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



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Dr. Markus J. Aschwanden
Solar & Astrophysics Laboratory
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