Z. H. Saleh

Co‐location of lightning leader x‐ray and electric field change sources

[2] Although X-ray emission from lightning was long predicted [Wilson, 1925], only recently was the production of X rays in cloud-to-ground lightning confirmed. Moore et al. [2001] first reported the detection of energetic radiation emissions immediately preceding the return stroke of natural cloud-to-ground negative lightning, followed by a similar discovery by Dwyer et al. [2003] for rockettriggered lightning. Dwyer et al. [2004] reported that these emissions were composed of multiple, brief bursts of X rays in the 30–250 keV range, with each burst typically lasting less than 1 ms. Further, they showed that the sources of the X-ray bursts traveled from the cloud toward the ground, supporting the view that the leader front is the source of the X rays. Dwyer et al. [2005] compared X-ray and electric field records simultaneously obtained during the stepped leaders of natural negative cloud-to-ground lightning. The conclusion from this analysis was that the production of X-rays is associated with the electric field changes accompanying the stepping of the leader that initiates the first return stroke. Although an obvious temporal correspondence was observed, uncertainties in measurement time delays and oscilloscope trigger times prevented any accurate determination of the exact temporal relationship between the X-ray bursts and the stepping of the leader. Observations of the similarity in X-ray emissions from natural and triggered lightning imply a common mechanism for different types of negative leaders [Dwyer et al., 2005]. The aforementioned discoveries have had an impact on views of lightning electrical breakdown in air, in that lightning can no longer necessarily be considered a conventional low-energy (eV) discharge, but often involves an electron distribution function that includes a significant high-energy (keV to MeV) component. These recent advancements highlight many unknowns regarding leader propagation, the stepping process, and their association with X rays. Among the most pressing of these issues are the intensity of the X rays at the source, the electric field at the leader front, the directionality and attenuation of the X-ray emissions, and the spatial and temporal relationship between the sources of X rays and leader steps. This paper addresses the issue of independently locating the sources of X-ray emissions and the corresponding leader step electric field changes via time-of-arrival (TOA) measurements, which may allow advancement on many of these issues. Leadersinbothnaturalandtriggeredlightningareconsidered.

The rarity of terrestrial gamma‐ray flashes

We report on the first search for Terrestrial Gamma-ray Flashes (TGFs) from altitudes where they are thought to be produced. The Airborne Detector for Energetic Lightning Emissions (ADELE), an array of gamma-ray detectors, was flown near the tops of Florida thunderstorms in August/September 2009. The plane passed within 10 km horizontal distance of 1213 lightning discharges and only once detected a TGF. If these discharges had produced TGFs of the same intensity as those seen from space, every one should have been seen by ADELE. Separate and significant nondetections are established for intracloud lightning, negative cloud-to-ground lightning, and narrow bipolar events. We conclude that TGFs are not a primary triggering mechanism for lightning. We estimate the TGF-to-flash ratio to be on the order of 10^(−2) to 10^(−3) and show that TGF intensities cannot follow the well-known power-law distribution seen in earthquakes and solar flares, due to our limits on the presence of faint events.