Adv. SpaceRes. Vol. 14, No. 4, pp. (4)29-(4)39, 1994 0273-1177/94 $6.00 + 0.00 Printed in GreatBritain.Allrightsreserved. Copyright © 1993COSPAR CORONAL PHYSICS FROM ECLIPSE OBSERVATIONS S. Koutchmy Institut d'Astrophysique de Paris*, CNRS 98 bis Boulevard Arago, F-75014 Paris, France ABSTRACT Solar total eclipses are rare occasions offering the opportunity to make a snapshot of the solar corona. Thanks to the availability of a large radiative flux in the optical region, sophisticated methods can be used from the ground to analyse all parts of the highly structured whlte-light corona. Coronal absolute intensities and line emissions, including their polarization, are also studied to analyse density and temperature inhomogeneities, velocities and magnetic fields. Detailed density distribution is directly extracted from fine coronal structures. During the July II, 1991 eclipse, the large 3.6m aperture CFH optical telescope was used to analyse time sequences over small coronal fields and to image the finest structures ; results from this experiment are presented with emphasis on small-scale dynamical plasma processes with possible inclusion of wave-phenomena. Finally, to prepare the SOHO mission on coronal physics, we give an overview of what is known on coronal structures from eclipse observations of the past solar cycles: temperatures, densities, velocities, occurence and structure of streamers, coronal holes, threads, overall variability. INTRODUCTION The corona is normally too faint to be seen from ground except at an eclipse of the Sun, because it is fainter than the normal blue sky, ///1//////24//. On the average, a solar total eclipse occurs somewhere in the world every year and a half, but often the band of totality crosses oceans. Its duration can be up to 7 min. The recent July 11, 1991 total eclipse was observed from populated areas including the big island of Hawaii, with the largest and best optical telescopes in the world. During total solar eclipses, when first the photosphere and then the chromosphere are completely hidden from view, a faint white polarized halo produced by the scattering of the solar light from electrons (the K- corona) becomes visible: the white-light (W.L.) corona. Around the moving disc of the Moon slightly larger than the Sun, the corona is accessible from the transition region (T.R.) up to the most extended structures, like streamers, during a few minutes. At low spatial resolution "snapshot" observations are made, including spectra, to analyse the temperature and velocities. No shadowing or vignetting of the W.L. corona is seen during a natural eclipse and in good observing conditions, the limitations in the field of view is essentially due to the dominance of the brightness of the dust F-corona, //2//10//. The approximately constant sky brightness can also be considerably reduced, by using the IR or airborne experiments. Absolute photometric measurements can be performed with a high accuracy, thanks to images of calibration stats ///1//, simultaneously observed on the same picture as the corona. Suitably attenuated solar light can also be used immediately before or after the totality. Progress made in the last decades includes the use of an accurately calibrated radial neutral density filter to compensate the large average radial gradient/1//2// of the W.L. corona and to precisely remove parasitic effects produced in the instrument and the sky (the aureola effect) ; the use of such filters considerably improves the spatial resolution by avoiding overexposures. At the 1991 eclipse, strictly linear and highly sensitive detector systems were used, especially for making time sequences with narrow passband filters. However, because of its simplicity, reliability and large storage capability, film is still largely in use at eclipses. In IR, new 2D arrays were used for the first time in 1991 to study the far corona (outside a radial distance of r : 5 solar radii from the center of the Sun). For this purpose the radial filter is replaced by an externally occulting system ("double" occultation !). * associated with the Pierre et Marie Curie University of Paris VI, France (4)29