[Loops] EIS AR Loops - Tripathi et al, 2009, ApJ/ some historical comments etc.

[Leon Golub]

Dear Serge,

It is useful to be reminded of the history and of the ground-based data. 
  Still, I don't think that this contradicts my assertion. There is some 
evidence from density diagnostics, from analysis of cooling loops in the 
different EUV wavelengths, and from the SOT C IV data, that finer 
structures exist in the hot corona below the level we currently can see 
in the EUV or soft x-ray. I'm afraid that the only way to settle this is 
to fly a higher resolution telescope, going at least 2X better than what 
we currently have. (Indeed, as Jean-Francois notes, EUI may do that on 
Solar Orbiter. But I'd very much like to see this done - even if it's 
only a snapshot on a sounding rocket - sooner.)

Leon


Serge Koutchmy wrote:
> Dear Leon, Helen, Markus, Hugh, Jean-Francois, Joan et al.,
> Regarding the fuzziness (also called the diffuseness, the lack of 
> resolution, the impression of smearing- I am not using here any 
> impressionistic terminology, just because colors are not involved in 
> your business), it could be interesting as well to recall some 
> historical observations and, apparently, basic facts. If you think this 
> is outside the scope of your Space and/or other research, please delete 
> this message and accept my apologize.
> Loops (like the corona) were first observed, more than a century ago, 
> during solar total eclipses, in W-L. and we thought that we understand 
> at least a part of their physics (like being in hydrostatic equilibrium 
> based on their radial density variations) when assuming they are 
> stationnary.
> Lyot coronagraphs permitted to easily image the inner corona in emission 
> lines and hundred of papers were written on loops. Lines were classified 
> in 3 classes (excellent representative are for class I: the red line of 
> FeX; for class II: green line of FeXIV and for class III: CaXV yellow 
> line which is seen, by the way, all the time in active regions- see the 
> routine measurements of SPO).
> Classes correspond to roughly temperatures differing by a factor 2, 
> starting with the red line at 1 MK, almost without overlap in space 
> (uncorrelated). When the first good (photographic) images were obtained, 
> immediately the fuzziness of the Yellow line was noticed and more 
> importantly, that the FeXIV is fuzzier than the simultaneously obtained 
> FeX line images was established. I was at the Pic du Midi observatory in 
> 1971-72 when these images were taken by J-L Leroy and we had rather hot 
> discussions on their interpretation. We decided it is probably due to 
> instrumental problems, including seeing problems.
> Again and after in the 80ies, thousands of FeXIV and FeX images were 
> taken with the former "one-shot" coronagraph of SPO, with improved 
> resolution (more than a solar cycle covered !) and discussing about that 
> with Ray Smartt and many others, we could not exclude an instrumental 
> effect. What was noticed is that interacting loops were seen more often 
> in FeXIV, and higher in the corona.
> Later, came the Norikura team, boosted by the success of the Yohkoh SXR 
> imager. They again confirm this famous effect. Let me give you the exact 
> reference of a paper already written in the "Space age" in ApJ: 
> Ichimoto, K. Hara, H. Takeda, A. et al 1995, ApJ 445, 978 and extract 
> the interesting statement made p. 979: "...because the 3 emission lines 
> (FeX; FeXIV and CaXV) are observed with the same optical system, we 
> beleive that the diffuseness of the hot components compared with the 
> cool components IS A FIRM RESULT."
> Everybody knows of course the wonderful Trace 171A images of the 1 MK 
> component of the corona, with loop systems, which were definitely better 
> of quality (sharper) than what is obtained in hotter lines, using THE 
> SAME instrument. Even FINER loops are seen in much cooler lines, like 
> the Hapha or Lyman alpha line, starting from the drawings made in 1870 
> ies (yes, not 1970 ies), and after, photographic images taken at 
> SacPeak, rocket images from TRC, etc. and more recently CCD images and 
> movies taken with the La Palma SST and Hinode.
> Everything written tells us that the impression we get from an image is 
> subject to a false interpretation when instrumental parameters are not 
> taken into account. Resolution is not a matter of just pixel size. It is 
> mainly a matter of signal/noise ratio over the feature you consider and 
> even the signal alone is "noisy", just because the lack of photons; this 
