What is lightning and why does it happen in volcanic plumes ?
Lightning is an discharge occurring between two regions with different electric charges. At one point, a channel of ionized air form and progress step by step, eventually forming branching ("stepped leader"). If a positive leader meets a negative leader, a continuous conductive path is formed. The intensity of the electricity discharge increase dramatically, generating a "return stroke", much brighter than the stepped leaders.
So, to generate lightning, electrical charges must be split in two distinct regions. In thunderstorms, charges are carried by water droplets, graupel and ice. In volcanic plumes, charges might be carried by ash particles or gas. In both cases, particles have to be charged, and then separated.
1. The particle charging:
There are two main ways to charge ash particles.
The most known is called "triboelectrification" and happens when two particles rub against each other. It is the same phenomenon that occurs when a balloon is rubbed against a wool pull-over to make rise up hair.
The second one may occur when a particle is fractured, a.k.a. "fractoelectrification". One fragment may hold more positive charges and the other one more negative charges.
One of the goal of our study will be to see which effect is predominant in the charging of ash clouds.
3. The discharge:
At one point, the difference of charging between two regions of the plume becomes so important that the air between them can no longer preserve the insulation. A channel of ionized air starts to form in the negatively charged region. Ionized air being more conductive than the surrounding, the electrical current will concentrate in this channel ("leader")... and even more if it reach the ground ("return strike") ! A flash has occurred !
2. The segregation:
For some non-well understood reasons, big particles tend to be positively charged while small ones tend to be charged negatively. Due to their different mass, these two kind of particles tend to behave differently in the jet.
Imagine that you launch vertically feathers and lead beads over a fan. Lead beads will go straight while feathers will follow the vertices. That is exactly the same for ash particles. The positively charged, heavy ones will go preferentially in the center and the top of the jet, while the lightest ones will remain in the side of the jet.