(Photos courtesy of (1) David Sidwell and (2) Gene Ott of South Carolina Reptiles and Amphibians, respectively) Upper photo: St. Elmo's-like fire (corona discharge) glowing from a 60,000-volt Tesla coil. Nikola Telsa was an inventor who, along with George Westinghouse, brought us alternating current power distribution. Bottom photo: A lightning-shattered sharp-pointed snag, similar to the one Chris saw. (Photos used with permission.)
A Saintly Light Explained
SEPTEMBER 10, 2009April Holladay, HappyNews ColumnistQ: Why would a lightning-struck tree glow after being hit? It is not on fire and does not give off heat, but gives off a very visible glow.
Chris, Calhoun, Missouri
A: It was a dark and stormy night... I wasn't there but Chris emails that he was walking in the woods towards dusk "a little after a thunderstorm" when he noticed the tree. One side of the tree, shattered by an earlier lightning stroke, stabbed the night like a broken pike. An eerie glow extended upward from the tree like tapered torchlight from the tip. But there was no heat. It was not on fire. Chris could clearly see the glow from about six feet away. The phenomenon persisted for many minutes.
If our atmosphere were made of neon, the tree would have glowed orangey-red, like a neon sign. Instead, the tree tips lit blue-violet, as electrons bombarded nitrogen and oxygen atoms of the air, exciting them and causing the air to glow.
Lucky Chris. Few land lubbers witness St. Elmo's fire in nature. It's a more common sight among sailors, who may give thanks to St. Elmo, patron saint of sailors, when they see blue fire off mast tips. They've survived a storm.
St Elmo's fire is a self-sustaining, continuous spark of electricity, called a coronal discharge.
It's a "very, very slow spark, but going out in all directions," emails physicist Erik Ramberg of Fermilab. When voltage gradients are just right, a clear ionization path doesn't form between the charged object and ground potential. Instead, the electrons "essentially meander out of the electrode."
We produce St. Elmo's fire routinely in a neon tube or a copy machine. And you can generate the phenomenon yourself.
On a dry day, put on a pair of rubber-soled shoes. Scuffle across an acrylic carpet. The friction of your shoes generates negative charge and raises your voltage to at least 3000 volts. When you move your hand closer than a tenth of a centimeter to a doorknob, the voltage gradient (which is a measure of the electric field) is greater than 30,000 volts per centimeter. This is the field strength that breaks down air. Zap! A spark discharges your negative charge through ionized air to the doorknob.
But that's not St. Elmo's fire because the spark wasn't sustained. Scuffling across the carpet, however, may not generate enough charge to do the job, especially if the air is too humid. You'll probably need a Tesla coil to generate high enough voltages. If you, like Bloomfield, built one in your garage, you're all set. The figure shows the results — St. Elmo's fire glowing from the sharp point. The action of the coil created a charge (for example, negative) and raised the voltage of the pin to a high value (60,000 volts). Like charges repel. So, the negative charges tried to get away from each other and those on the pin tended to crowd into the point of the pin, where they were blocked from further travel by the air gap.
The concentration of negative charge in the small space at the point caused the electric field to increase locally. So the voltage gradient and the electric field is strong — easily 30,000 V/cm just off the pinpoint.
The strong electric-field force at the pinpoint accelerates electrons and protons of the air molecules away from each other. The result (called a plasma) is a gas-like mixture of positively-charged atoms (ions) and electrons that conducts electricity. The process of separating charges is called ionization.
The negative charge conducts through the plasma and eventually to ground in a gradual manner (we'll discuss how in a moment). The discharge process imparts energy to the air molecules and atoms, which causes them to glow. Click for the details of what makes St. Elmo's fire glow.
The air only in the immediate vicinity of the pinpoint turns into plasma because the charge is intensely concentrated only there. Outside a small radius about the pinpoint the charges are too widely spaced to create an electric field gradient large enough to ionize the gas.
The spark to ground (St. Elmo's fire) persists, because the ionization process controls its own radius of ionization with a sort of negative feedback. If the field gets too large at the pinpoint and creates too much charge, the excess of free charges piles around the pinpoint. The free charges around the point essentially enlarge the conducting pinpoint because the charges can conduct electricity, too. Now the shape of the pinpoint conductor is less sharp and the resulting electric field weakens, which slows the production of new free charges.
This self-correcting generation of charges (too much charge lessens the field, which lessens the generation of charge) provides a nice, steady supply of charge in the air near the pinpoint, which sustains the ionization and the resulting glow.
Chris saw St. Elmo's fire emanating from the lightning-struck tree trunk. The violence of the thunderstorm created a strong electric field acting on the air — strong enough to generate St. Elmo's fire off the sharp points of the shattered tree trunk.
"I suspect," emails physicist Louis A. Bloomfield of the University of Virginia, "that one role of the lightning strike was to carbonize parts of the tree," which then became electrically conducting. Carbon is an electrical conductor. When lightning struck the tree it formed a carbon track through the wood. The electrically conducting tree standing in the strong electric field of the thunderstorm was really like a "giant pin staring up at the sky and forming corona discharges."
The ground below the storm was still electrically charged and a high voltage existed between the negatively-charged cloud overheaad and positively-charged ground. The tree, rooted in the damp ground and electrically conducting, became positively charged. The shattered ends of the tree provided sharp points. The positive charges of the tree crowded into the tips of the shattered tree points, as like charges repelled each other and spread out.
The electric field, consequently, was extremely strong off the tips, strong enough to ionize the air there and provide a conducting path toward the negatively-charged cloud.
"The tree is sending an electric current upwards towards the storm [cloud]," emails research engineer William Beaty of the University of Washington. "And since this current is made from positive-charged air, there'd also be a slight wind blowing upwards off the glowing 'flames' on the broken wood. Sometimes electric currents occur as actual moving substances."
The sharp-pointed tree tips enabled a self-sustaining spark that lasted many minutes. Long enough for Chris to see an eerie glow.
About, about, in reel and rout
The death-fires danced at night;
The water, like a witch's oils,
Burnt green, and blue, and white."
- Rime of the Ancient Mariner by Samuel Taylor Coleridge, 1798
Safety hazard: "Note that such corona only can exist when the electric-field in the air is extremely intense, and so another lightning bolt might occur at any moment. If you see corona flames, or if your hair rises, immediately take shelter in a building or car." -- William Beaty
I am indebted to physicist Louis A. Bloomfield of the University of Virginia for his discussion of how to generate an electric field by scuffling your feet and then using a pin to create a spark.
Phosphor glows: Why, What, How long, WonderQuest, 2003
Why electrons don’t crash into the nucleus, WonderQuest, 2003
How atoms got their energy, WonderQuest, 2006
How charge flows through metals, WonderQuest, 2006
Science Hobbyist by Bill Beaty
What causes the strange glow known as St. Elmo's Fire? Is this phenomenon related to ball lightning? by William Beaty, Scientific American, September 22, 1997
Happy NewsHow Everything Works: Making Physics Out of the Ordinary, by Louis Bloomfield (Wiley, 2008).
The Fire Of St. Elmo by Keith C. Heidorn, The Weather Doctor
Conceptual Physics, by Paul G. Hewitt (Addison Wesley, 2008)
(Answered Aug. 10, 2009)