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 University) on coupling radar with electrical and thermal data.
5) Project management
The project was primairly managed by Dr. Corrado Cimarelli and Dr. Damien Gaudin (technical and scientific part) and Mrs. Heggmaier (financial part) with the help of the staff from the department.
