Expedition of French Institutes to Iran

Institut d'Astrophysique de Paris- CNRS/ leading Organization/ :
[Dr. S. Koutchmy; J. Mouette; P-A. Grorod]
Institut d'Astrophysique Spatiale- CNRS, Univ. Paris XI & Obs. de Paris:
[Dr. F. Baudin; Dr. K. Bocchialini]
Laboratoire d'Astronomie Spatiale- Marseille (CNRS):
[Dr. Ph. Lamy}
Societe Astronomique de France (SAF):
[Mr R. Leguet; Mr. C. LeRoux; Mr. G. Mahoux; Dr. R. Robley]
in collaboration with a team from the Tabriz University (Iran):
[Dr. A. Adjabshirizadeh et al.]

Observations to be performed from a mountain site in Iran, near Ispahan- 100 km toward NWW. Probability of having a clear sky is 95%; altitude 2200 m; totality will occur at U.T. 12:03. The corona is expected to have the shape of a pre-maximum corona close to the polarity reversal at the N-pole but not at all in the S-pole region. Prominences and active regions at the limb would be observed, as well as streamers in projection over the N-pole/.

A-Main experiments
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1/ Coronal imaging
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We will concentrate on the INTERMEDIATE W-L corona (between the very inner corona, where loops and jets dominate, and the outer corona, where the outward flow dominates) which cannot be measured from space due to the vignetting by occulting systems.

We do absolute photometry using simultaneously imaged stars in the same field of view. Precise polarimetry is planned as well as the use of a radial gradient filter of 150 mm diameter. These data are required to complement spaceborne observations regarding the precise measurements of plasma densities in the main part of the corona. They will be compared to the values recently obtained during the solar minimum. High spatial resolution imaging is an other advantage of the method.

We also plan to do some deep imaging to look at the so-called cool corona around the occulting Moon, in H-alpha, K CaII lines and narrow-band continuum. For that, we use both radial filters of 50 mm diameter and narrow band interference filters with small telescopes. Both photographic film and CCD camera are used.

The hot topic considered with this last set up is the analysis of the recently discovered solar PROLATENESS (ovalization) at the high chromospheric level (e.g. AA:1998, 336, L57); we want to go further out in the low corona. Until now, the prolateness was measured only up to 6 to 7000 km heights, using EIT (SoHO)-HeII emissions and, of course, at lower levels in H-alpha, K CaII, etc. In H-alpha the prolateness is well seen up to the 'top' of spicules and could be a fundamental indicator of the mechanism converting the magnetic turbulence energy into convecting plasma flow near separatrix layers.

2/ Spectroscopic experiments
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We prepared spectrogaphs for both i) HIGH dispersion to look at the profile of the Fe XIV coronal emission line for different radial distances and different position angles, and ii) LOW dispersion analysis to simultaneously look at several lines and deduce the temperature inhomogeneities of the whole corona.

High sensitive film and CCD DV-cameras were selected as detectors. High dispersion will be used to look at the properties of propagating waves in the intermediate corona, thanks to the measurement of the Doppler effects, including the Doppler widths, and the determination of the so-called non-thermal velocities as a function of radial distances and latitudes. We plan to get data up to 2 solar radii from the sun's center, based on previous experiments (Chile 94 and Guadeloupe 98).

Low dispersion will be used to simultaneously measure several forbidden lines and look at the temperature inhomogeneities at large scale (+/- 3 solar radii) from the analysis of the distribution of the emission measures. Electron temperatures will also tentatively be studied, using the Grotrian's method over the b MgI and D NaI lines.

The use of new-technology fast CCD cameras should be relatively efficient at eclipses. We hope to significantly improve what has been done in the past with plates and films, although new fine-grain films are also available. Further, the coordinated effort (with ESA, NASA and ISAS) to perform well defined and selected observations using the spaceborne facilities, especially the CDS, SUMER, EIT and the LASCO experiments of the SoHO mission, the SXT of YOHKOH and possibly, the TRACE mission, makes our observations more effective than it was in the past. We have now an excellent coverage of coronal phenomena occuring before and after the eclipse, which makes eclipse results more reliable and more effective to process and to interpret.

During the total eclipse, a large number of photons are available (the corona is as bright as a full Moon) such that it is easier and evidently cheaper to conduct sophisticated investigations, especially at high resolution and dispersion. Of course, this is possible during just the short time (a few min) of totality! Only spaceborne observations and long duration missions permit a survey of coronal and solar phenomena.

B- Selected results of preceding eclipse observations
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The results of our 1991 observations are published in a series of papers in Astron. Astrophys., Astr. Rep. and ApJ. We discovered very fine white-lightstructures at the base of the intermediate corona (down to the 300 km scales); some of them have short lifetimes (like 40 sec). During more than 200 sec at high cadence we observed what we believe is a coronal plasmoid (small cloud of 2000 km diameter with changing shape, moving at 100 km/s speed) which travelled across more radial coronal structures (a spectacular movie was assembled), showing interactions leading to a splitting of its core, etc.

The dynamics of the corona at small scales was found surprisingly large; the so-called turbulence seen on coronal line profiles (20 to 30 km/s) is resolved in the inner corona at scales under 1". To give a good idea of what was seen in 1991, let us mention that the dynamics is already evidenced watching the original as a video movie of a narrow field of view (80" size), without introducing any acceleration. These CFHT data confirm our earlier observations of plasmoids and jets (spike) obtained since 1973.

From our large scale images taken in 1991 (active corona) with a radial filter from 2 different locations (Hawaii and Brazil) with a 1 hour 40 min separation in time, we compared the position of SHARP EDGES of streamers and we succeeded in using the effect of the so-called rigid rotation of the corona to look at the 3-D structure (stereo-effect) /results published in Nature 1992, 360, 717 in collaboration with Russian scientists/.

Results tell us that the coronal plasma at large scale is well confined in sheets and the topology of those sheets is a new very interesting topic we are trying to consider using numerical simulations (paper in press in collaboration with IZMIRAN).

We intend to repeat this experiment in collaboration with the team from Kiev; however, the Lasco/SoHO coronagraphs accumulated a large number of W-L images of the more external corona and we can now look at these images and improve our eclipse results.

From our 1994 spectroscopic experiment, using a long slit, we got the line profiles of the green line of FeXIV in several locations, with the following main conclusions:

Over one polar region, the profiles are broader but intensities are really low, although well measurable up to 0.2 solar radii (coronal hole boundaries; plumes, etc.) from the limb. They tell us that small regions with 2 millions kelvin ionic temperature definitely exist over the poles. These regions could be related to the locations where X-ray polar jetlets (Yohkoh observations, see AA, 1997, 320, L33) are observed and where magnetic reconnections are occuring.

Over the equatorial regions, we measured line profiles of large intensity, in streamers, and even some small but significant (#10 km/s) line shifts at radial distances of 0.5 and more from the limb.

We think that a large part of the broadening could be due to ubiquitous propagating waves which could explain the acceleration of the slow wind. Those propagating magneto-acoustic waves were quite recently seen using chromospheric lines formed in the network (AA:1995, 299, 893; AA:1996, 314, L9). However, it is not yet clear what is the best diagnostic to reveal reconnections (temperature effects; plasmoids...) and/or propagating waves (Doppler effects) and their dissipation, taking into account the brevity e.g. limitations of eclipse observations. More works is needed at the forthcoming eclipses of the next Millennium !!!