Radio Emission Reveals Inner Meter-Scale Structure of Negative Lightning Leader Steps

Natural sources of intense ultra-high-frequency radiation in high-voltage atmospheric discharges

We study the sources of intense ultra-high-frequency (UHF) radiation (in the frequency range 1-6 GHz) arising during the development of high-voltage atmospheric discharges. The discharges were initiated in a long discharge gap by applying an approximately 1-MV pulse with positive or negative polarity. By employing a radio registration system based on ultrawideband antennas, we managed to localize the UHF radiation sources in the discharge with centimeter accuracy and investigate their temporal and spatial correlation with the discharge structures. The vast majority of the localized sources turned out to be concentrated in the near-electrode regions. It is found that the generation mechanism of intense UHF radiation in a laboratory discharge cannot be unambiguously associated with such basic processes as the head-on collision of opposite-polarity streamers or the interaction of single streamers with the near-electrode plasma at the surface of metal electrodes. We discovered that the observed UHF emission appears basically as a precursor of the intense plasma development in a certain discharge region, whereinto a bright counterstreamer comes a bit later. The findings were confirmed by the statistical observations and results of imaging the dynamics of the discharge structures with a nanosecond temporal resolution.

Electromagnetic emissions in the MHz and GHz frequency ranges driven by the streamer formation processes

We provide comprehensive data on the spectral and temporal characteristics of low-frequency (LF) (MHz) and high-frequency (HF) (GHz) radio emissions and investigate their correlation with the streamer formation. We show that the propagation of streamers from the cathode is accompanied only by the LF radio emission (10-150 MHz). In contrast, the HF radio emission (1-4 GHz) arises during the travel of counterstreamers from the anode, which is also indicated by radio interferometric measurements. The power of the LF radio emission sharply increases almost synchronously with that of the HF radio emission. We find that the HF radio emission has a complex spectral and temporal structure and appears as multiple short (less than 1 ns) bursts characterized by various frequency components, existing in subnanosecond time intervals.

Branching Morphology of Negative Leaders with Different Propagation Directions in Natural Lightning

Comparing the branching features of negative leaders with different propagation directions could provide insight into the common tendency of development pathways and the formation pattern of branches in natural lightning. This paper reports an upward negative leader (UNL) and a downward negative leader (DNL), and their branching features are analyzed and compared. The UNL is classified into vertical (UNL-V) and horizontal (UNL-H) segments based on propagation directions at different stages. The downward negative leader (DNL) is classified into main (DNL-M) and secondary (DNL-S) channels based on whether the channel is ultimately connected to the upward connecting leader. The vital angle parameters characterizing the branching morphology are investigated. For the strong branch eventually forming a section of the main channel, its deflection angle conforms to the lognormal distribution with a mean range of 22–36°. The included angle between the branches and the deflection angle of weak branches conform to the normal distribution with means close to 40° and 60°, respectively. Moreover, the velocity for four categories of negative leaders decreases noticeably by two or more branching behaviors in a frame interval of about 80 μs. In particular, similarities in branching morphology have been found in UNL-H, UNL-V, and DNL-S, with a semblable distribution in deflection and included angles. Statistical results indicate that branches of DNL-M tend to follow the previous direction of leader development, and the branching behavior has minimal impact on its velocity.

Separate luminous structures leading positive leader steps

The physics governing the propagation of lightning leaders and long spark leaders is still not well understood. Positive and negative leaders seem to behave differently. Negative leaders develop in a step manner, guided by the separate luminous structures termed space stems and space leaders. Positive leaders, on the other hand, are generally thought to have no separate luminous structure involved in their propagation. However, a separate luminous structure observed in a positive leader discharge had been reported in recent literature, suggesting that positive leaders may similarly do steps to negative leaders under certain conditions. Here we report the observation of the positive leader step led by a separate luminous structure at high humidity in laboratory lightning-like discharges. We also found the streamer-like common zone connecting the primary leader channel with the separate luminous structure, as well as the bi-directional development of the separate luminous structure. We hope that these findings would contribute to a better understanding of the nature underlying positive long spark leaders and lightning leaders.

Streamer formation processes trigger intense x-ray and high-frequency radio emissions in a high-voltage discharge

For a laboratory discharge initiated in a long air gap by a microsecond megavolt pulse, we simultaneously register wideband high-frequency microwave and hard-x-ray emissions and thoroughly analyze the temporal relationship of the emissions depending on the discharge evolution. The temporal structure of microwave radiation is found to consist of numerous short intense bursts with high-frequency components. We directly show that x-ray and microwave emissions can appear almost synchronously in the discharge but only when a complex net of countless plasma channels forms and spans the entire discharge gap. The channel formation is closely related to the intense development of multiple streamers.

The Relationship of Lightning Radio Pulse Amplitudes and Source Altitudes as Observed by LOFAR

When a lightning flash is propagating in the atmosphere it is known that especially the negative leaders emit a large number of very high frequency (VHF) radio pulses. It is thought that this is due to streamer activity at the tip of the growing negative leader. In this work, we have investigated the dependence of the strength of this VHF emission on the altitude of such emission for two lightning flashes as observed by the Low Frequency ARray (LOFAR) radio telescope. We find for these two flashes that the extracted amplitude distributions are consistent with a power-law, and that the amplitude of the radio emissions decreases very strongly with source altitude, by more than a factor of 2 from 1 km altitude up to 5 km altitude. In addition, we do not find any dependence on the extracted power-law with altitude, and that the extracted power-law slope has an average around 3, for both flashes.

Synchrotron mechanism of X-ray and gamma-ray emissions in lightning and spark discharges

X-ray and γ-ray emissions observed in lightning and long sparks are usually connected with the bremsstrahlung of high-energy runaway electrons. Here, an alternative physical mechanism for producing X-ray and gamma-ray emissions caused by the polarization current and associated electromagnetic field moving with relativistic velocity along a curved discharge channel has been proposed. The existence of fast electromagnetic surface waves propagating along the lightning discharge channel at a speed close to the speed of light in vacuum is shown. The possibility of the production of microwave, X-ray and gamma-ray emissions by a polarization current pulse moving along a curved path via synchrotron radiation mechanism is pointed out. The existence of long tails in the power spectrum is shown, which explains observations of photon energies in the range of 10–100 MeV in the terrestrial gamma-ray flashes, as well as measured power spectrum of laboratory spark discharge.

A distinct negative leader propagation mode

The common phenomenon of lightning still harbors many secrets such as what are the conditions for lightning initiation and what is driving the discharge to propagate over several tens of kilometers through the atmosphere forming conducting ionized channels called leaders. Since lightning is an electric discharge phenomenon, there are positively and negatively charged leaders. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily build for radio-astronomy observations. It is observed that a negative leader rather suddenly changes, for a few milliseconds, into a mode where it radiates 100 times more VHF power than typical negative leaders after which it spawns a large number of more typical negative leaders. This mode occurs during the initial stage, soon after initiation, of all lightning flashes we have mapped (about 25). For some flashes this mode occurs also well after initiation and we show one case where it is triggered twice, some 100 ms apart. We postulate that this is indicative of a small (order of 5 km[Formula: see text]) high charge pocket. Lightning thus appears to be initiated exclusively in the vicinity of such a small but dense charge pocket.