First of all, a bit more eye candy. Redoubt Volcano has been erupting steadily, and whenever it’s clear (not right now) there’s something interesting to look at. I took this timelapse of a steady plume (including the image to the right) at sunset yesterday. I tacked on a couple other things at the end too.
After getting those incredible photos a few nights ago, I thought I’d try to understand a little bit about how volcanic lightning forms. I learned a lot… special thanks to the volcanic lightning scientists at New Mexico Tech for answering my questions. And in the course of looking about, I stumbled across some stunning photographs.
First of all, lightning is apparently not the result of friction inside an ash cloud. Though the contact between particles is most likely part of the story, it’s not actually the mechanical resistance (friction) during that contact that matters.
How lightning happens
Lightning is a flow of electricity between two masses that have different electric charge. Electric charge is the ratio of electrons to protons in some quantity of matter… if there are more electrons than protons, then the matter is negatively charged. And if there are more protons, then it is positively charged. Most matter, as well as the average of all matter, is electrically neutral. It has the same number of electrons as protons.
1. Starting state (particles might have already been charged by some previous process).
2. Collisions lead to charge separation. For this to happen there has to be some difference in the electrical properties of the particles in the collisions.
3. Some process, such as aerodynamic sorting, segregates the positively and negatively charged particles. This means that there are sections of the cloud that are more negative or positive than other sections.
4. When the charge separation becomes too great, electricity will flow between the positive and negative regions of the cloud forming lightning and neutralizing the charge separation.
Click on the figure to get a bigger version.
Two things have to happen to produce a bolt of lightning. There must be some process that causes charge separation between two large and widely separated masses. Also, some process must electrically connect those two masses to allow electricity to flow.
Generally the latter process is relatively simple. When the charge separation grows strong enough, it overcomes the resistance of the air and opens a channel of ionized air to conduct electricity through. The complicated part is how you get the charge separation in the first place.
How does charge separation happen? It’s fairly well understood that when two uncharged (neutral) objects with different electrical properties come into contact, that electrons can flow between them leaving one charged relative to the other. This is what happens when a balloon comes in contact with your hair. Rubbing it around ensures that more of the surface area of the balloon and your hair come into contact to maximize the amount of electricity that flows between them. This same process may well happen between particles in a lighting head or an ash cloud. Water, ice, and glassy ash particles might well exchange charge.
However, by itself this cannot produce lightning. If particles in a cloud exchange charge but remain close to each other, the cloud as a whole is still neutral. So there must be some process that separates the particles with positive charge from those with negative charge. This can happen if the particles have different aerodynamic properties.
For example, if positive charge tended to be concentrated in larger particles, those might fall faster than smaller particles. Gradually the positively and negatively charged particles would separate, with the larger particles lower in the cloud or toward the outsides of large eddies. This could create the charge separation that makes lightning possible.
The specific mechanism by which particles of differing charge are separated is unknown (step 3 in the figure). This idea of particle size segregation is just one possibility. It seems likely that other processes are involved. I exchanged email with a group of scientists at New Mexico Tech about their research on volcanic lighting. They believe they’ve observed lightning between the eruption plume and the volcano right at the start of an eruption, suggesting that there are processes that occur inside the volcano to lead to charge separation. The process by which the charge is separated is unlikely to be the aerodynamic effects described above, since the plume has little time for such processes to occur and the strong turbulence in the plume would likely re-mix the particles before much charge separation could occur. You can see more about New Mexico Tech’s Redoubt research here.
And that’s basically the story as it’s understood now. Nobody knows exactly what goes on when individual particles in the eruption plume interact, but somehow charge gets exchanged and the exchange is biased in some way so particles with different charge are segregated. This charge separation on a giant scale leads to lightning.
Sunset on the plume from Redoubt Volcano. I did some fairly major tweaking on this image, but I like the aesthetic. Might be a nice source image for artwork. Click on the image to get a bigger version.
As a reward for reading this far, or for skipping over the text, I put in a couple other time-lapse videos. I’m still experimenting with the best way to get this imagery, so you’ll see some flicker and vibration in some. For those who are wondering about the camera and so on, I’m using a Canon Digital Rebel XTi with a 70-200 L lens. The shutter timer software is EOS utilities (comes with the camera) and I’m using Sony Vegas to assemble and crop the videos. I’m hoping to use a friend’s EOS 5D for some shots this weekend when it’s supposed to be clear… we’ll see what the weather and the volcano bring!
Redoubt Volcano erupting at sunset on March 31 2009 from Bretwood Higman on Vimeo. Another nice sunset, with wind!
28 March Redoubt Eruption from Bretwood Higman on Vimeo. You can’t really see the volcano, but you can see the plume following a small explosion near the beginning.