Portable Antenna System for Electric Field Monitoring During Volcanic Eruptions: First Results from Sakurajima Volcano, Japan

Volcanic lightning and the electrification of ash plumes have the potential to significantly impact volcanic ash-induced hazards and the fluid dynamics of the eruption column. Despite being well-known phenomena, there is still a lack of systematic quantitative observations relating electrical variations to plume dynamics, and specifically tailored sensors for the electrical monitoring of volcanoes remain scarce (Cimarelli & Genareau, 2022). Regionally deployed Very Low Frequency (VLF) antennas are designed for long-range thunderstorm detection and often prove inadequate for detecting lower-intensity volcanic discharges near the vent (Vossen et al., 2021). While Very High Frequency (VHF) antennas are more efficient in volcanic lightning detection (Behnke et al., 2014), particularly when deployed as an array, they are typically custom-designed and expensive.

We present a newly developed low-cost, so-called slow antenna system designed to detect and quantify electric field changes associated with volcanic activity. The instrument utilizes a flat metal plate antenna coupled with a charge amplifier circuit that converts electrostatic induction into a proportional voltage. The signal is then digitized and logged via a Raspberry Pi equipped with a 32-bit analog-to-digital (AD) converter. The system currently achieves a sampling rate of 19.200 Hz, enabling the detection of electrical processes that exceed the resolution of conventional monitoring systems. Our design prioritizes portability, scalability and cost-efficiency to facilitate deployment at remote volcanoes.

To validate our system, we are preparing a two-week field campaign at Sakurajima volcano, characterized by persistent explosive activity and frequent generation of volcanic lightning. Our data processing workflow involves: (1) event recording; (2) data storage and retrieval; (3) de-drooping corrections to reconstruct true changes of the electrical field; and (4) instrumental response calibration to convert voltage measurements into absolute electric field values. Additionally, we are benchmarking our antenna against the commercially available Previstorm system.

Preliminary results demonstrate the instrument's capability to capture rapid (0.1 ms) electric field transients associated with explosive events. This work establishes a foundation for the broader deployment of cost-effective electric field monitoring. Specifically, deployment as an array would enable the temporal reconstruction of discharges within the eruption column (Behnke et al., 2014), providing a crucial complementary dataset for early warning systems and plume dynamics studies. Future work will focus on correlating electric field signatures with multiparametric monitoring data to better constrain eruption mechanisms and enhance hazard assessment.

 

Behnke, S. A., Thomas, R. J., Edens, H. E., Krehbiel, P. R., & Rison, W. (2014). The 2010 eruption of Eyjafjallajökull: Lightning and plume charge structure. Journal of Geophysical Research: Atmospheres, 119(2), 833–859.

Cimarelli, C., & Genareau, K. (2022). A review of volcanic electrification of the atmosphere and volcanic lightning. Journal of Volcanology and Geothermal Research, 422, 107449.

Vossen, C., Cimarelli, C., Bennet, A., Giesler, A., Gaudin, D., Miki, I., Iguchi, M., & Dingwell, D. B. D. (2021). Long-term observation of electrical discharges during persistent Vulcanian activity. Earth and Planetary Science Letters, 570, 117084.