Vent Discharges Produced by Explosive Volcanic Eruptions: Characteristics, Signatures, and Volcano Monitoring Applications

Explosive volcanic eruptions often produce swarms of streamer discharges (few meters in length) at or just above the vent of a volcano concurrent with the onset of eruption. These discharges - commonly referred to as "vent discharges" - are just one type of electrical discharge that occurs over the course of an eruption. While vent discharges occur simultaneously with the onset of an eruption, small lightning discharges (tens of meters to a few kilometers in length) occur within the rising ash column seconds or minutes after eruption onset and larger lightning discharges (tens of kilometers in length) occur in the buoyant ash plume minutes or tens of minutes after eruption onset. Unlike these other types of volcanic lightning, vent discharges occur at relatively high rates (thousands or tens of thousands per second) for seconds or tens of seconds, depending on the duration of eruption. Due to these characteristics, the radio frequency (RF) signal generated by vent discharges is referred to as continual RF. By contrast, the RF signal produced by the other types of volcanic or meteorological lightning occurs in discrete bursts, such that even at high discharge rates, it lacks the long-lasting continual nature that is characteristic of vent discharges. Thus, continual RF is a distinctive signature that can be exploited in volcano monitoring applications. In addition to providing a rapid indication of an explosive volcanic eruption, the detection of vent discharges can help characterize the intensity and duration of an eruption. However, in order to exploit the continual RF signal for volcano monitoring, advances in RF sensor design are required. To enable this, we are investigating the RF signatures of vent discharges. We present measurements of continual RF that were recently made at Sakurajima volcano in Japan. These measurements include log-detected narrow-band VHF waveforms and VHF lightning mapping data. We examine the characteristics and features of continual RF impulses and explore how they can be differentiated from RF signals produced by regular volcanic lightning.