What is Lightning?

Lightning is essentially a BIG electrical spark. Air is normally a very good electrical insulator. However, if the voltage is high enough, air can become an electrical conductor. Through a rather complex process, a path of electrically conducting air, called a plasma, can propagate from a high voltage terminal (or a highly charged region within a storm cloud) towards a region of lower potential. During propagation, the current flowing through the channel is typically amperes or tens of amperes. The conductive channel propagates at a rate that's much faster than the speed of sound. Once the plasma channel (called a leader) "connects" to the opposite terminal (or ground or another cloud), a huge current - typically tens of thousands of amperes in the case of lightning- then flows through the spark channel, intensely heating the plasma within the channel to temperatures as high as 10,000 - 50,000 degrees K.

For small sparks, the spark makes a small click. Larger currents can cause a snap, or even a loud bang. Lightning creates a BIG bang, but we hear the sound coming from various parts of the giant spark at different times as the longer lasting sound of rolling thunder. The intense heating in the center of the spark channel causes the gases to radiate light and heat as "black body radiation" - a continuous spectrum of light that spans the infrared, visible, and ultraviolet. The heated molecules and atoms generate the light you see in lightning. Because of the supersonic breakdown process and huge pulse of current that flows when the leader "connects", lightning always generate a shock wave and thunder in any gaseous atmosphere. The gas mixture can be air (on Earth) or other mixtures of gases. Lightning has been observed on a number of other planets in the solar system

It IS possible for a spark to jump across a vacuum, but the process is significantly different than for a discharge within a gas. If the electrical field is great enough, electrons can literally be ejected from the negative terminal (cathode) in a process called field emission. This is often a sudden, explosive process that occurs on small high-field points (called asperities) on the surface of the cathode. Once free, these electrons are accelerated towards the opposite electrode (anode), acquiring lots of kinetic energy (velocity) in the process. When the high velocity electrons collide with the anode, they can be energetic enough to dislodge additional electrons and ions (positively charged metal atoms) from the anode. The ions are attracted back to the cathode and acquire energy from the electrical field. When they collide with the catjode, they can liberate more ions and electrons. Once the gap is full of a plasma made from free electrons and metal ions, a visible conductive plasma path between the electrodes is formed. This can pass large amounts of current (a vacuum arc) that radiates light and heat.

A simple spark discharge or even a hot arc is not sufficient to trigger nuclear reactions. However, under the right conditions, a high energy flow of charged particles can induce nuclear reactions. For example, a particle accelerator uses electrical fields to accelerate negative or positively charged particles. Note that a flow of charged particles is (by definition) an "electrical current". This flow (or particle beam) can consist of electrons, protons, larger ions, positrons, or other more exotic charged particles.

If you accelerate charged particles to a sufficiently high energy and then allow them to slam into matter, you can induce nuclear reactions. For example, electrons accelerated to 8 MeV or higher will induce neutron activation within carbon, forming the radioactive isotope C11, with a half-life of 20 minutes. Higher beam energies have even greater "impact" on target nuclei, stimulating higher-energy reactions, including fission, transmutation, or creation of other short-lived particles. Note that it's not the high voltage or heating per se, but the absorbed kinetic energy of the accelerated particles that triggers these nuclear reactions.