A catalogue of solar rotation-scale variations in the first Schumann resonance frequency at high latitudes, 2004–2024

Abstract Schumann resonances (SR) are extremely low-frequency (ELF) electromagnetic resonances of the Earth–ionosphere cavity, excited primarily by global lightning activity. Their modal frequencies can vary with the effective electrical properties and dimensions of the cavity, including lower-ionospheric conditions. Solar-rotation-scale variability has been reported in SR parameters, but its appearance in long-term SR frequency time series has not yet been systematically studied. We analyzed daily peak-frequency measurements of the first-mode SR (F1) from Hornsund (Svalbard), Ivalo, and Rovaniemi (both in northern Finland) from 2004/2005 to 2024. The north–south and east–west component time series were examined for time-localized wavelet power in the predefined 25–29-day, 12–15-day, and 8–11-day bands. Temporally overlapping detections in the same target band were grouped across the six F1 time series and labeled as events. Events detected at two or more stations were defined as multi-station events. For these events, we estimated band-limited F1 amplitudes and examined their relationships with solar-wind speed, geomagnetic indices, solar-radiation proxies, and lightning counts using the cross-wavelet transform and wavelet coherence. We identified 8, 29, and 68 time-localized events in the respective bands, of which 2, 4, and 25 were multi-station events. The 25–29-day activity was episodic rather than persistent, with the clearest case occurring across all three stations in late 2020. Same-band, phase-stable coherence was found mainly with solar-wind speed and the AE and Kp geomagnetic indices. Many events coincided with high-speed streams, stream interaction regions, or ICME/sheath intervals, although only a subset showed same-band, phase-stable coherence in individual F1 time series. After correction for multiple comparisons, the timing tests did not establish individual event–driver associations or an excess of solar-wind event overlaps. Comparisons with simulated records provided limited statistical support for an excess of events, with results depending on the assumed background. No single driver could be established. Within these statistical limits, the findings indicate that solar-rotation-timescale variability can appear in high-latitude SR F1 as small, time-localized modulations rather than as a persistent 27-day oscillation.