Invisible lightning – a signal of an imminent eruption?

dim., 4 juil. 2021, 19:28
Volcanic lightning in an ash-rich plume during the 2010 Eyjafjallajökull eruption in Iceland (Image: Marco Fulle / Stromboli Online)
Volcanic lightning in an ash-rich plume during the 2010 Eyjafjallajökull eruption in Iceland (Image: Marco Fulle / Stromboli Online)
Explosive ash plumes often result in awe-inspiring displays of volcanic lightning. This occurs when silicate materials from rocks in a volcanic eruption break apart and they encounter each other in the middle of the turbulent eruption plume, or when ash particles rub together after being ejected. The ash particles exchange electrons, creating an imbalance in the positive and negative charges – to neutralise this imbalanced electrical field, a bolt of lightning zaps through the charged clusters, creating the spectacular volcanic lightning bolts we often observe during an explosive, ash-rich eruption.

A lesser-known type of volcanic electrical activity, known as a 'vent discharge', comprises invisible spikes of electrical activity that often occurs during the early stages of the eruptive phase. This phenomenon is a relatively new discovery in the field of volcanology and is measured using specialised equipment. Vent discharges generate a continual radio frequency (CRF) signal and are important as these invisible electrical bursts occur early in an eruption, thus may provide an early warning of impending explosions.

A recent study published in Geophysical Research Letters by Smith et al. investigated the possibility of vent discharges as an early indicator, using Sakurajima volcano in Japan as a case study.

The study analysed 97 carefully monitored explosive plumes (less than 3km high with durations less than 5 minutes) from Sakurajima volcano, from June 2015, to help show when eruptions produce visible lightning bolts versus when they produce these mysterious, invisible bursts of electrical activity (vent discharges). Smith et al. found that lightning at Sakurajima occurred in ash-rich plumes. Vent discharges, however, occurred only when ash-rich plumes with volcanic lightning rocketed skyward at velocities greater than approximately 55 meters per second.

Monitoring such discharges could be particularly helpful for early identification of ash-rich eruptions. Tracking ash is critical as it poses a major hazard to local communities and the aviation industry. As stated by Smith, the lead author of the study, electrical activity signals an ash-rich plume no matter the weather or time of day, and vent discharges provide a measure of an eruption's intensity, which could help observatories model the plume's trajectory. An improved understanding of plume dynamics through lightning and CRF from vent discharges may lead to better plume modelling and therefore advances to both civilian and aviation warnings in the long term.

Using the electrical activity of the volcanic plume to monitor eruptions is certainly not a way to replace any previous early-warning methods but rather a valid, complementary source of information that can provide additional insight about the developing eruption cloud.

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