Modification of lightning quasi-electrostatic signal by mesospheric halo generation

• Measurements presented of 1–50 Hz electrostatic field change from lightning.
• Lightning detection sites separated by 120 km observe the same flashes.
• The shape of lightning flash signals occasionally differed between sites.
• Difference explained by generation of mesospheric halo.
• Halo detection when conditions unfavourable for optical instrumentation.

Current induced by the electrostatic component of lightning on a conductor exposed to the atmosphere was measured at 100 Hz by detectors at two sites in southern England. These detectors were separated by 120 km and capable of lightning detection to a range of approximately 100 km. Signals from the same flash detected by both receivers were generally of the same shape, as expected if the source was charge neutralisation by lightning of relatively small horizontal extent. For approximately 1% of flashes detected the signal shape received at both sites was considerably different, indicating that the small vertical dipole approximation was not sufficient to explain the detected charge reconfiguration during the flash. This discrepancy was explained by the addition of a short pulse of horizontally extensive charge above the flash, consistent with a mesospheric halo. The effect of this halo superimposed on the signal from the parent lightning is described. The observations support previous evidence that quasi-static current measurement can be used to identify flashes which produce halos and so complement existing optical halo detection instrumentation.