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Atmospheric science in a bolt of lightning

Lightning during a thunderstorm in Toronto, Canada. [Credit: John R. Southern, via Wikimedia Commons]

Lightning is beautiful, frightening, and destructive. When a difference in electrical charge builds up between clouds and the ground (or another cloud), that charge can balance itself by discharging suddenly and violently. It’s one of the most powerful electrical phenomena we can encounter in our daily lives (at least those of us who live in regions where thunderstorms aren’t too rare). During the discharge, the atmosphere along the lightning bolt can reach 30,000º C, several times hotter than the surface of the Sun. That’s an incredible amount of electrical energy.

Air is a very poor conductor of electric current. Partly that’s a matter of density — the nitrogen and oxygen molecules in air are spaced too widely to allow electrons to pass between them — but it’s also the structures of the atoms. Conductors have very loosely-bound electrons, so they can move around with a bit of encouragement, allowing current to flow. But air doesn’t admit electrical currents: it’s an insulator, also known as a dielectric.

Dielectrics are commonly used in capacitors: devices for storing and manipulating electric charge. Capacitors consist of two layers separated in space with a difference in charge between them, so a cloud and the ground serve as a ginormous capacitor. Under ordinary circumstances, the charge stays put: nothing passes between the capacitor layers. However, if the voltage is large enough, the dielectric can’t hold anymore, and charge flows through it catastrophically. That phenomenon is known as dielectric breakdown, which is also my techno dance music stage name.

In a solid insulator, the discharge can destroy the material; in air, dielectric breakdown is lightning. The effect is (ha!) striking: the electrical energy is sufficient to split molecules apart, separate electrons from atoms, and heat the air to incandescence.

I was prompted to think and write about this topic when Mike Brown shared a remarkable photo, taken by French astrophotographer Denis Joye. (Unfortunately, I haven’t figured out a way to get permission to repost his photo here, so follow the link and look. I’ll wait.) Joye didn’t just photograph lightning: he used a diffraction grating to split the light into its component colors. When he did so, he was able to identify emissions from several types of atoms that make up our atmosphere: nitrogen, oxygen, and hydrogen.

Under ordinary conditions, we wouldn’t see emissions from any of these atoms. To make them glow, you need either a strong electric current flowing through them, or very high temperatures, neither of which are present ordinarily. But it’s also a matter of atmospheric chemistry: our air is made primarily of molecular nitrogen (N2), with most of the rest comprised of molecular oxygen (O2) and water vapor (H2O). Yet, Joye’s spectra showed atomic nitrogen, atomic oxygen, ionized versions of both of them (meaning they were missing one electron), and atomic hydrogen. None of these ordinarily exist in our atmosphere, because temperatures are cold enough that atoms like to clump into molecules, and Earth’s gravity is too weak to hold in hydrogen unless it’s stuck to a heavier atom like carbon or oxygen.

Molecules are bound together by the electric force, which also keeps electrons in atoms. Lightning is powerful enough to break those bonds, ionizing nitrogen (the lines labeled NII in the image) and splitting molecules into their component atoms. The hydrogen alpha (Hα) and beta (Hβ) lines are more often seen in the spectra of stars or nebulas, yet here they are in our own atmosphere — for a few moments, at least. This photo is an astounding illustration of how we can identify chemical components through light alone.

Lightning on Jupiter, as seen by the Pluto-bound New Horizons probe. [Credit: GSFC/NASA]

Mike Brown is best known to most of us for killing Pluto, but he also studies Jupiter and its moons. He was hunting for spectral photographs of lightning while investigating to see if it could be done for Jupiter. The giant planets, like Earth, have common lightning storms, which can reveal some of the details about their atmospheres and clouds. (Mars also has lightning, though it’s associated with dust storms, since clouds are rare on the Red Planet.) I suspect the answer to Mike’s question is yes, our telescopes are good enough to analyze the spectra of light created by lightning on Jupiter. What will we learn? Let’s look and find out.

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1 Response to “Atmospheric science in a bolt of lightning”



  1. 1 The week in review (August 25-31) | Bowler Hat Science Trackback on August 31, 2013 at 09:05
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