The shadow band phenomenon

Abstract

Shadow bands are a phenomenon observed during eclipses of the Sun and visible only in the minutes immediately preceding and following totality. Prior to the solar eclipse of 7 March 1970, these bands had never been recorded by an experimental apparatus, accurately measured nor well analyzed. Data were gathered at the site established at Wallops Island, Virginia, in two ways: 1. (1) Recording onto magnetic tape of the output from six collimated photocells whose spectral responses ranged from the ultraviolet to the infrared; 2. (2) visual and mechanical measurement of the orientation and motion of the bands. The recorded data were later processed using the output from a frequency spectrum analyzer. Most of the energy of the shadow bands ranges between 1 and 25 Hz, and appears not unlike plots of scintillation spectra. The onset, amplitude, and duration of the shadow bands seem to be spectrally related to the limb darkening of the Sun's photosphere. Other results of the experiment conclusively indicate that the shadow band phenomenon is a manifestation of atmospheric turbulence in the form of air packets of different density from that of their environment (density schlieren), made visible by the light from the crescent sun.