On the occurrence of volcanic lightning during vulcanian eruptions at Sakurajima volcano, Japan

Volcanic eruptions are enigmatic, not in the least because of volcanic lightning during eruptions, which is still not fully understood. In an effort to better constrain conditions under which electrical discharges occur, we use data from a multicomponent geophysical network installed in 2019 at Sakurajima volcano, Japan. The network includes one vertically scanning Doppler radar to resolve the internal structure of the eruption column at a low temporal resolution, and two fixed Doppler radar systems to resolve the dynamics of the eruption near the vent at high temporal resolution. Eruption onset times were determined using a total of 5 infrasound stations. Electrical discharges were detected by three electric field mills (EFM), a thunderstorm detector from Biral (BTD), a lightning mapping array sensor (LMA) as well as a fast antenna (FA). In addition, meteorological conditions are monitored by a local weather station and eruptions were recorded by two different cameras.

 

Sakurajima volcano is known to exhibit frequent complex eruptions/explosions often exhibiting a sequence of single pulses at intervals down to a few minutes. From our network, we generated an eruption catalog spanning 7 months of activity between June and Dec 2019. Out of this catalog, we select representative examples of multiple pulse events of different strength in terms of erupted mass, plume height, and eruption velocities. For each pulse within such a sequence, we analyzed the electrical activity as well as extracted eruption velocities, eruption volumes, plume heights, and eruption onsets among others. We find that the timing of the electrical discharges is correlated with the onsets of single pulses within a sequence. Furthermore, we can detect changes in the electrical activity ranging from times of CRF signals from small streamer discharges up to volcanic lightning while being able to track the erupted mass and plume extend at the same time. While electrical activity can increase together with eruptive strength, we also observe quite “strong” eruptions with little detectable discharges. On the other hand, less energetic eruptions are observed to produce electrical discharges, especially when the plume of a pulse rises through pre-existing ash clouds in the crater region.