Broadband and Narrowband Multi-site VLF Studies of Global Terrestrial Gamma-ray Flashes

Recent observations of a large number of terrestrial gamma-ray flashes (TGFs) aboard the RHESSI spacecraft have enabled new studies on the nature of high electric fields associated with lightning. Ground based measurements of ELF/VLF radio atmospherics associated with TGFs have found some distinguishing properties of the lightning associated with TGFs, including a high peak current and a possible land/ocean variability. 75% of TGFs studied show detectable sferics at long distances. Still unknown, however, are the altitude and physical mechanism responsible for TGFs, the nature of the remaining 25% of TGFs, and a precise global occurrence rate. It is useful, therefore, to expand previous studies made with only one VLF receiver, to include several globally distributed quiet receivers from Stanford University's ELF/VLF receiver network. Using these data, TGFs can be accurately triangulated, so that the effective detection distribution below RHESSI can be determined. In addition, VLF transmitter signals crossing over TGF regions are studied for evidence of ionospheric disturbances in association with TGFs. Regional variations, and implications to the various proposed physical mechanisms, are discussed.