I have written a post before about the initiation and development of lightning in clouds. This previous piece of writing focuses on the photographic evidence provided as a way of analysing lightning production while this current study focuses more on looking at accompanying events to lightning storms to see if current theories hold up. To this day we still do not know what physical mechanism lies behind lightning production in thunderclouds and we also don’t know why different types of lightning leaders (channels of ionised air which lightning will follow) are created and then propagate. Considering that lightning is not a very rare natural occurrence its creation has been described as “probably one of the biggest mysteries in the atmospheric sciences.”
One of the standing theories is that electrons are accelerated within the thundercloud, as has been an accepted fact for over a decade, which can, provided they are moving at relativistic speeds, far exceed the 1cm mean free path limit in air. When a collision with an air particle does occur it results in more high energy relativistic electrons being produced which leads to an avalanche of electrons conducting the lightning down to Earth. This relies on a primer gamma ray to start the process by providing a large energy push however. The experiment performed was to measure the secondary ground enhancement, the technical way of saying the increase in secondary cosmic rays that occur whenever a thunderstorm writhes overhead. In particular the times when the particle flux was cut short by a lightning flash were looked into (it has also been reported for particle and X-ray rates to grow up until a flash where they suddenly drop back to their original level).
Although the number of examples that could be collected was quite low the researchers feel confident enough to give a conclusion from the data. First electric fields begin to build up within a storm cloud about 4 km to 8 km off the ground. Because the number of high energy electrons produced from cosmic rays is large at this height, enough have a significant energy and so move fast enough to be sufficiently accelerated even more by the clouds (if they moved slowly their mean free path would not allow for significant acceleration). These particles, zooming through the cloud, ionise a huge volume, which eventually leads the build up of charge in certain locations. When this discharge occurs (through a still mysterious mechanism) it is directed towards the ground, making all the ionised cloud molecules neutral again. This halts the cosmic particles as they are forces to ionise the air again.
Paper links: On the initiation of lightning in thunderclouds