From MariachiWiki

by Josh Seidman

Summary

Lightning is a common natural occurring phenomenon that has intrigued mankind for thousands of years. Yet, the exact mechanism of how it happens is vastly unknown. Even though Benjamin Franklin successfully proved that lightning is an electrical phenomenon, scientists to this day have yet to ascertain what process actually causes lightning. The electric field necessary to spontaneously ionize air and conduct electricity, which is 3000 kV/m (Hong, 2000) far exceeds the electric field of a typical thundercloud, which is 100 – 300 kV/m, (Uman, 1969). How can discharge happen at electric fields that are one order of magnitude lower than required? A possible explanation is that cosmic ray showers create the necessary ionization to start the process (Gurevich and Zybin, 2005).

It is believed that clouds charge by a process known as induction charging. Within clouds warm water droplets rise while cold ice crystals sink. When particles, such as these, collide they transfer electrons leaving some of the particles positively charged and others negatively charged. Due to the forces of updrafts and gravity these opposing charges become separated leading to the production of an electric field (Wahlin, 1989). A cosmic ray shower can provide enough ionization to lower the threshold to induce a lightning discharge. Cosmic rays are atomic nuclei with a wide range of energies traveling at nearly the speed of light throughout the universe (Friedlander, 2000). During the decent of these cosmic rays through the Earth’s atmosphere they collide with molecules, typically nitrogen or oxygen causing a shower of secondary and tertiary particles, some of which fall through storm clouds (Dwyer, 2005). A powerful electric field, such as the electric field formed within a thundercloud, can greatly accelerate electrons produced by ionization during the passage of millions of secondary and tertiary particles in the cosmic ray shower. These electrons, called runaway electrons, can be accelerated to high velocities causing a discharge called runaway breakdown.

Once lightning is initiated, the electrons create an ionized channel called a stepped leader. At each step the electrons accumulate at the leaders tip, creating an intense localized field that accelerates more runaway electrons. Once the stepped leader, which can split into branches, dives into the ground, a large pulse of current can flow through this channel. The current heats the air up to 30,000 degrees Celsius, causing the flash of visible radiation called the return stroke (Dwyer, 2005).

The purpose of this research is to search for a relationship between cosmic ray and lighting activity. We use components of the MARIACHI detector for this purpose. A scintillator based shower detector is used to detect particles while a radio receiver and an Electric Field monitor detects electrically induced events such as lightning. Any simultaneous events seen by the two systems would validate the hypothesis that cosmic rays are responsible for triggering lightning.