Impulsive Volcanic Plumes Generate Volcanic Lightning and Vent Discharges: A Statistical Analysis of Sakurajima Volcano in 2015.

The origin of electrical activity accompanying volcanic ash plumes is an area of heightened interest in volcanology. However, it is unclear how intense an eruption needs to be to produce lightning flashes as opposed to "vent discharges," which represent the smallest scale of electrical activity. This study targets 97 carefully monitored plumes <3 km high from Sakurajima volcano in Japan, from June 1 to 7, 2015. We use multiparametric measurements from sensors including a nine-station lightning mapping array and an infrared camera to characterize plume ascent. Findings demonstrate that the impulsive, high velocity plumes (>55 m/s) were most likely to create vent discharges, whereas lightning flashes occurred in plumes with high volume flux. We identified conditions where volcanic lightning occurred without detectable vent discharges, highlighting their independent source mechanisms. Our results imply that plume dynamics govern the charging for volcanic lightning, while the characteristics of the source explosion control vent discharges. Plain Language Summary There are different types of electrical activity that occur within volcanic ash plumes. One well-known type is volcanic lightning, which creates the familiar flashes of visible light. Another, lesser-known type is called "vent discharges," which are tiny and invisible but create a peculiar signal known as continual radio frequency. Vent discharges are important because they sometimes occur very early in an eruption and may provide a way to give early warning of volcanic hazards. For this study, we used a specialized instrument called a lightning mapping array to measure the electrical activity during small eruptions at Sakurajima volcano. We explored when lightning flashes and vent discharges occurred during 97 different explosions. First, we found that the two types did not always happen in the same explosion, showing they are caused by different processes. Second, we found that vent discharges were more likely to occur when the explosive plume rose quickly, possibly because rock breakup and particles rubbing together create a static charge close to the vent. In contrast, lightning flashes occurred when the plume had high volume flux. These findings give clues about how to link electrical signals to eruptive processes for volcano monitoring. [ABSTRACT FROM AUTHOR]