Pete's Stuff

Neat Stuff on the Intertubes: Ka-Boom!

So what the heck is this, caught on video one night in August 2007? Hint: it's not a evening fireworks display.

Click through and play the video to find out. (NB: I have lost the original source for the video and would love to give credit to the author, if I could! Unfortunately there is no name mentioned in the video.)

It's a fantastic example of a form of upper atmospheric lightning called "gigantic jets". These are electric discharges associated with thunderstorms that move into the rarified upper atmosphere, instead of downwards into other clouds or the earth's surface. There's currently a debate on Wikipedia about how to name this and related phenomena; since the vastly different physical conditions produce displays that don't resemble traditional lightning at all, the term "Transient Luminous Event", or TLE, has been proposed.

Whatever they're called and however impressive they look, it's not particularly mysterious why they occur. A thunderstorm is a violent place, with strong updrafts and downdrafts, uncountable raindrops and chunks of hail smashing into each other. All this friction tends to release electrons from the combatants' surfaces; the same effect as briskly rubbing a balloon on a wool sweater. The larger pieces tend to accumulate electrons and become negatively charged. Of course, these are exactly the same pieces that are most affected by gravity, and as they fall to earth they carry electrons along with them. The net result is that the earth's surface below a storm becomes negatively charged, and the cloud above it becomes positively charged. This is precisely why lightning happens, and why lightning is most intense in or near areas that have both a great deal of turbulence (more friction = more electrons) and heavy precipitation (which separates the electrons from the cloud). It also explains why the main bolt of a lightning strike travels from the ground to the cloud - the electrons are just going back where they came from.

This isn't the only way that electrons can be separated from their original homes, though. The same principle can take place inside a cloud, leading to the top of the cloud being more positively charged than the bottom (and strong updrafts can even reverse this). So lightning can also occur between two parts of the same cloud, or between the positive part of one cloud and the negative part of a different one. This is no surprise to anyone - in fact, it's estimated 80% of lightning occurs intra- or inter- cloud. However, there is another type of lightning that's far rarer, but still observed. It happens when the arc avoids the complex interior of the cloud and connects directly from the top of a positively charged cloud to the ground, often many miles away from the actual precipitation. These so-called "superbolts" can often be ten times as powerful as a typical bolt, or more, due to the large amount of charge separation that's being rectified.

Schematic of known TLEs (From Wikipedia)

But wait, there's more! It turns out that the upper layers of the earth's atmosphere, particularly in the upper mesosphere to the lower thermosphere (also known as the ionosphere) also carry a small net negative charge. So in rare cases, that's the target of an electric discharge from a positively charged cloud (or to the cloud, technically). The physical conditions are very different from regular lightning, though - the distances are much greater and the air is extremely rarified and becomes ionized by solar and cosmic radiation at an altitude above 60 miles. This gives a very different appearance to these events, having much more in common with the glow of neon tubes and auroras than with our familiar lightning.

The two most common types of events are jets and sprites. Jets come in many sizes and shapes, some intense and comet-like as in the diagram to the right, and some taller and branching as in the video. In general jets begin near the top of the thunderstorm cloud and tend to be blue or green in color due to the fluoresence of nitrogen molecules. Sprites occur at higher elevations and tend to be larger, with complex shapes and a reddish color, due to atomic oxygen. These are the same two colors most commonly seen in the aurora borealis, and for good reason, as it's due to the molecular and atomic makeup of the upper atmosphere. The final TLE shown above is the ELVES - manifested as an expanding ring of fluoresence at the base of the thermosphere. However, they're extremely faint and short-lived, on the order of a few milliseconds.

Sprites and jets have an interesting history. Ever since the radical increase in the volume of air travel in the 1950s, commercial pilots, who intentionally fly at cruising altitudes above most weather, had reported seeing flashes of light above storm systems during night flights. For many years, these reports were explained away as optical illusions, tired eyes, or pure fabrications - but doubtless they retained some credence among cockpit occupants. It wasn't until 1994 that the first reliable video was taken of a luminous event above a storm system, and in fact that video was taken here at the University of Minnesota, of a summer thunderstorm located close to Duluth. The 150-mile separation provided a better, side-on perspective on the storm, enhanced by the curvature of the earth's surface and paralleled in the low-horizon location of the gigantic jets in the video. Since then, TLEs have been repeatedly captured on video and firmly established as a real, studyable phenomenon.

However, there's still not a great deal of available data, so it's not clear how common these events are and how bright they can become. Nevertheless, it's probably worth keeping on eye on any visible thunderstorms the next time you're on a redeye flight. You don't want the pilots to have all the fun, do you?