Article: Experimental generation of volcanic lightning
Authors: C. Cimarelli, M.A. Alatorre-Ibargüengoitia, U.
Kueppers, B. Scheu, and D.B. Dingwell
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Figure 1: Volcanic lightning from an eruption of Puyehue
volcano, Chile. Image
source
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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.
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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.
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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 8th, 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.
