The Problem of The Problem of Lightning InitiationLightning Initiation

Dr.-to-be Danyal Petersen

Desert Research Institute

Reno, NV

What is lightning?What is lightning?

• Large-scale spark-type discharge of thundercloud space charge

How does a thundercloud become How does a thundercloud become charged?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 How is charge distributed in a typical thundercloud?typical thundercloud?

from Stolzenburg (1998)

Where does lightning typically Where does lightning typically occur in a thundercloud?occur in a thundercloud?

from Thomas (2001)

Lightning InitiationLightning Initiation

Observation: Maximum observed thundercloud electric fields are 1/10th the dielectric strength of air

Hypothesis: Lightning initiation begins with corona discharges on polarized hydrometeors

CoronaCorona

Electron avalanches occur in high-field region near point electrode

Positive StreamerPositive Streamer

•Electron avalanche liberates positive space charge

•Electron avalanches ahead of space charge effectively extend positive space into electrode gap

Positive Streamer SystemPositive 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 Phelps and Griffiths model of positive streamer systempositive streamer system

Electric field Electric field modification modification

due to due to multiple multiple

overlapping overlapping streamer streamer systemssystems

Sounds Great! So, what’s the Sounds Great! So, what’s the problem?problem?

0.0

3.0

6.0

9.0

12.0

0.0 100.0 200.0 300.0 400.0

E (kV/m)

Alti

tude (

km)

Thundercloud fields still too weak!

New Idea: New Idea: Runaway Runaway

BreakdownBreakdown

Local intensification Local intensification of the thundercloud of the thundercloud electric field near electric field near polarized plasma polarized plasma extremitiesextremities

One possible outcome of Runaway Breakdown

Another possible outcome of Another possible outcome of runaway breakdownrunaway breakdown

Local intensification Local intensification of the thundercloud of the thundercloud electric field at the electric field at the base of the base of the discharge regiondischarge region

A ‘Hybrid’ HypothesisA ‘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 Hybrid mechanism for local electric field intensificationfield intensification

So, how does this initiate lightning?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 leaderMorphology of a lightning leader

Lightning as a bipolar leaderLightning as a bipolar leader

• Simultaneous positive and negative leaders, conductively connected

• Require large currents in order to maintain channel heating

Great! (waving hands)Great! (waving hands) So a piping hot bipolar So a piping hot bipolar leader suddenly forms! leader suddenly forms!

Piece of cake, right? Piece of cake, right? (well, not quite…)(well, not quite…)

Pilot ProcessPilot Process

‘‘Space’ leaderSpace’ 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 Hypothesis: Embryonic lightning leader as an isolated space leaderleader as an isolated space leader

A pilot/space leader A pilot/space leader process could process could develop in intensified develop in intensified field region such as field region such as near positive near positive streamer system or streamer system or strong runaway strong runaway avalancheavalanche

RecapRecapRunaway breakdown

Hydrometeor-initiated positive streamer systems

Pilot/space leader process

How can this hypothesis be How can this hypothesis be verified?verified?

• In-situ studies – observation of physical processes in thunderstorms

• Laboratory studies – reproduction of physical processes in controlled environment

An ideaAn idea

• Large parallel-plate electrode gap with suspended simulation ice crystal

• Interested in reproducing streamer system mechanism followed by space leader formation