Contrasting effects of the 3-5 November 2021 geomagnetic storm on reception in Colorado of WSPR transmissions from North-Eastern North America with those from Australia

Solar wind particles from three M-class flares hit the Earth's magnetic field around 19:30 UTC on 3 November 2021. The planetary geomagnetic disturbance index (Kp) peaked at 7 that evening and the following morning. At the USGS Boulder Geomagnetic Observatory, Colorado the vertical magnetic field anomaly was below -40 nT between 07:38 UTC and 12:56 UTC on 4 November, dipping briefly to -75 nT. These dramatic space weather events are examined using WSPR spots at N6GN, near Fort Collins, Colorado. Between 10:30 UTC and 11:00 UTC the 7 MHz WSPR spot count showed a ~90% drop compared with previous days at that time interval. Second, the median distance for remaining spots increased to 7089 km from ~2500 km of previous days. Furthermore, the noise level dropped about 4 dB. At that time of day the noise at N6GN's remote receiver is limited by propagated-in noise rather than local or receiver noise. Central to the observed spot count decrease and median distance increase was a 98% reduction in spots received from grid FN, North Eastern North America: down to 5 spots from a typical 245 on other days in the same interval. But what caused that precipitous drop? We look at signal levels of individual transmissions to try and understand whether received signal levels dropped below the noise or whether Doppler flutter spread the signals beyond the bandwidth of the WSPR decoder. We also seek to understand the increase in spots from Australia compared with previous days. During the storm itself, signal levels from Australia were unchanged; it was not until the following day that levels and the number of spots decreased. We caution and investigate that the very narrow band transmissions may not be decoded more due to spectral distortion and spreading rather than the more usual lack of signal to noise ratio. This analysis provides a valuable use case for WSPR transmissions, reporting via wsprnet.org, augmented with noise estimates and on-line access via the WsprDaemon database with quick-look Grafana and animated Octave visualizations.