  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 the light patterns before and after totality were collected.
These data were then analyzed using spectrograms. Both at the altitude of the
balloon 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
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 and 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 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 and inferred that their propagation velocity was about v ~
59 cm /s. 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. 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.
