By Ethan Siegel email@example.com, ScienceBlogs » Highlighted – April 19, 2010 at 09:30PM
If you are caught on a golf course during a storm and are afraid of lightning, hold up a 1-iron. Not even God can hit a 1-iron.
–Lee Trevino, golfer who actually has been struck by lightning.
Today’s astronomy picture of the day is absolutely gorgeous.
Of course, this is the Icelandic volcano, Eyjafjallajökull, erupting and spewing up volcanic ash. But, of course, the most interesting part of this picture is the lightning, which looks like it both originates and ends in the Volcanic ash itself!
(Image credit for both images above: Marco Fulle, via helicopter.)
First off, I’d like to be concrete about this. The way it looks — that the lightning originates in the volcanic ash — is exactly the way it is! This isn’t some “lucky shot” by a photographer, either. Check out this picture of Eyjafjallajökull from April 17th.
(Image credit: Snaevarr Gudmundsson, through Universe Today.)
Is it just this volcano? Eyjafjallajökull, it turns out, is not remarkable at all for having volcanic lightning. If we look at some other major recent eruptions, we can see volcanic lightning in the ash there as well. Here’s Chile’s Chaitin volcano, from its 2008 eruption.
(Image courtesy of National Geographic.)
And here’s another one: Sakurajima, from its eruption in 1991.
(Image credit: Sakurajima Volcananological Observatory.)
All told, there have been more than 150 different eruptions over the past couple of centuries where volcanic lightning has been recorded. In fact, I’ve managed to dig up some photographs of volcanic lightning from before I was born! Here’s Mount Vesuvius — and the accompanying volcanic lightning — from 1944!
(Courtesy of an old tripod.com website.)
Okay, so now you’re convinced that lightning happens in volcanic ash all the time. Yes, it makes for a spectacular picture, but how do you get this in the first place?
Well, what is lightning? You create a big enough electric potential difference between two places, and you can get all of these excess charges to “jump down” to the lower potential. In air, it takes a voltage of about 33,000 Volts (!) to get a spark to jump even one centimeter! Lightning that goes from a high cloud down to the ground can have a voltage difference in excess of a billion Volts!
You can do this because you can get a huge amount of charge separation. For example, in a big lightning strike, you separate out about 1020 electrons! But ash and rocks — even molten rock — are electrically neutral, right? So how to we get a big voltage from neutral matter?
Thankfully, the ash that comes out is hot enough so that not every particle is neutral: many are positively charged ions and many are negatively charged ions.
If you can make something push the positive ions differently than it pushes the negative ones, you can create a charge separation! If you get enough charges separated, you can make a large enough voltage to give you lightning!
This should be easy, because when you get charged particles moving around, you make electric and magnetic fields, which is exactly the ingredient you need to separate these charges. As long as these fields are here, differently charged ions are doomed to separate away from one another! And as soon as you get a large enough charge accumulated in different parts of the sky, that’s the tipping point, and then you get lightning!
And for those of you who like details with your pretty pictures and explanations, there is some uncertainty as to the exact mechanism that gives you this separation of charge. (Some ideas are here, and some research into the topic is available here.) But this really is lightning from within the volcanic ash! It certainly makes for quite a show, so enjoy it, but enjoy it from a safe distance!
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Also check out the featured ScienceBlog of the week: Eruptions