Cloud‐to‐stratosphere lightning discharges: A radio emission model

Recent observations of rare cloud‐to‐stratospheric lightning discharges suggest the events are inherently “slow‐rising”, with the emitted energy reaching peak values in about 10 milliseconds. Applying a dipole radiation model, we demonstrate that the emitted radio wave energy from such slow‐rising events is strongest below about 50 Hz, and possesses a significant rolloff at higher frequencies. In the analysis, various current distributions are considered in order to determine the effect on the radio spectrum. Near 10 kHz, the emission from cloud‐to‐stratospheric lightning is significantly reduced as compared to the typical cloud‐to‐ground return stroke, with amplitudes as much as 50 dB lower. This result may explain the lack of detection of VLF signals from recently observed long‐lasting discharge events.