Search

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 August 21, 2017 total solar eclipse, data revealing the light patterns before and after totality were collected at 600 Hz. These 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 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 16 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. These higher frequency signals, which we attribute to atmospheric scintillation, appeared mostly uncorrelated over a length scale of 10 cm. 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 scaled white screen. We judged that the bands were roughly parallel to the orientation of the bright thin crescent Sun before and after totality. Thus, if the ν ≈ 4.5 Hz frequency signal is identified with the peak-to-peak shadow band wavelength of λ ≈ 13 cm measured in the video, it can be inferred that their propagation velocity was about v ≈ 59 cm s − 1 (≈ 2.1 km h r − 1 ). 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 visual observers often report before and after totality. This signal cannot be due to atmospheric scintillation and we ran a check to make sure this signal is not an artifact of our electronics. We recommend that additional searches for eclipse shadow bands be made at high altitude in the future. • Ground and high altitude balloon based photo-diode arrays collected data around totality of solar eclipse. • Brief, chaotic signals, attributed to atmospheric scintillation found only in ground based array. • Data analyzed using spectrograms revealed sustained shadow band signal above and below the atmosphere. • The author(s) declare(s) that there is no conflict of interest. [ABSTRACT FROM AUTHOR]