Volcanic Lightning
May 2, 2009

Over the last twelve months, there have been several well-documented examples of volcanic lightning. It has become one of my favorite natural phenomena, evoking the titanic destructive power of our planet in a pretty striking way. The rest of this post is just pictures of volcanic lightning from the last twelve months and links to even more pictures. If you’re interested in how volcanic lightning works, here’s the post I wrote about it a few months ago.

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The Electrostatic Two-Step
February 22, 2009

chaiten

I went to a talk a few weeks ago by this guy, about the spontaneous electrostatic charging of particles. Here’s the talk summary:

The electrostatic charging that occurs when two surfaces rub (“triboelectric” or “contact” charging) is one of the most well known phenomena in science. Everyone has noticed it at some point when they walk across a rug and then get a shock when they touch a doorknob, and everyone is familiar with the elementary science demonstration of rubbing a balloon on their head and then seeing how it charges the balloon and the hair.

This electrostatic charging plays an important role in atmospheric and space sciences; e.g., [lightning] is caused by charged ice particles, particle charging alters the flows of wind blown sand and snow, and charged dust causes problems for space missions to the moon and other planets. Electrostatic charging also has consequences in industry, both beneficial and harmful; e.g., the operation of photocopiers and laser printers is based on charged toner particles, and explosions at gas pumps can be caused by sparks due to the charge that builds up on a person that has rubbed against a car seat while exiting the car.

Despite the widespread importance of triboelectric charging, there is no scientific explanation of how the charging occurs. Our work aims to improve this understanding.

In the talk, Professor Lacks presented a very straightforward and simple model for how a bunch of chemically identical electrically neutral dust particles can spontaneously build up huge amounts of charge through collisions. The basic requirement is a range of dust grain sizes, with each grain having electrons on its surface trapped in high-energy molecular states. The grains are therefore electrically neutral, but not quite in equilibrium: they want to still have the same amount of electrons, but to have them in lower-energy states.

The way to reach this equilibrium is through collisions. If two grains collide in the right way, they can transfer surface electrons back and forth, so the higher-energy electron on each grain can move to the other grain and settle in a lower-energy state there. One might expect this process to maintain a neutral charge on average, since each grain is both gaining and losing an electron in every collision. The trick is that, once a few collisions have occurred, the smaller grains will have given away most of their higher-energy electrons, so when they collide they no longer have anything to give, but they can still receive an electron from a larger grain. Thus, in the aggregate, the model predicts a tendency for smaller grains to spontaneously accrue negative charge, and for larger grains to accrue positive charge. This matches with observations, such as electric fields in dust devils and in volcanic plumes. Nice work, Lacks et al.!

Of course, their model describes only the first step to reaching equilibrium. The second step is for enough of this charge to build up to ionize the air around the dust, which allows for sparking and/or lightning. After the lightning, the charge is uniformly distributed again, and hopefully the electrons are in lower-energy states than when they started. It’s a rather shocking way of achieving equilibrium, but it works.