Lab volcano gives lightning clues
By Jonathan Amos Science correspondent, BBC News, Vienna
This slow-motion video shows the experiment in progress
Don't do this at home. Corrado Cimarelli makes his own volcanoes that spout ash vertically at hundreds of metres a second.
The Italian is studying the awesome sight of lightning that is often observed in eruption plumes.
His "lab volcano" allows him to recreate and study the processes that give rise to the necessary electrical conditions.
The hope eventually is to learn something about the nature of volcanoes purely from their lightning behaviour.
"That's the beauty of these experiments... things that are unconstrainable in nature can be constrained in the lab" Dr Corrado Cimarelli Ludwig Maximilian University of Munich
"The lightning can tell us a lot about the structure of the eruption plume and the ash particle sizes within it," Dr Cimarelli told BBC News.
Such information could give an indication of whether a particular eruption was likely to pose a risk to aviation, he added.
The Ludwig Maximilian University of Munich researcher was speaking here in Vienna at the European Geosciences Union General Assembly.
His lab volcano is no mountain; the vent is only about 3cm wide. But it is able to reproduce the conditions that trigger volcano lightning very successfully.
The set-up is essentially a hot, pressurised metal tube from which real volcano ash particles (from Popocatepetl in Mexico) are accelerated at high speed.
Slow-motion video captures mini-lighting strikes dancing around the exhaust jet.
To get discharges in a real volcano, there needs to be a large electrical potential between different regions of the eruption cloud.
Ash particles can be charged by fracturing them and by rubbing them together.
Sakurajima volcano in Japan produces regular, spectacular flashes If the charges are big enough and are located in the right places in the plume, a bolt can jump from one location to another.
It is clear from the experiments that particle size is a critical factor. The smaller the particles, the higher the number of bolts.
"That's the beauty of these experiments," said Dr Cimarelli.
"Things that are unconstrainable in nature can be constrained in the lab. And that's what we did. We changed systematically the sizes of the material we were using and we noticed that if we decreased the grain size of the ash, we produced more flashes."
Dr Cimarelli's team is now taking the lessons learned out into the field to study lighting at Sakurajima volcano in Japan. It is the type of volcano that produces regular, spectacular flashes.
The scientists want to test the idea that you could extrapolate ash size from the frequency of lightning events.
"The size of the particles determines the time of residence in the atmosphere and the smaller they are, the longer they stay up to be carried by the winds," said Dr Cimarelli.
"This means, of course, that if you have smaller particles, those particles can be carried long distances. And this is bad news for aviation, which we all know from the Eyjafjallajokull eruption in 2010."
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