Long-Baseline VLF Observations of Solar Flares from Antarctica

We present long-baseline Very Low Frequency (VLF) observations of solar flare-induced ionospheric disturbances obtained at the Bulgarian Polar Astronomical Observatory (St. Kliment Ohridski Base) on Livingston island, Antarctica, representing the first such measurements from this high-latitude Southern Hemisphere location. Using continuous VLF transmissions at 21.4 kHz (NPM, Hawaii) and 24.0 kHz (NAA, Maine), propagating over trans-hemispheric paths exceeding 11,000 km, we investigate the response of the ionospheric D-region to solar flares during the period 24 January–8 February 2025. After removing the strong diurnal signal via superposed epoch analysis, we analyse the flare-related perturbations in VLF amplitude and their correlation with GOES soft X-ray flux for 41 flares of class C7.0 and above. The long propagation paths provide enhanced sensitivity to flare-driven changes in D-region ionization. The observations reveal clear, frequency-dependent responses and measurable time delays between X-ray and VLF peaks. These delays, including cases of near-zero or negative lag for stronger events, highlight the role of flare spectral characteristics and D-region recombination processes. Our results demonstrate the scientific value of Antarctic VLF observations for probing solar-terrestrial processes coupling, and establish a new node in the global VLF monitoring network, with direct relevance for space weather research.