Balloon measurements at Stromboli suggest radioactivity contributes charge in volcanic plumes

By: Martin Airey

Volcanic lightning is an awe-inspiring and humbling display of nature’s power. It results from the breakdown of large electric fields that are generated within the volcanic plume. The processes that result in the accumulation of charge are varied and complex and by no means fully understood. Current knowledge of the key established mechanisms that are known to contribute to plume charging centre around the role played by ash. These mechanisms fall broadly into the categories of fractoemission and triboelectrification (Mather and Harrison, 2006). Fractoemission is the release of neutral and charged (electrons, positive ions, and photons) particles from fracture surfaces as magma fragments upon eruption (James et al., 2000); these particles may then interact with ash and aerosols to impart a net charge. Triboelectrification is a mechanism by which charge is transferred between the ash particles as they collide.

When charge has been produced, it must then be separated in order for an electric field to develop and a discharge to occur. The plume is a dynamic and chaotic environment, where primitive constituents of the magma, such as solid particles, gases, and metal species are mixed with atmospheric material as it is entrained by the plume. Above the initial jet region, thermal buoyancy-driven dynamics enable the plume to grow to an altitude at which neutral buoyancy is attained. Within this setting, charged aerosols and charged ash grains settle differently resulting in the separation of positively and negatively charged regions in the plume (Mather and Harrison, 2006), which can ultimately cause a discharge to occur.

But what if there are other additional mechanisms that contribute to either the charging or separation processes? As it is a complex, rapidly evolving, multiphase environment, there is the potential for many other chemical and physical interactions occurring within the plume that may currently be overlooked by this simplistic view. To test this, sensors and instrumentation developed at Reading over many years for deployment on weather balloons was combined through a NERC-funded project into a disposable modular payload called VOLCLAB (VOLCano LABoratory). The range of sensors that can be incorporated into the VOLCLAB package includes an optical backscatter droplet detector, a charge sensor, a sulphur dioxide sensor, an oscillating microbalance particle collector, and a turbulence sensor.


View from Stromboli’s summit into the vent complex showing the gas-rich plumes

In September 2017, a team of scientists from the University of Reading, Ludwig Maximillians Universität (Munich), and the University of Bath set off to Stromboli on fieldwork funded by National Geographic, equipped with VOLCLAB sensors, radiosondes, balloons, a thunderstorm detector, and lots of helium. Stromboli was an ideal choice for this expedition as it erupts frequently (several times an hour) and produces a wide range of plume types ranging from ash-rich to predominately gaseous. By launching these instruments directly into the plumes, in situ measurements may be acquired from all these plume types. The two-week long campaign required a daily hike to the summit at 900 m, often with very heavy kit. Many sensor-equipped balloons were launched from the summit with a range of success in encountering a plume, and VOLCLAB packages were deployed in fixed locations around the summit to continually record passing plumes.

   Martin Airey (holding VOLCLAB package) and Corrado Cimarelli

                 Keri Nicoll, Kuang Koh, and Martin Airey

Most interesting was the discovery of significant electric charge in plumes that contained negligible or no ash. This led to the investigation of what might be causing this unexpected charging mechanism. It is widely known that volcanoes emit a broad range of chemical products (Allard et al, 2000), one of which is radon, which is produced in high concentrations from all volcanoes. Radon is routinely monitored at many volcanoes, including Stromboli, which is known to constantly emit very large quantities through the soil near the vents, and even more during eruptions (Cigolini et al, 2009). As radon radioactively decays, it increases the charge present by ionising the air. This additional source of charge, inferred for the first time with these new direct measurements inside gaseous plumes, will inevitably contribute to the overall charge structure and may affect the likelihood of lightning strikes.

The original open access article, published in Geophysical Research Letters, may be found at: https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2019GL082211

And additional press can be found in the following links:
Science: https://www.sciencemag.org/news/2019/03/volcanic-lightning-may-be-partially-fed-earth-s-natural-radioactivity
New York Times: https://www.nytimes.com/2019/03/29/science/volcanoes-lightning-radon-gas.html
Atlas Obscura: https://www.atlasobscura.com/articles/mount-stromboli-volcano-science
The VOLCLAB package covered in Meteorological Technology magazine: 
http://viewer.zmags.com/publication/235ac328#/235ac328/56
Some footage of the fieldwork was also included in the Arte documentary “Living with Volcanoes” from around 7 minutes: 
French: https://www.arte.tv/fr/videos/069786-010-A/des-volcans-et-des-hommes-iles-eoliennes/
German: https://www.arte.tv/de/videos/069786-010-A/leben-mit-vulkanen/

References:

Allard, P., Aiuppa, A., Loyer, H., Carrot, F., Gaudry, A., Pinte, G., et al. (2000). Acid gas and metal emission rates during long‐lived basalt degassing at Stromboli volcano. Geophysical Research Letters, 27(8), 1207–1210. https://doi.org/10.1029/1999GL008413

Cigolini, C., Poggi, P., Ripepe, M., Laiolo, M., Ciamberlini, C., Delle Donne, D., et al. (2009). Radon surveys and real‐time monitoring at Stromboli volcano: Influence of soil temperature, atmospheric pressure and tidal forces on 222Rn degassing. Journal of Volcanology and Geothermal Research, 184(3–4), 381–388. https://doi.org/10.1016/j.jvolgeores.2009.04.019

James, M. R., Lane, S. J., & Gilbert, J. S. (2000). Volcanic plume electrification—Experimental investigation of fracture charging mechanism. Journal of Geophysical Research, 105(B7), 16,641–16,649. https://doi.org/10.1029/2000JB900068

Mather, T. A., & Harrison, R. G. (2006). Electrification of volcanic plumes. Surveys in Geophysics, 27(4), 387–432. https://doi.org/10.1007/s10712‐006‐9007‐2

 

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