Thunderstorms are natural laboratories for studying electrical discharges in air, where the vast temporal, spatial, and energy scales available can spawn surprising phenomena that reveal deficiencies in our understanding of dielectric breakdown. Recent discoveries, such as sprites, jets, terrestrial gamma ray flashes, and fast positive breakdown, highlight the diversity of complex phenomena that thunderstorms can produce, and point to the possibility for electrical breakdown/discharge mechanisms beyond dielectric breakdown theory based mainly on laboratory experiments. Here we present one such confounding discovery, termed fast negative breakdown, that does not fit with our current understanding of dielectric breakdown. Our adaptation of radio astronomy imaging techniques to study extremely transient lightning-associated events confirms that electrical breakdown in thunderstorms can begin with oppositely-directed fast breakdown of negative polarity, similar and in addition to fast positive breakdown expected from conventional dielectric theory and recent observations. The discovery of fast negative breakdown calls for an addendum to the physical description of electrical discharges in air.

This paper reports a study to understand the radio spectrum of thunderstorm narrow bipolar events (NBEs) or compact intracloud discharges, which are powerful sources of high-frequency (HF) and very high frequency (VHF) electromagnetic radiation. The radio spectra from 10 kHz to about 100 MHz are obtained for three NBEs, including one caused by fast positive breakdown and two by fast negative breakdown. The results indicate that the two polarities of fast breakdown have similar spectra, with a relatively flat spectrum in the HF and VHF band. The ratio of energy spectral densities in the very low frequency and HF bands is (0.9-5) × 10 5 . We develop a statistical modeling approach to investigate if a system of streamers can explain the main features of fast breakdown. Assuming that the current moment peak and charge moment change of individual streamers vary in the ranges of 5-10 A-m and 5-20 μC-m, respectively, the modeling results indicate that a system of 10 7 -10 8 streamers can reproduce the current moment, charge transfer, and radio spectrum of fast breakdown. The rapid current variation on a time scale of nanoseconds required for fast breakdown to produce strong HF/VHF emissions is provided by exponentially accelerating and expanding streamers. Our study therefore supports the hypothesis that fast breakdown is a system of streamers. Finally, suggestions are given regarding future streamer simulations and NBE measurements in order to further develop our understanding of NBEs and lightning initiation.

Terrestrial gamma ray flashes (TGFs) are brief bursts of energetic gammy‐ray photons generated during thunderstorms, which have been detected almost exclusively by satellite‐based instruments. Here we present three lines of evidence which includes the three out of three simultaneously observed pairs, the same occurrence contexts, and the consistent estimated occurrence rate, which indicate a direct relationship between a subset of TGFs and a class of energetic radio signal easily detectable by ground‐based sensors. This connection indicates that these gamma ray and radio emissions are two views of the same phenomenon and further enable detection of these TGFs from ground distant radio signals alone. Besides dramatically increasing the detection rate of TGFs, this ground detection approach can identify TGFs in continental and coastal areas that are at latitudes too high for present TGF‐detecting satellites and will provide more insights into the mechanism of TGF production.

The production mechanism for terrestrial gamma ray flashes (TGFs) is not entirely understood, and details of the corresponding lightning activity and thunderstorm charge structure have yet to be fully characterized. Here we examine sub-microsecond VHF (14-88 MHz) radio interferometer observations of a 247-kA peak-current EIP, or energetic in-cloud pulse, a reliable radio signature of a subset of TGFs. The EIP consisted of three high-amplitude sferic pulses lasting ≃60 μs in total, which peaked during the second (main) pulse. The EIP occurred during a normal-polarity intracloud lightning flash that was highly unusual, in that the initial upward negative leader was particularly fast propagating and discharged a highly concentrated region of upper-positive storm charge. The flash was initiated by a high-power (46 kW) narrow bipolar event (NBE), and the EIP occurred about 3 ms later after ≃3 km upward flash development. The EIP was preceded ≃200 μs by a fast 6 × 10 6 m/s upward negative breakdown and immediately preceded and accompanied by repeated sequences of fast (10 7-10 8 m/s) downward then upward streamer events each lasting 10 to 20 μs, which repeatedly discharged a large volume of positive charge. Although the repeated streamer sequences appeared to be a characteristic feature of the EIP and were presumably involved in initiating it, the EIP sferic evolved independently of VHF-producing activity, supporting the idea that the sferic was produced by relativistic discharge currents. Moreover, the relativistic currents during the main sferic pulse initiated a strong NBE-like event comparable in VHF power (115 kW) to the highest-power NBEs.

The NASA Kennedy Space Center (KSC) and Air Force Eastern Range (ER) use data from two cloud‐to‐ground (CG) lightning detection networks, the Cloud‐to‐Ground Lightning Surveillance System (CGLSS) and the U.S. National Lightning Detection Network™ (NLDN), and a volumetric lightning mapping array, the Lightning Detection and Ranging (LDAR) system, to monitor and characterize lightning that is potentially hazardous to launch or ground operations. Data obtained from these systems during June–August 2006 have been examined to check the classification of small, negative CGLSS reports that have an estimated peak current, ∣Ip∣ less than 7 kA, and to determine the smallest values of Ip that are produced by first strokes, by subsequent strokes that create a new ground contact (NGC), and by subsequent strokes that remain in a preexisting channel (PEC). The results show that within 20 km of the KSC‐ER, 21% of the low‐amplitude negative CGLSS reports were produced by first strokes, with a minimum Ip of −2.9 kA; 31% were by NGCs, with a minimum Ip of −2.0 kA; and 14% were by PECs, with a minimum Ip of −2.2 kA. The remaining 34% were produced by cloud pulses or lightning events that we were not able to classify.