[Loops] Loops and AR temperatures

[Giulio Del Zanna]

Dear all,

if you go to
http://solar.physics.montana.edu/cgi-bin/eprint/index.pl

you will find three papers relevant to the last two loop
workshop meetings.

Those of you present during my talk at the loop meeting in
Santorini might recall the Hinode EIS observations which I used to
cross-calibrate EIS with the TRACE 195 A band. I made the point that
at upper transition region temperatures there is a host of lines, many
still not identified, and which can become important in any modelling
of this TRACE band. After two years I finally finished the `preliminary'
work, and identified a large number of lines [the TRACE calibration
work has been postponed for now].

A nice by-product of this work is the fact that with newly-identified lines from 
Fe VII and Fe VIII (together with lines from Fe IX, X), it is in principle 
possible to provide good constraints on the electron temperatures in the
`upper transition region'. I say in  principle because I think that the 
radiometric calibration of the EIS instrument needs some revisions.
And because a lot more work still needs to be 
done on the atomic data [I found some surprising results which need
to be confirmed]. In all the `leg' of loops I looked at, I found
`near-isothermal' emission measures (confirming my SOHO CDS results),
and electron temperatures far off from those where ions have peak abundance
[not surprisingly].

I fully agree on the word of caution given recently by Phil Judge, however
combining different diagnostics can provide reliable measurements.
When I first applied the `good old' EM Loci method to AR loops in 2003,
I did it to show that AR plasma is composed of near-isothermal structures,
and not of multi-thermal ones as many people still think.
However, I have always warned that better
direct temperature measurements are needed and large uncertainties are
present in any EM technique.


Finally, those of you present at the Florence meeting might recall me saying 
that most Hinode/XRT filters in AR observations would be dominated by Fe XVII
emission lines, and that a host of Fe XVII lines are present in the
Hinode EIS spectra, so some direct cross-calibration is in principle
possible. Well again after a number of years I have finally finished
the identification work for the Fe XVII EUV lines.
Results on the XRT calibration will follow at some point.
The Fe XVII lines, once blends and various problems are taken into
account, can also be used to constrain emission measures in an interesting
temperature region.


cheers

-- 
                    Giulio Del Zanna

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*                   STFC Advanced Fellow                    *
* Department of Applied Mathematics and Theoretical Physics *
*              CMS,  University of Cambridge                *
*           Wilberforce Road, Cambridge, CB3 0WA  UK        *
*           E.mail: GDelZanna at spd.aas.org                   *
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