Detecting volcanic lightning with different methods: a comparison

The origin of volcanic lightning is still a matter of intense research because it may complement other monitoring techniques in detecting volcanic eruptions at remote or not well monitored volcanoes. Various methods have been used to detect electrical discharges over the last decades and here we compare four different methods for the detection of electrical discharges which were operated in late 2019 at Sakurajima volcano. The electrical activity was observed by an electric field mill (EFM, which measures the static electrical field at 10 S/s (samples/second)), a thunderstorm detector from Biral (BTD, also measures the static electrical field at 100 S/s), as well as a lightning mapping array station (LMA, measuring VHF electromagnetic radiation) and a fast antenna FA (time constant τ = 10^-4 s⁻¹, 180MS/s). We developed special algorithms to identify electrical discharges in the EFM and BTD data sets because those were the most continuous data sets available to us. From a period of high volcanic activity during which also data from the other instruments were available, we then picked several eruptions characterized by different types of electrical discharges. We analyzed the number of discharges detected by each instrument as well as how well the amplitudes (electrical field changes) are resolved by the different instruments. We find good correlations of detected signal strength between the FA and EFM data, the correlation between these data and the BTD is especially for larger events not as perfect. In order to analyze this problem, we use the FA measurements to downsample and synthetically reconstruct the BTD data set, showing deviations for larger amplitudes as well. Overall, the instruments show good agreement on detecting electrical discharges within volcanic eruptions with differences on the detection of „assumable weaker“ signals. We highlight the advantages and disadvantages of the different techniques in terms of detecting electrical discharges during volcanic eruptions. We finish discussing how the different instruments can be used to determine electric field changes as well as further investigate the type of electrical discharges detected.