwand/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/ <http://www.lmsal.com/%7Easchwand/>
> e-mail: aschwanden at lmsal.com <mailto:aschwanden at lmsal.com>
> _______________________________________
> ____________________________________
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