AbstractThis work applies a new remote sensing approach to indirectly probe the electric fields in the discharge region of fast breakdown (FB) processes during lightning initiation. A VHF‐UHF broadband (1–750 MHz) measurement system is used to record the radiation spectra of FB. The average spectrum over 4 FB events exhibits a −10 dB spectral cutoff at 430 MHz, which yields an electrical field of 1.96–2.02 times the conventional breakdown field at 8–9 km altitudes. This in turn implies a localized field enhancement inside the region dominating the VHF‐UHF emissions of >7 times the expected pre‐discharge background field for lightning initiation. Such a high field has implications for the dynamics of individual streamers and their impact on the immediate environment, and also constrains the geometry of parts of the FB structure, suggesting that streamers initiate continuously in the enhanced field ahead of a narrow (aspect ratio >4) and conducting region.
