@proceedings{bibcite_487,
  author = {Gwyn Griffiths},
  title = {Post-sunset sporadic F propagation: A sign of electron density isopleth convergence?},
  abstract = {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.\ },
  year = {2025},
  journal = {HamSCI Workshop},
  month = {03/2025},
  publisher = {HamSCI},
  address = {Newark, NJ},
  language = {eng},
}