> is at least my understanding after observing during more than 40 Years 
> and you can disagree.
> Now coming back to the W-L observations let me notice that:
> 1- The hotter is the loop, the higher it is (see also the V-R scaling 
> law). The heating makes the temperature higher when going radially 
> outwardly up to 0.3 to 1 solar radius from the surface. The important 
> factor is that the radial gradient is higher in cooler loops compared to 
> hotter loops. It is of course what the hydrostatic law shows (on W-L 
> images). The scale height of cool loops (FeX; 171A etc) is roughly 50000 
> km; it is 100 000 for FeXIV. This is what you get analyzing W-L images 
> assuming the loop iso-thermal and computing the radial gradient. Of 
> course the higher is the gradient, the sharper is the image.
> 2- Talking now about the transverse gradients, the interplay between the 
> magnetic pressure and the gaz pressure should be considered. The beta is 
> decreasing when going up to the heights of hot loops (and further 
> opening up due to the wind, see eclipse images). It makes transverse 
> gradient weaker for hot loops. Accordingly, cool loops will look sharper.
> This is instrumental effect.
> 
> In this naive analysis made by a lambda eclipse/coronagraphic observer, 
> I do not see nothing wrong in having hotter loops fuzzier and I do not 
> see the urgent need to introduce an additional filling factor, as far as 
> we use good images giving large aspect ratio of loops. What is needed is 
> a more sophisticated diagnostic, like the spectroscopic diagnostic which 
> would include the line profile analysis to measure the Doppler-Fizeau 
> effect. In addition we need the temporal resolution to look at 
> transverse waves of shorter periods... there is a bright future for a 
> new groundbased large aperture coronagraph which would permit the access 
> to these diagnostics. Improving the W-L and visible monochromatic 
> imaging in Space (inner corona; fast imaging; higher resolution; 
> polarization) with an ASPIICS space mission which produces artificial 
> eclipses during several Years could be the ultimate. For the moment, I 
> guess SDO with its 4K coronal imagers of improved resolution will bring 
> a lot and I am sure it will indeed bring a complete renewal of the 
> topic. This is what you told us and let's hope it will be launched in 2009.
> 
> Thank you for reading this up to the end (almost). Let me wish you all 
> the best for 2009 and use the attached season's greeting coronal image 
> for that, including the caption I put after.
> (apologize to those of you who already got it).
> S.
> -----------------------------------------
> 
> BEST WISHES FOR A HAPPY 2009 NEW YEAR !
> 
> Enclosed is a seasons’greetings compressed composite that we did using
> some new observations from the Pic du Midi Observatory CLIMSO
> instrument: a CaII-K line disk and a coronagraphic H-alpha image, same
> scale, but obtained 2 s later, in the very late afternoon. No activity
> during this surprisingly extended solar minimum. However here the solar
> disk was near the W horizon, at the time when the daily Madrid to Zurich
> Airbus 320 was passing by… It is the 1^st time the infamous aircraft
> contrails are imaged in “emission”, using grazing incidence rays near
> the H-alpha line. Would it be solid particles producing additional
> pollution ?
> 
> We believe some new type of propagating waves was revealed using these
> unique coronal observations, although a definite Doppler-Fizeau
> signature is missing. Note that the heating process due to their
> dissipation in the Earth atmosphere was not taken into account to
> explain the not less infamous global warming because waves possibly also
> accelerate the rather cool Mistral (a well known wind in the South of
> France)...
> 
> Take care.
> 
> 
> 
> 
> 
> Leon Golub a écrit :
>> Helen et al.,
>>
>> I can't let this go uncommented. Even though this paper was accepted 
>> (I know, I was the editor) I disagree with the conclusion
>> that hot loops are fuzzier. Yes, that's what you see, but it is
>> also explainable if the hot parts of the AR have many fine threads
>> (unresolved at present resolution) with a large filling factor.
>> That as the conclusion Joan and I came to from analyzing 284A
>> data.
>>
>> Anyway, this won't be settled until we fly an imager with higher
>> resolution. We're proposing one this year and I'm hoping we get
>> the chance to clear this up, finally.
>> Cheers,
>>
>> Leon
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>>   
> 
> 
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