Measuring Streamer Producing Thundercloud Electric Fields Using Radio Observations of Narrow Bipolar Events

The strong VHF radiation from compact intra-cloud discharges (CIDs) is attributed to streamers. An analytical model, taking altitude and applied electric field as input, is developed for effective representation of current for a double-headed exponentially growing streamer. The decay of streamer current is attributed to two- and three-body attachment of electrons to molecular oxygen. The model predicts streamers of growing strength and spatial scales at altitudes where electron losses due to three-body attachment are suppressed with reducing air pressure. We show that CIDs at higher altitudes develop during a longer period such that the spectral content of recorded sferics shifts towards lower frequencies. The model is used to interpret the recorded sferics of two CIDs originating from ~9.5 km altitude in terms of radio signals emanating from an ensemble of streamers. The driving thundercloud electric fields are found to be ~0.4Ek, where Ek is conventional breakdown threshold field.