Observations of X-ray bursts from natural lightning flashes by TEROS

This work presents the first multi-point observations of natural Lightning Energetic Radiation (LER) in China. In summer of 2021, 3 LER events were recorded by the independently-developed Thunderstorm Energetic Radiation Observation System (TEROS). Combining with the lightning location data, low-frequency fast electric field change waveforms and return stroke peak currents, the energetic radiation characteristics of the 3 events are investigated in detail. All of the 3 LER events were detected during the last stages of the downward leaders before the negative cloud-to-ground lighting return strokes, which were typical X-ray bursts having photons with energy of MeV, and the maximum energy exceeded 2. 2 MeV. The durations of the 2 X-ray bursts accompanying the stepped leaders were hundreds of microseconds, while the duration of the other one accompanying the chaotic dart leader was tens of microseconds, which can be attributed to the difference of the developing speeds of various types of leaders. The X-ray burst with the largest return stroke peak current had the highest single photon energy, the largest total deposited energy per unit area and the largest number of burst process. While approaching the return stroke, the intensity of this event also showed an increasing trend consistent with the electric field change, indicating that the electric field at the leader tip is an important factor affecting the production of energetic radiation. With the decrease of the arrival intensity, the energetic radiation signals gradually changed from the beam mode to the single photon mode, showing the phenomenological characteristics similar to Terrestrial Gamma-ray Flashes (TGEs), and more or even all of the burst processes could not be detected. We suggest that energetic radiation may be a common phenomenon during the development of lightning.