A comparative study on the lightning sferics associated with terrestrial gamma-ray flashes observed in Americas and Asia

We examine the broadband (<1 Hz to 400 kHz) electromagnetic lightning signals associated with Terrestrial Gamma-ray Flashes (TGFs) detected by the Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI) and the Gamma-ray Burst Monitor (GBM) on the Fermi Gamma-ray Space Telescope during 2010/2011. The TGF-associated lightning sferics are mainly recorded at two stations located near National Cheng-Kung University in Taiwan and near Duke University in the United States, respectively. The general features exhibited by the TGF-associated lightning sferics are consistent with previous findings that gamma-rays in TGFs are typically produced during a slow process that creates a considerable (but not necessarily) charge moment change within several milliseconds. In some cases, this slow process can be attributed to the upward negative leader during the initial stage of normal intra-cloud (IC) lightning, and it is usually punctuated by one or several fast discharges, the major one of which (i.e. TGF-related discharge) is closely involved in the gamma-ray production. The equivalent peak current of TGF-related discharges could be as high as >+500 kA, and the associated charge transfer is also considerable (typically >+20 C km). The observed complexity of TGF-associated lightning emissions can also be interpreted in the context of the initial development of normal IC lightning flashes, where the upward negative leader drives the millisecond-scale current and may also provide the seed electrons for avalanche multiplication in the upper part of active thunderstorms. Our analyses show that the thunderstorms in the land area of South China produce TGFs that can be readily observed by Fermi/GBM, and the future ground-based coordinated observations in this area would be rather promising to gain more insights into the physical mechanism of TGFs.