The Problem of Lightning Initiation Dr.-to-be Danyal Petersen Desert Research Institute Reno, NV

What is lightning? Large-scale spark-type discharge of thundercloud space charge

How does a thundercloud become charged? Charging occurs at updraft/downdraft interface, where mixed phase exists Involves collisions between rimed ice (graupel) and pristine ice Rime charging Saunders (1998)

How is charge distributed in a typical thundercloud? from Stolzenburg (1998)

Where does lightning typically occur in a thundercloud? from Thomas (2001)

Lightning Initiation Observation: Maximum observed thundercloud electric fields are 1/10 th the dielectric strength of air Hypothesis: Lightning initiation begins with corona discharges on polarized hydrometeors

Corona Electron avalanches occur in high- field region near point electrode

Positive Streamer Electron avalanche liberates positive space charge Electron avalanches ahead of space charge effectively extend positive space into electrode gap

Positive Streamer System In strong E fields, positive streamers intensify and branch As streamer system accumulates positive charge, equal negative charge is deposited in wake

Phelps and Griffiths model of positive streamer system

Electric field modification due to multiple overlapping streamer systems

Sounds Great! So, what’s the problem? Thundercloud fields still too weak!

New Idea: Runaway Breakdown

Local intensification of the thundercloud electric field near polarized plasma extremities One possible outcome of Runaway Breakdown

Another possible outcome of runaway breakdown Local intensification of the thundercloud electric field at the base of the discharge region

A ‘Hybrid’ Hypothesis Runaway breakdown followed by hydrometeor-initiated positive streamer systems This relaxes the required field intensification due to runaway breakdown, allowing positive streamer systems to make up the difference

Hybrid mechanism for local electric field intensification

So, how does this initiate lightning? This is the ‘hand waving’ part of most lightning initiation explanations, going something like …”And then a miracle (lightning) happens”. Clearly, local intensification of the electric field can happen. But is it really that simple?

Morphology of a lightning leader

Lightning as a bipolar leader Simultaneous positive and negative leaders, conductively connected Require large currents in order to maintain channel heating

Great! (waving hands) So a piping hot bipolar leader suddenly forms! Piece of cake, right? (well, not quite…)

Pilot Process

‘Space’ leader One of the pilots becomes heated enough to remain conductive Continued bipolar streamers further heat and extend pilot, becoming ‘space’ leader

Hypothesis: Embryonic lightning leader as an isolated space leader A pilot/space leader process could develop in intensified field region such as near positive streamer system or strong runaway avalanche A pilot/space leader process could develop in intensified field region such as near positive streamer system or strong runaway avalanche

Recap Runaway breakdown Hydrometeor- initiated positive streamer systems Pilot/space leader process

How can this hypothesis be verified? In-situ studies – observation of physical processes in thunderstorms Laboratory studies – reproduction of physical processes in controlled environment

An idea Large parallel- plate electrode gap with suspended simulation ice crystal Interested in reproducing streamer system mechanism followed by space leader formation