Optical and electromagnetic observations of wave shapes of continuing currents in sprite-producing positive cloud-to-ground lightning

Abstract:

A charge transfer from cloud to ground in lightning flashes is carried out by three different mechanisms: lightning return strokes, continuing currents and M-components. The continuing current is a continuous mode of charge transfer, which usually lasts tens of milliseconds. The M-component is observed as an increase in luminosity of the channel during the occurrence of the continuing current. The charge transferred to the ground from the cloud altitude is reflected in the value of the charge moment change, which plays a crucial role in setting conditions for the initiation of sprites.

We selected several sprite events captured by a low-light camera installed in Nýdek, Czechia. We studied details of luminosity curves for their causative positive lightning strokes recorded by an array of high-speed photometers deployed at 14 places in Czechia in the frame of the European fireball network. All selected events were simultaneously detected by several photometer stations. We have found that all luminosity curves belonging to sprite producing lightning strokes contain clear signatures of the continuing current. Sometimes, we can also clearly identify the luminosity enhancements related to the M-components. We combine the local optical measurements with the remote electromagnetic recordings in the ELF (Hylaty station, Poland) and VLF (Rustrel, France) bands. We have found that the optical wave shapes of the continuing current significantly alter from event to event. We have also found a strong similarity of the luminosity and current moment curves derived from the ELF magnetic field measurements for all analyzed events. It indicates that it might be possible to predict the occurrence of sprites based on the wave shapes of lightning luminosity.