Scientists make volcanic lightning in a laboratory
Experiments help show what triggers the dramatic displays.
Few scenes in nature give off menacing vibes as effectively as flashes of lightning dancing around a tower of ash being spewed angrily from the fiery maw of a volcano. It’s enough to catch the Eye of Sauron. Any scientist seeking to replicate this phenomenon in a laboratory might reasonably be asked to prove that his or her last name isn’t Frankenstein.
While the ability to make measurements high in the eruption plume has allowed researchers to learn about mechanisms that cause lightning there, the understandable difficulty of working near the vent of the volcano has kept science at bay when it comes to looking into the lower regions of an eruption. A team of researchers (none named Frankenstein) at Germany’s Ludwig Maximilian University led by Corrado Cimarelli built an experiment that allowed them to study the conditions that trigger lightning at the base of the eruption plume.
Their apparatus consisted of a tube that connected a chamber filled with pressurized argon gas and ash with a tank of air at atmospheric pressure and temperature. Where the tube meets that tank, a pair of ring-shaped antennas measured electric potential, an instrument recorded pressure, and a high-speed camera filmed the eruption at 50,000 frames per second.
Ash from several volcanoes was separated into different particle sizes, and the material was used in the laboratory eruptions, along with some tiny glass beads. Experiments were run with only small particles, only large particles, and both. The different ash samples, as well as the glass beads, allowed them to see whether the chemical make-up of the particles made an important difference.
There were three phases in each of their experimental eruptions, as you can see in the video above. First, the pressurized argon would start gushing out of the tube after it was released. As the ash particles followed, they spread out from the main jet because of the pressure difference. While most of the particles traveled in the same direction, smaller particles around the outside of the jet would be caught up in turbulence. As the pressure difference decreased, this turbulent region would eventually dissipate, leaving the orderly core of the jet.
It was during the second, turbulent, phase that the miniature lightning discharges flickered around the outside of the jet in large numbers. When only large particles were used, none were scattered by the turbulence and no lightning occurred. The behavior was the same no matter what the particles were made of.
So what is it about the turbulent motion? There are a number of ways in which individual particles build positive or negative charges—mostly collision interactions—but those charges need to be separated to trigger an electric discharge. The turbulence creates a lot of complex motion that inevitably clusters particles together. If one cluster happens to be dominated by particles with negative charge, and another is positive, you get lightning. When there are multiple sizes of particles, each behaves differently during the eruption, which can also help create charge separation.
Beyond understanding the lightning in eruption plumes, the researchers point out that this knowledge could help in forecasting the behavior of the ash clouds that disrupt air traffic, for example. An important factor in making accurate forecasts is knowing how much fine ash is in the plume. If tracking lightning activity near the vent could provide accurate information about changes in ash content, it would make for better forecasts.
Geology, 2014. DOI: 10.1130/G34802.1 (About DOIs).