Introduction

St. Elmo's fire is a popular name for so-called coronal discharge, which frequently happens during thunderstorms. Unlike lightning, coronal discharge is much less spectacular and less transient in nature. It looks like a blueish flame or glow engulfing tall sharp objects, such as powerlines, roof pinnacles, chimneys, and lightning rods. Unless the electric field is extremely strong, the glow is only visible at night, but can still be heard during the day as a hissing or crackling sound.

St Elmo's fire, or coronal discharge, is a weakly luminous continuous discharge caused by strong electric fields near sharp objects.

Cause of coronal discharge

All matter consists of positive and negative charges. The positive charges are protons and the negative charges are the electrons. Electrons may be free in the air, while protons are usually bound to atoms or molecules, giving those atoms or molecules a positive net charge (forming ions) if some electrons are missing.

Opposite charges attract eachother, while like charges repel. At many times, the lower part of a thunderstorm is negatively charged, while the top is positively charged. The negative charge, which is nearest the ground, has most influence on the charges in the ground. The negative charges are repelled a bit, making the ground under a storm positively charged. The different charges (at ground and in the cloud) create an electric field in the air.

The electric field, when it becomes strong enough, is able to pull so hard on electrons and the positive charges in atoms that the electrons are freed from the atoms, leaving the atom ionized (an ion). In the case of coronal discharge, a strong electric field tears the molecules in the air apart (ionization of the air).

Since the electric field is greatly enhanced near sharp, conductive, protruding objects, this is the place where coronal discharge is most likely. The air in the direct vicinity of a sharp object will become ionized, and therefore slightly conductive.

Electron avalanches

The electrons freed from the ions are accelerated by the electric field. These electrons may get so much energy by being accelerated that they can ionize subsequent molecules when they collide with them. Thus, more free electrons are created that also start accelerating. A chain reaction develops, called an electron avalanche. One single electron may free thousands of other electrons, before these electrons move so far away from the region of high electric field that the reaction stops.

There can be many electron avalanches occurring simultaneously in a coronal discharge.

Electrons also recombine with molecules or ions, making negatively charged ions or neutral molecules again, and are then 'lost' in the ionization process. The recombination process produces light, and this light becomes visible as the blueish glow of corona discharge.

Danger of coronal discharge

Coronal discharge is a gradual electric discharge into the air. The current that feeds the discharge is so low that coronal discharge by itself is not particularly dangerous. However, it frequently occurs under thunderstorms when the electric field is high, and lightning is likely.

Coronal discharge also occurs briefly in the near vicinity of a cloud to ground lightning flash. If you ever felt a jerking or pulling on your arms, clothes and hair the instant lightning would strike very close by, you may have had coronal discharges developing over your body as the result of the strong electric field created by the downward propagating lightning leader. Consider yourself lucky.

As a rule, whenever you notice or hear coronal discharge nearby, run for cover immediately.

Photographing coronal discharge

Not many photos of natural St. Elmo's fire exist, so this subject is very interesting to try to photograph. The glow from the discharge is very faint however, so you should try photography at night, and relatively close to the discharge. This may be unsafe, so great care must be taken.

Try photographing at 800 or 1600 ISO, lowest f/ratio as possible (largest aperture) and use bulb (B) mode, exposing as long as possible without lightning illuminating the field. Even if you don't see any coronal discharge but suspect it, it may be worth doing a time exposure. The glow may be too faint to see with the naked eye.

Alternative ways to photograph coronal discharge is to make or buy a high voltage generator, such as a Walton-Cockcroft cascade, Van de Graaff generator, Tesla coil, Whimshurst machine, or a Marx generator. Of these, the Van de Graaff generator is easiest and probably cheapest to build and the least unsafe. The generator must be able to continuously provide the discharge current.