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During the solar eclipse of 21 August 2017, the University of Florida monitored very low frequency (3-30kHz) radio activity at ten locations, enabling the remote sensing of the D region ionosphere (similar to 50- to 100-km altitude). Two of the locations were far enough away (Antarctica) that no response to the eclipse was detected despite the fact that the eclipse significantly shadowed the transmitter. A particularly rare observation is presented: Three receivers were at locations where the Moon blocked at least some of the solar X-ray flare that occurred near 1800UT, providing high-resolution observations of solar flare occultation by the Moon. The remainder of the sites registered meaningful responses to both the solar eclipse and the solar flare. Observations indicate that very low frequency signal propagation is affected once 30-55% obscuration is achieved anywhere along the propagation path. No long-term or delayed effects, such as atmospheric gravity waves, were observed.

Plain Language Summary We present and analyze observations of the 21 August 2017 solar eclipse and a coincident solar X-ray flare. Observations are performed across the Western Hemisphere. Observations provide solid experimental evidence that the effects of the solar flare and the solar eclipse can be distinguished from each other, and that the solar flare observed at some locations were diminished by the presence of the Moon. Analysis indicates that it may be possible to image the X-ray source on the Sun using ground-based very low frequency observations during a solar eclipse.