Impact of a Solar X-Flare on NVIS Propagation: Daytime characteristic wave refraction and nighttime scattering

An experiment to investigate several characteristics of near vertical incidence skywave (NVIS) propagation (polarization, characteristic waves and their isolation, elevation angles, and radio noise) over a distance of 105 km and at 7 MHz was performed in The Netherlands. Daytime characteristic wave propagation with almost perfectly circular polarization and high isolation values had been expected from our earlier work and could now be demonstrated. Simultaneous elevation angle measurements showed pure single-hop F2-layer reflection and no ground wave component. Nighttime scattering with loss of polarization was observed when the critical frequency (fxF2) dropped below the operating frequency. The "Happy Hour" propagation, shown in our previous work [2], was confirmed with new measurements. The occurrence of a solar X-ray flare during the measurements provided a unique opportunity to compare the observations in a quiet ionosphere with those during a short wave fadeout (SWF). During the SWF, the signal strength of the ordinary wave dropped 35 dB and that of the extraordinary wave 45 dB. This lasted 15 min; recovery to normal signal levels took 35 min. The radio noise level, which was already low in the quiet rural measurement location, dropped another 12 dB during the SWF, with the remainder being generated locally. This proves that in a quiet rural location the major part of the radio noise arrives via the ionosphere.