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 _______________________________________________ Loops mailing list Loops@solar.physics.montana.edu https://mithra.physics.montana.edu/mailman/listinfo/loops
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