Diagnostic study of geomagnetic storm-induced ionospheric changes over very low-frequency signal propagation paths in the mid-latitude D region

Abstract

Abstract. We performed a diagnostic study of geomagnetic storm-induced disturbances that are coupled to the mid-latitude D region by quantifying the propagation characteristics of very low-frequency (VLF) radio signals from transmitters located in Cumbria, UK (call sign GQD), and Rhauderfehn, Germany (DHO), and received in southern France (A118). We characterised the diurnal VLF amplitudes from two propagation paths into five metrics, namely the mean amplitude before sunrise (MBSR), the midday amplitude peak (MDP), the mean amplitude after sunset (MASS), the sunrise terminator (SRT) and the sunset terminator (SST). We analysed and monitored trends in the variation of signal metrics for up to 20 storms to relate the deviations in the signal amplitudes that were attributable to the storms. Five storms and their effects on the signals were examined in further detail. Our results indicate that relative to pre-storm levels the storm day MDP exhibited characteristic decreases in about 80 % (67 %) of the events for the DHO-A118 (GQD-A118) propagation path. The MBSR showed decreases of about 60 % (77 %), whereas the MASS decreased by 67 % (58 %). Conversely, the SRT and SST showed amplitude decreases of 33 % (25 %) and 47 % (42 %), respectively. Of the two propagation paths, the amplitude decreases for the DHO-A118 propagation path signal were greater, as previously noted by Nwankwo et al. (2016). To better understand the state of the ionosphere over the signal propagation paths and how it might have affected the VLF amplitudes, we further analysed the virtual heights (h'E, h'F1 and h'F2) and critical frequencies (foE, foF1 and foF2) from ionosondes located near the transmitters. The results of this analysis showed significant increases and fluctuations in both the F-region critical frequencies and virtual heights during the geomagnetic storms. The largest increases in the virtual heights occurred near the DHO transmitter in Rhauderfehn (Germany), suggesting a strong storm response over the region which might account for the larger MDP decrease along the DHO-A118 propagation path.

Empirical Parameterization of Broadband VLF Attenuation in the Earth‐Ionosphere Waveguide

“…Multiple sources of transient ionization can also temporally perturb the diurnal electron profile cycle and thus significantly alter the reflection properties. These include X‐rays from solar flares (Anderson et al., 2020; Gołkowski et al., 2018; Quan et al., 2021), perturbations caused by geomagnetic storms (Nwankwo et al., 2022), and secondary ionization due to electrodynamic heating from VLF waves emitted by nearby lightning (Gołkowski et al., 2014; Inan et al., 1991). The present work aims to capture factors controlling sferic attenuation patterns that can be inferred from properties of the source‐receiver path only.…”

Reply on RC2

“…These changes in the D-region electron number density also affect VLF propagation. During solar flare events (McRae and Thomson, 2004) and geomagnetic storms (Nwankwo, 2022) rapid temporal changes may occur in the D-region electron density and vertical structure (and thus collision frequency with neutral constituents) which influences VLFpropagation.…”