Shock tactics

From aircraft safety to extra-terrestrial life – how understanding lightning associated with a volcanic eruption is key to many breakthroughs. Here we learn how Dr Karen Aplin is beginning to uncover the mechanism of these awesome displays

Volcanic plumes play host to some of the most spectacular displays of lightning on the planet but, whilst there are many theories, the exact mechanisms behind these natural light shows, and why some volcanoes see more lightning than others, are a mystery.

In a recent study published in Physical Review Letters researchers from Oxford University, and the Universities of Bristol and Reading, investigated what role volcanic ash particles rubbing together might play in making lightning bolts.

Lightning in volcanic plumes is not completely understood, but there are several ideas about how it might work. Lightning is a giant spark that fires when large quantities of electric charge build up. Thunderclouds are made of water, ice and hail, and the electrification arises from collisions between light particles (ice) moving upwards in the cloud and heavier water drops falling. One way lightning in volcanic plumes could be generated is from ash particles colliding with each other. This is similar to the electric charge that can be generated by rubbing a balloon on a jumper.

Another way lightning can occur in volcanic plumes is from some volcanoes, like Eyjafjallajökull in Iceland, that have glaciers on top of them. When the volcano erupts, the glacier melts which both causes huge floods and makes a big cloud of ice and water that becomes mixed with the volcanic plume. This mixture of cloud and plume can generate volcanic lightning by ice, water, and ash collisions in what's called a 'dirty thunderstorm'.

We were interested in measuring the electric charging from ash particles colliding with each other. To do this, we dropped small samples of ash through a tube and measured the electric charge on the ash when it lands on a detector at the bottom of the tube. As the ash falls it rubs against other ash particles, and transfers electric charge.

This experiment, carried out under controlled conditions in the lab at Oxford Physics, is the closest we can get to copying how the ash behaves in a volcanic plume. We took special care to make sure unwanted effects, such as the ash rubbing against the sides of its holder, were as small as possible so that we only measured the electric charge generated from the ash rubbing against itself.

The ash samples we tested were from the Icelandic volcanoes Grímsvötn and Eyjafjallajökull, and were generously given to us by the Icelandic Met Office. With the help of our colleagues in Earth Sciences and Geography at Oxford we measured the sizes of the ash particles, and sieved the ash to separate out the particles of different sizes, so we could understand how the size of the ash affected the electric charge transferred.

We found that some ash particles became more electrically charged than others, and that the size of the ash particles was important. If the particles have a wide range of sizes, they charge better than particles that are all of similar size.

[caption id="attachment_35842" align="alignright" width="200"] Credit rachelcifelli/Flickr[/caption]

Particles with the biggest difference in size from the largest to the smallest charged best of all. This suggests that frictional charging from particles rubbing together will be quite efficient in the plume near the volcano, where lightning is observed.

We also found that ash from different volcanoes had different electrical properties. For example, the ash from Grímsvötn charged up much more easily than the ash from Eyjafjallajökull. This is particularly interesting, as the Grímsvötn eruption produced much more lightning than Eyjafjallajökull. We don’t yet know why this is, but our lab measurements may be a first step towards understanding what part the properties of the ash play in volcanic lightning.

These findings could help in the remote sensing of volcanic activity. Firstly, motivated by the dramatic lightning from the Grímsvötn eruption, the Icelandic Met Office is experimenting with lightning detection to provide additional early warning of eruptions. A better understanding of how volcanic plumes become charged could reduce false detections of volcanic lightning, and perhaps provide more information on the progress of the eruption.

[caption id="attachment_35955" align="alignleft" width="200"] Ash particles from Icelandic volcanoes used in the study.[/caption]

Secondly, our results explain our previous curious observations, obtained with weather balloons, of electric charge within plumes distant from the volcano. This charge was unexpected, since the electricity on the particles when they were near the volcano should decay away relatively quickly.

Our results show that when the ash particles of different sizes rub against each other anywhere in the plume, they become electrified. Knowing that all plumes are very likely to be charged, no matter how far away from the volcano, means that we can start to develop new techniques to distinguish between clouds and volcanic plumes, the separation of which is crucial in mitigating hazards to aircraft.

Our findings are not limited to just terrestrial volcanoes. They are another step towards confirming that volcanic lightning occurs in planetary atmospheres. One reason why this is important is that chemicals generated by lightning have been linked to the origins of life, so any planet that might have lightning may yield fundamental answers to some of the biggest questions there are. Electromagnetic signals from lightning also look promising for detecting volcanic activity as space probes fly past.

Adapted from The Oxford Science Blog