VHF broadband digital interferometer and mapping of lightning discharges

The Lightning Research Group of Osaka University (LRGOU) has been developing two- (2D) and three-dimensional (3D) VHF source mapping systems of electromagnetic (EM) waves emitted by lightning discharge progression by a unique technique based on broadband digital interferometry. Through our field observations real time monitoring in terms of 2D are realized and 3D images of lightning channels by post processing are obtained. Since VHF impulses are radiated from mainly negative breakdowns their source location gives the positive charge distribution. In other words a broadband interferometer (ITF) may not only visualize negative leader progressions but also give information on positive charge distribution inside thunderclouds. In leader progression of negative cloud-to-ground (CG) flashes two categories are identified. One is characterized that negative leader begins at an altitude of around 8 km agl and goes down to the ground for several tens of milliseconds. The other in contrast has some intermittent VHF radiation at 10 km agl about 1 ms before continuous leader development. The ignitions at high altitudes could be powerful events and relate to transionospheric pulse pairs (TIPP) and/or compact intracloud discharges (CID).