Mid-latitude ionospheric disturbances during the major Sudden Stratospheric Warming event of 2018 observed by sub-ionospheric VLF/LF signals

Abstract

This study examines the impact of a major sudden stratospheric warming (SSW) event in February 2018 on the ionosphere of the mid-to–high latitudes in the Northern Hemisphere. The SSW event commenced on February 11 and persisted for about 18 days. The polar stratospheric temperature at 10 hPa increased by about 50 K during February 17 (daytime) and February 18 (nighttime). To detect lower ionospheric variations, VLF/LF signals from four transmitters received by three receivers in the European sector and one receiver in the eastern USA are analyzed along the five propagation paths. The analysis of the diurnal variations in the European sector indicates a significant fluctuation in signal amplitude exceeding ∼5 dB during nighttime on the most disturbed day (February 13). The 1-h averaged nighttime amplitude also showed a maximum deviation of more than ±2σ on February 13. Notably, this occurrence took place two days after the onset of the SSW event at the European receiver locations. No delay in the occurrence of the maximum anomaly was observed in the westward propagation path in the eastern USA. We also analyzed the critical frequency of the F2 layer (foF2) of the ionosphere during February 2018, recorded by Lowell digisonde stations located near the VLF/LF receivers. The foF2 data shows a significant maximum anomaly of ∼1 MHz during nighttime on February 13 at the European stations, coinciding with the SSW period. This anomaly is consistent with the observed VLF/LF anomaly and aligns with the observed temperature variations. However, the deviations in foF2 at the USA stations are not as prominent as those observed at the European stations.