Lightning is one of the most impressive, normaly experienced geophysical phenomenon. It produces the brightest light and the loudest sound commonly occurring on Earth. It is estimated that there are 30–100 cloud and cloud-to-ground lightning discharges per second worldwide. This means approximately 9 million discharges per day worldwide. Even if the first research on lightning dates back to the half of 1700 with Benjamin Franklin, this phenomenon is not yet well understood. Recent studies suggest new theories on the starting and the dark-time of lightning before reaching the Earth.
Lightning’s seemingly random occurrence in space and time and the wide range of its significant time variation, from tens of nanoseconds for many individual processes to almost a second for the total discharge, and its obscuration by the thundercloud producing it makes lightning particularly difficult to study [1].
In 1746 Franklin began his laboratory experiments with electricity, made possible by a familiarity with the frictional charge-separation mechanism occurring when dissimilar materials are rubbed together and the invention earlier that year of the Leyden jar, a capacitor to store electrical charge. In 1749 Franklin described the similarities between lightning and the laboratory sparks he had created, and in 1750 he published the design of an experiment involving vertical metal rods insulated from the Earth intended to prove that lightning was electrical. Among Franklin’s many accomplishments was his measurement showing that the major charge in the lower part of a thunderstorm was negative, his laboratory experiments having led him to define the sign of electrical charge [1].
Until the late nineteenth century understanding of the main aspects of the phenomenology of cloud-to-ground lightning were unknown, consider that it was only about 30 years ago that transient luminous events were discovered to occur in the rarified air above thundercloud tops; and only about 20 years ago that it was found that high-energy phenomena (runaway electrons, X-rays, and gamma rays including the terrestrial gamma ray flashes observed on orbiting satellites) are produced by both thunderclouds and lightning [1].
Lightning can be defined as a very long electrical spark. Most lightning is generated in thunderstorms and is characterized by a length of 5–10 km, at the extreme over 100 km. In a thunderstorm, the primary charge transfer process is thought to involve collisions between soft hail (graupel) particles that are heavy enough to fall or remain stationary in the thunderstorm’s updrafts and small crystals of ice that are light enough to be carried upward in those updrafts, all in the presence of super-cooled water droplets [1].
To produce the primary thundercloud these ice-hail interactions must take place at altitudes where the temperature is generally between -10°C and -20°C. After charge has been transferred between the colliding ice and hail particles, the positively charged ice crystals are carried further upward in updrafts to the top part of the thundercloud, to an altitude near 10 km above sea level in temperate summer storms; while the negatively charged hail resides at an altitude of 6–8 km. Thus, the idealized primary charge structure of an isolated, mature thundercloud consists of many tens of Coulombs of positive charge in its upper portions and a more or less equal negative charge in its lower levels. In a typical thundercloud, a small positive charge is also found below the main negative charge, at altitudes where the temperature is near or warmer than freezing.
The most common cloud-to-ground discharge, downward lightning carrying negative charge, may well begin as a local discharge between the bottom of the main negative charge region and the small lower positive charge region beneath it. This local discharge would serve to provide free electrons previously immobilized by virtue of their attachment to hail and other heavy particles. Because of the electron’s small mass, free electrons are extremely mobile, compared to the heavier ionized air atoms or molecules, or to charged hail, ice, or water particles which are essentially stationary on the time scale of lightning. Hence free electrons are the primary contributor to the lightning current. In negative cloud-to-ground lightning, the free electrons over-run the lower positive charge region, neutralizing a significant fraction of its small positive charge, and then continue their trip toward ground. The physical mechanism for moving the negative charge to Earth is a propagating electrical discharge called the “stepped leader”. The stepped leader’s movement from cloud to ground is not continuous, but rather it moves downward in discrete luminous segments of tens of meters length, then pauses, then moves another “step”, and so on.
Recently, an interesting explanation for the “stepped-leader” has been proposed [2] to understand why lightning proceeding to the ground by successive luminous steps separated by ‘dark’ times of many microseconds and why the structure of the dark column connecting the streamer step with the cloud that can be km in length, is electrically conducting, yet has a very low sustaining electric field.
These phenomena can be explained by the accumulation of singlet delta metastable oxygen molecules excited in the corona pulses of lightning. The step time is necessary for the excitation of large metastable densities to produce significant metastable detachment of electrons from negative ions. The detached electrons form a highly conducting step, initially luminous, that causes a redistribution of electric fields and an increase in the potential and electric fields at the end of the step to make possible a further step by ionization.
The conducting regions develop within them very low electric fields. Successive steps are likely to combine to form the long conductive columns of lightning that exist before the return stroke.
These results have important practical implications, in the design of new methods for protect people and object from lightning, especially in times of climate change, when thunderstorm and lightning are everyday more frequent.
[1] The physics of lightning; Joseph R.Dwyer and Martin A.Uman – Physics Reports; Volume 534, Issue 4, 30 January 2014, Pages 147-241.
[2] Toward a theory of ‘stepped-leaders’ in lightning; John J Lowke and Endre J Szili – Journal of Physics D: Applied Physics; 2023 – 56, 045201.
