Spectral Hardness of X‐ and Gamma‐Ray Emissions From Lightning Stepped and Dart Leaders.

During the 2022 New Mexico monsoon season, we deployed two X‐ray scintillation detectors, coupled with a 180 MHz data acquisition system to detect X‐rays from natural lightning at the Langmuir Lab mountain‐top facility, located at 3.3 km above mean sea level. Data acquisition was triggered by an electric field antenna calibrated to pick up lightning within a few km of the X‐ray detectors. We report the energies of over 240 individual photons, ranging between 13 keV and 3.8 MeV, as registered by the LaBr3(Ce) scintillation detector. These detections were associated with four lightning flashes. Particularly, four‐stepped leaders and seven dart leaders produced energetic radiation. The reported photon energies allowed us to confirm that the X‐ray energy distribution of natural stepped and dart leaders follows a power‐law distribution with an exponent ranging between 1.09 and 1.96, with stepped leaders having a harder spectrum. Characterization of the associated leaders and return strokes was done with four different electric field sensing antennas, which can measure a wide range of time scales, from the static storm field to the fast change associated with dart leaders. Plain Language Summary: During the 2022 monsoon season in New Mexico, Langmuir Lab scientists used special X‐ray detectors to study natural lightning over a mountain‐top facility. The team detected X‐ray emissions from 4 natural lightning flashes. The X‐rays are produced by the lightning precursor stage, called leader. Leaders travel long distances in the atmosphere and carve the way for lightning to transfer charge from the cloud to ground. After careful inspection of the energy content of the X‐ray photons, the team concluded that leaders traveling in virgin air, known as stepped leaders, emit more energetic X‐rays than leaders retracing previously established lightning channels, called dart leaders. Key Points: 5‐ns resolution allowed for the measurement of individual X‐ray photon energies from natural lightning and to confirm its power‐law spectrumStepped leader X‐rays come in a train of bursts lasting milliseconds, while dart leader X‐rays come effectively in a single burstStepped leaders have a harder energy spectrum than dart leaders, with a larger population of >1 MeV photons [ABSTRACT FROM AUTHOR]