In 1957, the author has described the morphology of the monochromatic corona, using synoptic charts of the green line intensity. He drew attention to regions of very low emission that could be identified on several consecutive rotations. For these features, he coined the term 'L6cher' what means 'holes'. Such a hole appeared in the SW-quadrant of the Sun at the total eclipse of March 7, 1970. From the photometry in white light, the density distribution in the hole has been calculated under the assumption of a circular cross section. The result is presented in Figure 2. In the central part, whose diameter is roughly half that of the whole hole, the density is less than 10~ of the density of the undisturbed corona at the same distance from the Sun. The hole did appear in a region free of solar activity for three consecutive rotations (Figure 3).
1. Some Preliminary Remarks on Coronal Holes
In the last few years, several papers have dealt with a new phenomenon in the structure of the corona, which has been named coronal hole (Munro and Withbr oe (1972); Altschuler et al. (1972); Krieger et al. (1972); Noci (1973); Vaiana et al. (1973); Gurman et al. (1974)). Holes are extended areas of the corona which show a very low emission over a long period of time. They have been 'discovered' on pictures of the corona which were taken in the EUV-and in the X-ray-region. These pictures show the holes not only on the corona at the limb, like opdcal pictures, but also in front of the solar disc. The mentioned emissions are proportional to the square of the electron density and, therefore, depending strongly on density variations. For that reason, coronal holes are primarily areas of low density. Furthermore, the coronal temperature rises with the density, as Waldmeier (1971a) stated. Therefore, coronal holes are of low temperature, a reason more for their weak emission.
Optical lines (e.g., the green coronal line 5303 A) show the same behaviour. Their intensity increases with the square of the density and with the temperature. In the coronal holes, therefore, such lines are very weak. The intensity of the line 6374 A, however, decreases with increasing temperature. The influences of density and temperature compensate each other so that in this line coronal holes do not stand out clearly. The optical lines can only be observed at the Sun's limb; however, from observations on consecutive days, heliographic maps of the monochromatic corona can be drawn. Waldmeier (1957) has published a large number of such maps. In many of them, coronal holes appear; frequently they can be recognized on maps of consecutive rotations. Therefore, coronal holes are of long duration. Waldmeier (1957, S. 275, 276, Figures 258a, 259a, 260a) drew attention to these features and called them 'L6cher',