Richard Sonnenfeld

Physics Department & Langmuir Laboratory for Atmospheric Physics

New Mexico Institute of Mining and Technology

(Photo courtesy of Harald Edens)

Understanding the Lightning Leader

APS 4Corners 10/22/2011

Abstract P1.00001 (1:30 pm)

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Lightning statistical parameters● 40 flashes/second on Earth.● I_peak=100,000 amps● V_cloud=30 Megavolts● Arc Temperature = 10,000 K● Charge transfer Q=20 coulombs● Max dI/dt ~ 10^11 Amps/s● Channel radius r ~ 1 cm● Stepped Leader velocity <0.001c● Dart Leader velocity 0.1c● Return Stroke velocity 0.5c

(Photo courtesy of Harald Edens)

(From Uman, “All AboutLightning “)

(NASA Photo)

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There are only two things to study about lightning physics …

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There are only two things to study about lightning physics …

Charging andDischarging

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Charging● How are charges distributed in storms?

● How are charges created on hydrometeors?

– Cloud physics – freezing processes● What is link between lightning and severe weather?

– Updrafts, fronts, supercells

– Hurricanes / tornadoes / volcanoes● How is lightning linked to climate?

– Land / ocean differences

– Seasonal variations

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Electrical structure of stormsCharging

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∇⋅E⃗=ρϵ0

ρ=ϵ0ΔEΔ z

ρ=ϵ0+60kV /m1000m

∼0.5nCm3

Charging

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Charges separate because

Of different terminal velocities.

Updrafts feed moisture into

The freezing zone and drive

Collisional charging

Charging

7 a

Charging is driven by updrafts – Linked to severe weather (tornadoes, hurricane eye-walls)

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Discharging● What is the speed of a leader (in the

clouds)● What is the current in a leader?● What is the charge on a length of channel?● How does leader stay hot

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● Streamers● Tendrils of ionized air● Cold processes● Driven by electric field, impact and

photo-ionization..● Quenched by recombination.

EB∼3 (MV /matm

)×p (atm)

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Leaders● Are low current

(100 Ampere) arcs.● Streamers keep

leaders hot.● Leaders are nearly

equi-potentials.● Positive leaders move

smoothly.● Negative leaders step,

and are RF noisy.

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Leader – Arc properties

● Balance self-heating,

Conductive cooling,

Recombination rate.

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B

A

Discharging● What is the speed of a leader (in the

clouds)● What is the current in a leader?● What is the charge on a length of channel?● How does leader stay hot

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Instruments

● Lightning Mapping Array.● Electric field sonde.● High Speed Video

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Discharging● What is the speed of a leader (in the clouds)● What is the current in a leader?● What is the charge on a length of channel?● How does leader stay hot

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– 10,000 Samples/s– 16-bits/Sample,– Measure 8 channels

● E-field (Channels 0-3)● Timing (Channel 4)● B-field (Channels 5-7)

A vector field-change sonde

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BA

Modeling – TDMD method

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A

B

Modeling – TDMD method160 km/s

100 km/s

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DC

-0.5 mC/m

-2.4 mC/m-1.1 mC/m

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DC

Modeling – TDMD method● “Time-dependent, multi-dipole”● RF emissions are assumed to be associated with

charge movement.● A negative charge is assumed to be deposited at the

location of the latest RF source.● A positive charge is placed on some previous source to

give the best-fit to the electric field change.

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Modeling – TDMD method

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Discharging● What is the speed of a leader (in the clouds)● What is the current in a leader?● What is the charge on a length of channel?● How does leader stay hot

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Lightning passes 200 m from ESonde

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Horizontal E-fields show negative charge passing by and creating

new leader

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Step-recoil waveClockwise

“C” on vector

plot shows negative chargemoving towardright

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Step-recoil wave

CCW “C” on vector

plot shows positive chargemoving toward

left

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Step-recoil wave

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Triggered Lightning

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091,564 us

112,657 us

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Triggered Lightning

116,563 us

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Triggered Lightning

120,157 us

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Triggered Lightning

122,813 us

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Triggered Lightning

122,969 us

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Triggered Lightning

129,688 us

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Triggered Lightning

129,844 us

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Triggered Lightning

130,000 us

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Triggered Lightning

130,156 us

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Triggered Lightning

Integrated High-Speed Video

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Luminosity vs. Channel

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Summary and Conclusions● Currents on leaders in clouds are in hundreds of

amperes (in contrast to tens of thousands in CG)● Leaders in the clouds move very slowly (about

10^5 m/s)● Leaders seem to create recoil waves as they step

forward. These waves are required by charge conservation and also probably help keep channel hot.

● Channels in intracloud lightning seem to act independently

● The wonderful complexity of natural lightning is beginning to yield to study by coordinated techniques.43

Supporting slides ...

Modeling – TDMD method

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Currents, Fields, and Energy in an IC leader

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