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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - VOLTAIC (VOLcanic lighTning: a lAb and fIeld ApproaCh)

Teaser

Summary

Ash plumes are a major hazard in explosive volcanism, because they impact air traffic, infrastructures and health of the population. Detecting and quantifying remotely ash plumes is challenging, in particular in bad weather and/or at remote volcanoes. This context raised the awareness of the community to develop innovative detection techniques. One of the most promising uses monitoring of lightning produced by the plume. Several explosive eruptions (Augustine 2006, Redoubt 2009, EyjafjallajÃ¶kull 2010, Bogoslof 2017) prove the ubiquitousness of electrical activity in volcanic plumes. In addition, volcanic lightning is easily detectable remotely by antenna networks recording electrical potential changes around volcanoes, or the radio-frequencies they produce. A key issue is to determine the sensitivity of such systems. Additionally, being able to derive the volume of ash, exit velocity and grain size produced by an explosion is of major interest for the modelling of atmospheric ash dispersion.

These objectives were the core of VOLTAIC.The experimental results showed that the key parameter to monitor is the total amount of charge carried by the flashes (i.e. the cumulative magnitude of the discharges). The cumulative magnitude of the discharges is proportional to the volume of erupted ash and the pressure release during the explosion. Another key parameter is the ash grain size which controls how particles can segregate and generate clusters of opposite polarity. Field studies demonstrate that these conclusions remain valid at the scale of a real volcano, and allow an estimation of the lightning generation threshold in the specific case of Sakurajima volcano.

Work performed

1) Field studies and analysis
The weak activity of Sakurajima volcano at the beginning of the project lead to postpone the field campaigns there in May and July 2018. Following the contingency plan, this delay was used (i) to work on data from previous field campaigns and (ii) to extend the analysis to other volcanoes, namely Stromboli (Italy).
Field instrumentation consisted in plume detection setups (thermal infrared cameras and occasionally infrasound and seismic sensors) and lightning sensors (high-speed cameras, BTDs and a radio frequency antenna network). In addition to these temporary setups, we have installed 2 permanent BTDs at Sakurajima, which constitute the first attempt of continuous electrical monitoring of volcanoes.
The analysis has focused on the October 2015 Sakurajima field mission (when a radio frequency antenna network was deployed). We worked on a small dataset demonstrated the link between electrical activity and volume and jet velocity of the ash plume. Preliminary results have been presented at EGU General assembly in 2017; the rest of the analysis will be performed by our collaborator at University of South Florida, C. Smith in the scope of her PhD (expected publication on EPSL in 2019). Another extensive analysis was performed in September 2017 at Stromboli, where additional sensors were used to determine the charge of the plume, and showed that the gas itself can be charged. This changes the consolidated paradigm that used to consider ash as the only charge carrier (expected publication on GRL in 2018).

2) Laboratory experiments
Experiments involved extensive redesigning of the setup and the analysis procedure. In particular, a Faraday cage has been used to quantify the charge flux linked to the particle flow and the electrical discharges.
A fully numerical method has been developed to compute total charge borne by the experimental ash-laden jets. A database of the experimental discharges, including timing, polarity and magnitude was generated (expected publication on PANGEA in 2018). This technique, associated to the new setup, has demonstrated its reliability on experiment replicates.
The net charge of the particles flow is determined as negative and linked to the mass of particles and explosion energy. However, this quantity is not determinant to constrain the number and intensity of observed discharges, because it doesn\'t describe how charges are distributed among particles.
Another key information is that although pressure, mass and % of fines are all positively correlated with the total amount of neutralized charges, they contribute to it in different ways. While pressure and mass mainly control the discharge magnitude, the % of fines in the flow rather increases their number. This has important implications, not only on the charge-discharge within jets, but also on the use of electrical monitoring at active volcanoes. In particular, it demonstrates that the way electricity is discharged from a plume brings information on the characteristics and the behavior of the eruptive column. Not only the detection of electrical discharges can be used to locate volcanic plumes but, being able to characterize discharge properties such as length and magnitude would gain information on prevalent grain size and erupted mass in real time.
3) Training and career development of the researcher.
As scheduled in the career development plan, the researcher has been trained for designing and running experiments by the supervision and the responsible of the laboratory. The project has also allowed to develop soft skills, such as student supervision, installation of monitoring stations or project management. A skill assessment of the researcher has been performed by an external company (OPTIMEXPAT) at the end of the project.

4) Planned secondment
For e a better efficiency, the secondment has been done in the framework of a field mission at Sakurajima, where the researcher could work with Pr. Hort (Hamburg Univers

Final results

The main results show a simple relation between the eruptive parameters -namely the mass of ash in a plume, the grain size distribution, the pressure released by the eruption- and the electrical activity. Although the results of our experiments cannot be directly quantitatively extrapolated to the natural case, they largely match with field observations on the direct relationship between explosion magnitude and electrical discharge, highlighting the future potential of lightning observation for quantitative volcano monitoring.
Another major result of this project is the development of a new experimental setup and analytical method, which enables to perform robust and repeatable experiments. This will allow to explore the effects of other parameters -such as the magma temperature, gas composition, ash particle shape- on the electrical activity of a volcanic plume and can be extended at the electrification of granular flow which are relevant for geophysical flows and industrial processing.
By being the first project focused on quantifying the electrical activity of plumes, VOLTAIC has been a major step forward towards the quantitative use of lightning in volcano monitoring. Nonetheless, more work is still needed to make this technique operational, both from an experimental point of view (to explore the effects of other parameters on the lightning generation) and from a field point of view (to generalise our work to other volcanoes).