Post-sunset sporadic F propagation: A sign of electron density isopleth convergence?

Reception of WSPR spots enables alerts of propagation anomalies such as the outlier Doppler shifts on 14 MHz seen across North America during a post-sunset Travelling Ionospheric Disturbance on 26 July 2024. For an hour this event opened one-hop paths and added two-hop propagation to already-open one-hop paths. For analysis and interpretation I used the multiband milliHertz precision Doppler measurements of WWV at HamSCI RX888 station N8GA as it showed one- and two-hop signals. Two-hop propagation began 4 minutes after an inflexion on the fall-from-peak of one-hop Doppler. The prior rise from 0.5 Hz to 0.8 Hz took 8 minutes. After the inflexion the fall back to 0.5 Hz took 22 minutes, with a distorted flat top. If Doppler shift was solely attributable to advection, reflecting region vertical motion, crest to trough height reduction was ~69 km. However, it is not clear how electron density isopleth advection could increase electron density, hence critical frequency, to allow two-hop propagation. If part of the Doppler arose from the plausible, but less often considered, convergence of electron density isopleths a reasonable hypothesis is that electron density could, temporarily, be sufficiently enhanced to support two-hop propagation. To try and identify the two Doppler sources I examined the LF and HF skirts of the two-hop Doppler spectrum immediately on the appearance of great circle propagation with its narrow spectral peak. The skirts, 13 dB to 25 dB below the peak, were from forward-scattered paths. I modeled Doppler shift on forward-scattered paths using 3D PyLap ray tracing and assumed pseudo-reciprocity. On these longer than great circle paths Doppler shift was less for the same reflection height change. This simple model suggests that Doppler on forward-scattered paths explained the LF skirt but not the HF skirt. Rather than simply attributing the HF skirt to convergence I consider other methods to test the assumption of advection only. Future studies may need to explicitly test whether attributing the Doppler shift purely to advection is warranted.