2025-03-06 ロスアラモス国立研究所 (LANL)
＜関連情報＞
- https://www.lanl.gov/media/news/0306-lightning-origins
- https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024JD042549
3次元電波マッピングと偏光観測により、稲妻の閃光は宇宙線シャワーによって発火したことが判明 3D Radio Frequency Mapping and Polarization Observations Show Lightning Flashes Were Ignited by Cosmic-Ray Showers
Xuan-Min Shao, Daniel P. Jensen, Cheng Ho, Michael P. Caffrey, Eric Y. Raby, Paul S. Graham, W. Brian Haynes, William G. Blaine
Journal of Geophysical Research: Atmospheres Published: 03 March 2025
DOI:https://doi.org/10.1029/2024JD042549
Abstract
Previous 2D radio frequency interferometric observations showed that lightning is commonly started with a positive fast discharge (+FD) before a normal negative leader continues from the origin of the +FD to the ensuing lightning flash. However, the inception and development of the +FD cannot be convincingly explained by existing discharge theories. With our new 3D broadband interferometric mapping and polarization system, we observed that the +FD was sometimes followed by an even faster and more extensive negative discharge (−FD) that propagated backward and overshoot the origin by a few hundred meters, as reported by an earlier 2D study. Surprisingly, the signal polarization, which measures the orientation of the discharge current, systematically slanted from the discharge propagation direction and rotated between the two opposite discharges, showing the +FD and −FD were driven by other storm-independent factors in addition to the storm electric field, or otherwise all would align in the same direction. Assuming a cosmic-ray shower (CRS) piercing through the cloud immediately before the +FD/−FD discharge, we found that their path is consistent with a pre-ionized path by the CRS, and the polarizations for the two opposite discharges are consistent with the respective deflected trajectories of high-energy positrons and electrons in the geomagnetic and an electric field. We further analyzed the more commonly observed +FD and showed that it is consistent with the CRS interpretation, suggesting these flashes in thunderstorms were ignited by cosmic-ray showers.
Key Points
- Lightning starts with a positive fast discharge, followed by an even faster and more extensive negative discharge observed in 3D
- Signal polarizations slant from the discharge propagation direction and rotate between the two opposite fast discharges
- These features can only be due to a cosmic-ray shower that preconditioned the discharge path and guided the discharge current direction
Plain Language Summary
How lightning is started in thunderstorms is still unclear. With our newly developed 3D mapping and polarization system, we observed that some lightning flashes were not only started with the positive fast discharge, but it was followed immediately by an even faster and more extensive negative discharge. Surprisingly, the signal polarization is slanted from the discharge propagation direction, and the polarization for the two opposite discharges rotate from each other, showing that the initiating fast discharges were not solely driven by the storm electric field. We analyzed these observations with a cosmic-ray shower (CRS) and found that these seemingly strange features can be consistently explained. A CRS provides an ionized path in the cloud that directs and enables the fast propagation of the discharges. The shower’s high-energy positrons and electrons are deflected from the shower direction in the electric and the geomagnetic field, leading to slanted discharge current, that is, the slanted polarization. The positrons and electrons are deflected differently in the electromagnetic field, and they respectively affect the two opposite discharges, explaining the polarization rotation between the two discharges. This concept can also explain the common cases that only involve the fast positive discharge, and therefore the initiation for most lightning flashes.
