Ionospheric Disturbances at Dawn, Dusk, and During the 2017 Eclipse

The author recently participated in the HamSCI propagation experiments during the August 2017 total solar eclipse, and the ARRL November 2017 Frequency Measuring Test (FMT). This article presents some interesting propagation phenomena observed during both activities. For the eclipse experiment, well-defined propagation enhancements of both 60 kHz WWVB and 5 MHz WWV for a path between Ft. Collins, CO and San Antonio, TX were documented. Additionally, deep propagation nulls of WWVB over this path were observed to occur every morning and evening, suggesting predictable multipath interference between competing daytime and nighttime modes. During the Frequency Measuring Test, propagation-induced frequency variations of 5 MHz WWV were observed at night and especially during dawn and dusk. One observed dawn frequency perturbation was particularly interesting because it occurred at a fundamental frequency shift plus two harmonically related overtones, indicating a nonlinear ionospheric response to rapidly increasing solar radiation.