Characteristics of broadband lightning emissions associated with terrestrial gamma ray flashes

To characterize lightning processes that produce terrestrial gamma ray flashes (TGFs), we have analyzed broadband (< 1 Hz to 30 kHz) lightning magnetic fields for TGFs detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) satellite in 2004-2009. The majority (96%) of 56 TGF-associated lightning signals contain single or multiple VLF impulses superposed on a slow pulse that reflects a process raising considerable negative charge within 2-6 ms. Some TGF lightning emissions also contain VLF signals that precede any appreciable slow pulse and that we term precursor sferics. The analyses of 9 TGFs related to lightning discharges with location uncertainty < 100 km consistently indicate that TGFs are temporally linked to the early portion of the slow process and associated VLF impulses, and not to precursor sferics. The nearly universal presence of a slow pulse suggests that the slow process plays an important role in gamma ray production. In all cases the slow process raises negative charge with a typical mean current moment of +30 kA km. The resulting charge moment change ranges from small values below +10 C km to a maximum of +200 C km, with an average of +64 C km. The current moment waveform extracted from TGF sferics with single or multiple VLF impulses also shows that the slow process initiates shortly before the major TGF-associated fast discharge. These features are generally consistent with the TGF-lightning sequence reported by Lu et al. (2010), suggesting that the majority of RHESSI TGFs are produced during the upward negative leader progression prevalent in normal polarity intracloud flashes.