A new study led by researchers from the Institute of Atmospheric Physics of the Chinese Academy of Sciences (CAS) has uncovered the first observational evidence of lateral negative re-discharges occurring on negative leader channels. Published recently in Geophysical Research Letters, the finding offers new insights into how lightning channels remain electrically active, and how their structures evolve before and after a return stroke.

Prior to this research, negative-polarity lateral breakdowns had only been observed near the tips of positive leaders—never documented along negative leader channels.

Using a self-developed very-high-frequency (VHF) lightning interferometer, which boasts sub-microsecond temporal resolution and spatial accuracy on the order of several tens of meters, the research team captured this phenomenon during a positive cloud-to-ground lightning event on the Qinghai-Tibet Plateau. By combining VHF interferometric imaging with ground-based electromagnetic measurements, the team confirmed that short-duration lateral re-discharges repeatedly appeared along pre-ionized negative channels—both before and after the return stroke.

Before the return stroke, the positive leader advanced steadily while the negative leader weakened. During this phase, small needle-like lateral re-discharges formed along nearly the entire horizontal negative channel. These short bursts propagated toward the negative leader tip at an approximate speed of 8 × 10⁴ m/s, with VHF radiation intensity comparable to that of needle discharges typically seen near positive leader tips.

After the return stroke, the data revealed rapid discharges along existing negative channels, and new lateral breakdowns extending into previously un-ionized air. These post-return-stroke events quickly lengthened the negative channels and helped sustain long-duration continuing current by preserving channel conductivity.

"What really caught our attention," said Prof. QIE Xiushu, the study's corresponding author, "was the link between these re-discharges and the evolution of channel potential. Gradual potential changes before the return stroke tended to reactivate weakened branches, while sharp increases afterward triggered new breakdown paths."

The results indicate that the rate or magnitude of channel potential change plays a controlling role in determining whether a lateral re-discharge reactivates an existing path or initiates a new one. This mechanism resolves uncertainties about how negative-polarity channels evolve around the return stroke, and how conductivity is maintained during the continuing current phase.

The study received support from the CAS Strategic Priority Research Program, the National Natural Science Foundation of China, and other funding sources.

Field installation of the lightning observation instrument by the research team. (Image by XU Chen)