[Loops] EM loci

[Petrus Martens]

Hello Scott et al.,

 > I have drawn my line in the sand with emission measures as a
> practical diagnostic - once we have a DEM/EM, etc what can we do with
> it?

Well, I respectfully disagree.  Once you have reliable DEMs for every
pixel -- as AIA hopefully will deliver some time -- and you have dealt
with the (complicated) background subtraction issues, you have
completely characterized the hydrostatics for the loop plasma.  (I am
thinking about studying coronal loops here.  Now that leaves out the
dynamics, I agree on that.)

However, one can do all sorts of interesting things with DEMs.  For
example, Markus and Nariaki had a very insightful paper several years
ago that demonstrated that a loop consisting of multiple strands, each
isothermal along the axis, could still appear as a loop with a T
increase along its axis, because of the differences in scale heights.
I am sure most of you remember.  However, that result in itself does
not demonstrate that that is what is the case inside real loops.  In
order to decide that one needs to have DEMs at several locations along
the loop.  If it is so that the DEMs are broad enough, and the hot
part of the DEM does not change much along the loop, but the cool edge
decreases as one go up, that would be consistent with the Ansatz of
Markus and Nariaki.  But if there is hot material near the loop top
that is not present in the DEMs along the legs, the assumption breaks
down.  Very simple to decide once you have the DEMs.

More generally, it can be demonstrated that different heating mechanisms
result in different DEMs along the loop, even if the temperature
structure does not change that much between one heating mechanism and
another.  So DEMs are a diagnostic, perhaps the best one, for the
coronal heating mechanism.

As for line or passband ratios and EM-loci, these methods can easily
lead to completely incorrect results and should be avoided in my
opinion.  Enrico and Jim just demonstrated very convincingly that even
if a pixel were perfectly isothermal, the EM-loci method probably still
would not bear that out because of the uncertainties in all the
parameters involved.  But the only point of the EM-loci method -- as
well as the line/passband ratio method -- is to determine a single
temperature!

Well, with a little bit of luck I let my opinion be known without
insulting anyone this time.

Cheers,

Piet




As far as I can tell there are only a few papers in the literature
> that deal with that particular issue in a real sense and those are from
> further back in the 80s [look for papers by Carole Jordan].
>
> I am all for learning something about the solar plasma from a zeroth
> order approach. However, no EM/DEM/Line Ratio estimation deals with the
> real time-varying corona (or outer solar atmosphere as we need to deal
> with a intrinsically coupled system). By definition, these diagnostic
> deal with equilibrium processes and thermal populations - I am not
> entirely sure that we have that in place and if we do is, is that the
> process doing the "heavy lifting" or just the after effect procing the
> apparent stationary state.....there is strong evidence of non-thermal
> processes affecting the chromospheric, TR and coronal plasmas in the
> very same spectra from which such diagnostics are derived [probably even
> worse for broadband EUV imagers].
>
> Sorry, to be a bummer!
>
> -S.
>
> **---------------------------------------------------------------------------------------------**
> Dr. Scott W. McIntosh
>
> National Center for Atmospheric Research Phone: 303 497 1544
> (Room 3624) High Altitude Observatory Fax: 303 497 1589
> 3080 Center Green Drive - CG1 email: mscott at ucar.edu
> <mailto:mscott at ucar.edu>
> Boulder, CO 80301
> http://people.hao.ucar.edu/mscott/
>
>
>
>
>
>
>
> On Sep 23, 2010, at 7:25 AM, Klimchuk, James A. (GSFC-6710) wrote:
>
>> Friends,
>> The EM loci technique is frequently used to access whether a plasma is
>> isothermal. The attached paper addresses the uncertainty in this
>> technique and proposes a way to determine the range of thermal
>> distribution widths that are consistent with a set of emission line data.
>> Cheers,
>> Jim
>> ********************************************************************************
>> James A. Klimchuk
>> NASA Goddard Space Flight Center
>> Solar Physics Lab, Code 671
>> Bldg. 21, Rm. 158
>> Greenbelt, MD 20771
>> USA
>> Phone: 1-301-286-9060
>> Fax: 1-301-286-7194
>> E-mail: James.A.Klimchuk at nasa.gov <mailto:James.A.Klimchuk at nasa.gov>
>> Home page:
>> http://science.gsfc.nasa.gov/671/staff/bios/cs/James_Klimchuk_ssi.html
>> No endorsement by NASA is implied for any correspondence related to my
>> role as an officer of professional organizations (American Geophysical
>> Union, American Astronomical Society, International Astronomical Union).
>> ********************************************************************************
>> <em_loci.pdf>_______________________________________________
>> Loops mailing list
>> Loops at solar.physics.montana.edu <mailto:Loops at solar.physics.montana.edu>
>> https://mithra.physics.montana.edu/mailman/listinfo/loops
>
>
>
> _______________________________________________
> Loops mailing list
> Loops at solar.physics.montana.edu
> https://mithra.physics.montana.edu/mailman/listinfo/loops