# Experimental Simulation of Lightning Current Discharge on Rocks
## 1 Introduction  
Lightning is a transient, high-current discharge whose path is measured in kilometers [1]. Lightning is one of nature's most spectacular sights, but unexpected and menacing at the same time. It can be extremely dangerous, presenting a major natural hazard in many different environments, from power utility companies to civil aviation, and more [2]. In recent years, with great interest in renewable energy, wind turbines have become extremely vulnerable to lightning damage [3].  
Lightning impacts not only economical activities, but also daily life. Thousands of people are killed every year by lightning bolts, while tens of thousands are injured [4; 5]. In the United States alone, damages due to lightning strikes amount to tens of millions of dollars annually [6].  
[PERSON] [7] showed that thunderstorm cells of 2 km radius would be able to produce a succession of lightning flashes at intervals of about 30 s and very intense lightning activity, with flashes at intervals of less than 10 s, would require storm cells exceeding 5 km in radius. Any cloud type can potentially cause lightning or some related form of electrical discharge, as can snowstorms, volcanoes, and dust storms [1]. Over half of flashes occur within the cloud and are called intra-cloud (IC) discharges. Cloud-to-ground (CG) lightning is the most studied because of their impact in human life [1]. Lightning strikes reach the ground on Earth as many as 8 million times per day or 100 times per second, according to the U.S. National Severe Storms Laboratory [8]. CG lightning strikes transfer about 10\({}^{9}\)-10\({}^{10}\) joules of energy. Most of the lightning energy is spent to produce thunder, hot air, light, and radio waves [9; 10]. Lightning strikes reach a speed of 94,000 km/s [9; 10; 11; 12; 13]. The peak temperature of the lightning channel is 10,000-30,000 K, which is up to five times the temperature of the surface of the Sun (the temperature of the solar interior is 10\({}^{7}\) K) [14; 15].  
When CG lightning strikes an appropriate target material such as sand, soil, or rock, the current flows through the target, heating the material to temperatures that exceed its melting point, followed by rapid cooling that results in quenching to form a glass called fulgurite [16]. About 90% of lightning flashes occur over continental landmasses as opposed to the open ocean [10; 17], with up to 10 fulgurites formed globally per second [16]. The lightning peak temperature on a target material is considerably higher than the melting point of silica (1600-2000 \({}^{\circ}\)C, depending on the moisture content [10; 18]).  
Our research on lightning-rock interactions is motivated by the desire to prevent accidents owing to lightning strikes and by the need to protect advanced ground-based and air-borne systems that use low voltage, solid-state electronics. As lightning strikes are natural phenomena, it is difficult to investigate the electrical and physical properties of lightning strikes. Rock fulgurites can be used to study the properties of the lightning strikes that created them, and in turn, help to elucidate information such as the energy distribution of CG lightning. However, the mechanism of natural rock fulgurite formation is not yet clarified. The purpose of this paper is to contribute to the study of rock fulgurite formation by: (i) simulating the lightning current discharges to generate rock fulgurites in basalt and granite specimens; (ii) measuring electrical arc properties that are relevant to rock fulgurite formation, including power, energy, and duration of discharge; and (iii) exploring the lightning-rock interaction and the origin of shock-impact glasses.