Volcanic Lightning: Where does it come from?

Article: Experimental generation of volcanic lightning

Authors: C. Cimarelli, M.A. Alatorre-Ibargüengoitia, U. Kueppers, B. Scheu, and D.B. Dingwell

Article URL: http://geology.gsapubs.org/content/42/1/79.full.pdf+html

Figure 1: Volcanic lightning from an eruption of Puyehue volcano, Chile. Image source

All types of lightning, especially volcanic lightning (Figure 1), are poorly understood by scientists. Because of our inability to observe lightning from within the thundercloud or ash plume, we do not know the specifics for their formation. The high concentration of ash in the eruption column prohibits direct, visual observation of the electric potential near the volcanic vent, where lightning is first observed. Once we know how lightning generation relates to volcanic eruptions, we can determine first-hand information on eruption location, eruption column structure, and the amount of ash erupted.

A group out of Ludwig Maximillian University, Germany successfully generated volcanic lightning under controlled laboratory conditions. The scientists wanted to figure out the dominant mechanism that controls particle electrification during the onset of explosive volcanism. Lightning was generated in rapid decompression experiments where concentrated ash was accelerated into a large tank of air, analogous to an active volcanic eruption. Throughout the experiment, high-speed cameras were used to visually capture lightning as it was generated within the ash cloud (Figure 2). Finally, the electric potential at the vent (where the ash was ejected into a tank of air) was measured with a pressure transducer and copper antennas.

Figure 2: A still-frame capture of lightning during the experiment. The concentrated ash at the bottom of the eruption column spreads out with increasing height. This spreading out, or decompression, of fine ash particles creates an electrical charge. This is similar to the generation of static electricity. Width of lower jet is about 6 cm. Image is from the original article. 

The experiments revealed that the number of electrical discharges directly relates to the amount of fine particles ejected. Thus, an increase in the number of fine particles ejected during an eruption results in an increase in the number of lightning strikes in the ash column. Additionally, they found that the clustering of fine particles provide a more efficient way for charge generation and lightning discharge in the eruption column. Another interesting find is that when there are two dominant particles sizes, the larger ash particles focused near the center of the jet while the finer particles were accelerated to the edge. This is an effective mechanism for the separation of positive and negative charges and for subsequent lightning generation. In an eruption, the concentrated ash particles near the vent rapidly spread apart with height and generate different types of lightning, where short lightning is generated near the vent and longer, more luminous lightning is generated higher up in the eruption column. This matches with observations of Sakurajima volcano in Japan, where an impulsive explosive eruption on February 8^th, 2010 generated short-lived, frequent lightning near the crater and longer, luminous lightning hundreds of meters high.

These experiments have opened a new way to investigate the generation of lightning within volcanic eruptions. Combining high-speed camera observations with other tools like Doppler radar, which can detect lightning hundreds of miles away in real-time, can provide early information about lightning generation at an active volcano. Measuring the lightning frequency at an erupting volcano can help us determine some eruption characteristics in real-time, such as the amount of fine ash being ejected. These ash estimates can then be input into ash forecast models for an early determination of ash transport, providing more time for hazard assessments and potential evacuations.