Different types of corona discharges associated with high-altitude
positive Narrow Bipolar Events nearby cloud top
Abstract
Single- and multi-pulse blue corona discharges are frequently observed
in thunderstorm clouds. Although we know they often correlate with
Narrow Bipolar Events (NBEs) in Very Low Frequency/Low Frequency
(VLF/LF) radio signals, their physics is not well understood. Here, we
report a detailed analysis of different types of blue corona discharges
observed by the Atmosphere-Space Interactions Monitor (ASIM) during an
overpass of a thundercloud cell nearby Malaysia. Both single- and
multi-pulse blue corona discharges were associated with positive NBEs at
the top of the cloud, reaching about 18 km altitude. We find that the
primary pulses of multi-pulse discharges have weaker current moments
than the single-pulse discharges, suggesting that the multi-pulse
discharges either have shorter vertical channels or have weaker currents
than the single-pulse discharges. The subsequent pulse trains of the
multi-pulse discharges delayed some milliseconds are likely from
horizontally oriented electrical discharges, but some NBEs, correlated
with both single-and multi-pulse discharges, include small-amplitude
oscillations within a few microseconds inside their waveforms, which are
unresolved in the optical observation and yet to be understood.
Furthermore, by jointly analyzing the optical and radio observations, we
estimate the photon free mean path at the cloud top to be
~ 6 m.