[Loops] EM loci

[Klimchuk, James A. (GSFC-6710)]

Thanks, Giulio.  You make many excellent points.  I have two quick comments.   First, the DEM distributions for dynamic and static loops can be vastly different (theoretically).  Second, I agree completely that we need to pay more attention to the diffuse emission, which is much stronger than the emission contained in discrete loop features.  I hope people realize that the Loops Workshops are not restricted to discrete loops.  Every closed magnetic flux tube can be thought of as a loop, even those that make up the diffuse corona.

Thanks again, 
Jim

> -----Original Message-----
> From: loops-bounces at solar.physics.montana.edu [mailto:loops-
> bounces at solar.physics.montana.edu] On Behalf Of Giulio Del Zanna
> Sent: Tuesday, September 28, 2010 5:10 AM
> To: A mailing list for scientists involved in the observation and modeling of
> solar loop structures
> Subject: Re: [Loops] EM loci
> 
> 
> Dear Jim et al.,
> 
> there seems to be a bit of confusion and some elephants in the room,
> so I list a few clarifications below.
> 
> - I believe it was my 'fault' to re-introduce in 2003 the old EM loci method to
>    study AR loops for the first time, and to show that *most* 'warm' loops
>    are nearly isothermal at each location.
>    I have to agree that, despite the limitations of the method, this is an useful
>    SOHO/CDS result, that so far has been confirmed by Hinode/EIS (in general,
>    iso-thermality appears to be the norm in AR structures, rather than
>    multi-thermality).
> 
> - I did not and would not 'trust' too much any EM method, however forward
>    modelling the line intensities in the loops (see Del Zanna & Mason 2003)
>    gave consistent results (though still static).
> 
> - Of course, it has been known for a very long time that any dynamics in the
>   loops will change the shape of any DEM/EM distributions, but by not too
> much I believe.
>    I've shown in 2007 how clear patterns of dopplershifts are present in
> coronal
>   loops from  Hinode/EIS, so we now have some extra observational
> constraints.
> 
> - One elehpant in the room is the problem that large (up to a factor of 10)
>    discrepancies are present in any  EM modelling, when the lines from the
>    'anomalous' ions are considered. This was obvious even in the Pottasch
> (1963)
>   results but went un-noticed until 1971 and has since been considered by
> very
>   few people (Phil Judge is one of them). There are a lot of erroneous
> statements
>   in the literature by the way (interested readers should look at some of my
>   papers).
> 
> - Even when considering lines from ions 'well behaved', the largest variations
>    in results are coming from the use of different ion abundances.
>    The latest ionization and recombination rates we have in CHIANTI are giving
>    results for some ions that are quite different, for example.
> 
> - The EM loci plots were first introduced by K. Strong in 1978 I believe, and
>    not by Pottasch who did something very different. The method is often not
> explained (I am afraid to say understood) properly.
> 
> - As any EM method, the EM loci will not give you an unique solution.
> 
> - To measure the electron temperature, there are better methods.
>    In the last couple of years I showed that some diagnostics (athough very
> limited)
>    for coronal loops are available from Fe VII, Fe VIII, Fe IX and Fe XI lines
>    observed by Hinode/EIS. It is a start.
>    When proposing for new instruments I always suggest that some
> temperature
> diagnostics  should be included..
> 
> - Another elphant in the room is the unresolved AR emission. It was ignored
>   until the first loop meeting when I and Peter Young showed how dominant it
> is.
>   I am glad to see that it is now a comonly-accepted issue.
>   All EUV imagers are inherently multi-thermal, so it is more
>   difficult to use them to measure the  unresolved AR emission, compared to a
>   spectrometer.
>   Ideally, we would need two spectrometers with high spatial resolution and
>   very different viewing angles.
> 
> 
> 
> Giulio
> --
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