Observation of Eclipse Shadow Bands Using High Altitude Balloon and Ground-Based Photodiode Arrays

The results of an investigation into whether or not eclipse shadow bands have an atmospheric origin are presented. Using high altitude balloon and ground-based photodiode arrays during the 21 August 2017 total solar eclipse, data revealing light patterns before and after totality were collected at 600 Hz. The data were then analyzed using spectrograms, which provide information on intensity fluctuations in the frequency space time domain. Both at the altitude of the balloon (~ 25 km) and on the ground, a sustained 4.5 Hz signal, that was confirmed to not be electronic in origin, was detected a few minutes before and after totality. This signal was coherent over a scale greater than 10 cm and detected in four separate balloon photodiodes and 12 separate ground photodiodes. At higher frequencies, up to at least 30 Hz, brief chaotic signals that were disorganized as a function of time were detected on the ground, but not at the altitude of the balloon. Higher frequency signals were uncorrelated over a length scale of 10 cm. We attribute these chaotic signals to atmospheric scintillation. Some of our ground arrays utilized red and blue filters, but neither the sustained 4.5 Hz signal nor the chaotic higher frequency signals showed a strong dependence on filter color. On the ground we made a video of the shadow bands on a white board labeled with a scale and judged that the bands were roughly parallel to the orientation of the bright thin crescent Sun before and after totality, tangent to the Moon's shadow on Earth. Their peak-to-peak wavelength was estimated to be ~ 13 cm from the video, which indicates their velocity was about ~ 59 cm/s (~ 2.1 km/hr). Shadow band signals other than the sustained signal at ~ 4.5 Hz are consistent with atmospheric scintillation theory. These results are surprising. Based on accounts in the literature we expected to confirm the atmospheric scintillation theory of eclipse shadow bands, but instead we detected a sustained ~ 4.5 Hz signal at both high altitude and on the ground, consistent with the type of shadow band signal observers often reported before and after totality. This signal can not be due to atmospheric scintillation. We recommend that additional searches for eclipse shadow bands be made at high altitude in the future.