As a result of the observations of solar eclipse on February 15, 1961 at wavelength 21 cm the lower brightness region of corona was discovered (detail N5 on figure 1) and it's location on east nimbus of the Sun was determined (figure 2). As far as this region was coincided with large solar prominence, the phenomenon of radio flux reduction was interpreted (1,2) as the coronal radiation absorption by more dense matter of solar prominence. Subsegently such phenomena were named "Coronal Holes".

Figure 1. Derivative of eclipse curve on February 15, 1961. The detail N5 is Coronal Hole.

Figure 2. The location on disk of the Sun radiation details of corona at wavelength 21 cm.