What makes St. Elmo's fire glow

Energy levels of a hydrogen atom (yellow is lowest energy, red highest). The nucleus is shown as the center blue blob, and an electron (green) is shown at its highest energy state, farthest from the nucleus. It will not last long in this state; soon it will emit a photon, and return to its lowest energy state (yellow ring).

First, a word about the nature of electrons.  We used to picture electrons orbiting around the nucleus like planets about the Sun.   In 1913, Neils Bohr used Einstein’s quantum idea to devise a better atom model. Bohr said only discrete electron orbits are allowed —the ones corresponding to the discrete values of the electron’s energy.  But electrons don't have to stay in a particular orbit. 

An atom can absorb energy easily enough if a light particle (photon) hits it with just the right amount of energy to kick an electron from one of its energy levels to a higher one. (That’s how a material blocks light.) Electrons in the next band of energy levels, the conduction band (orange in the drawing), are free to move about within the lattice of atoms (leaving their "parent" atoms, and forming ions).  The electrons drift in the direction of the potential difference, towards the positive charge.  That's how charge moves through a conductor. 

The electron doesn’t get to keep the energy, though. Pretty soon the excited electron loses the extra energy by giving off a photon (or many lower-energy photons) and, in one (or many steps), comes back down to its original rest-energy level — the only stable place for it to exist. 

A photon (pink wavy line) zaps an air atom and bumps the atom's electron to a higher energy level. Drawing courtesy of Ernest Galbrun.

Back to St. Elmo's fire.  The drawing depicts a magnified view of the pin, charged to a high voltage.   (How to build up charge on the pinpoint:   You'll probably need a Tesla coil to generate high enough voltages. The action of the coil creates a charge (for example, negative) and raises the voltage of the pin to a high value (60,000 volts, for example).  Like charges repel.  So, the negative charges try to get away from each other and those on the pin tend to crowd into the point of the pin, where they were blocked from further travel by the air gap.)  (Click for details why charges pack into sharp points.) 

The concentration of negative charge in the small space at the point causes the electric field to increase locally to at least 30,000 volts per centimeter in a tiny region (less than 0.1 cm diameter, shown by the dotted-line circle) near the pinpoint between the pin and ground. An electric field of 30,000 V/cm is enough to break down air

The air starts to breakdown when a photon with the right amount of energy hits an air atom and kicks one of the atom's electrons into a higher energy level where it becomes a free electron able to move within a conductor.  With the loss of one of its electrons, the atom becomes a positively-charged ion. 

The strong electric field is a force that acts on charges much like gravity acts on masses.  The electric field accelerates the charges  — the small-mass electron to high speeds, and the larger ion not so fast.  Moreover the field from the negatively-charged pinpoint accelerates the positive ion toward the pinpoint and the electron away from the pinpoint.  Thus, the field separates the electron from the ion, and the separation prevents the electron and ion from recombining (just yet). 

Electron avalanche.  Drawing courtesy of Ernest Galbrun.

The accelerating electron hurtles into a nearby atom and blasts another electron into a higher-level orbit, thereby producing another free electron and, consequently, producing another ion.  The field then accelerates and separates the new pair.  The separation of charge / ionization process repeats. 

Soon this repeated generation of electron-ion pairs cascades into an electron avalanche, which creates a gas-like mixture of positively-charged ions and negatively-charged electrons in the small region.  This mixture is called plasma.   In the drawing, the orange stars illustrate inelastic collisions that cause secondary atoms' electrons to become excited.

Atom recombination and upkeep of the discharge.  Drawing courtesy of Ernest Galbrun.

The massive positive ions (pink dots in the drawing) slowly migrate toward ground, as they discharge the negative charge on the pin.

As ions and electrons eventually recombine (where the field is weaker) outside the tiny region off the pinpoint  to form atoms, the electrons give off energy (in the form of photons) to return to their original lower rest-energy levels.  The light radiates into the surrounding air.  That glow is the light of St. Elmo's fire.

(Answered Aug. 10, 2009)