Veldhoven -1 ERW July 2005 Problems in Lightning Physics: The Role of Polarity Asymmetry Earle Williams MIT XXVII International Conference on Phenomena in Ionized Gases (ICPIG) Veldhoven, The Netherlands July 18-22, 2005

Veldhoven -2 ERW July 2005 Outline Basic mechanism of the thundercloud Polarity asymmetry in discharges at laboratory scale and at thundercloud scale Polarity asymmetry in the behavior of cloud-to-ground lightning Lightning initiation; electron runaways and gamma radiation Polarity asymmetry in lightning flashes to seawater

Veldhoven -3 ERW July 2005 Thundercloud Charge Separation by Ice Particle Collisions

Veldhoven -4 ERW July 2005 Related Questions about Polarity Why are thunderclouds worldwide positive dipoles? Why does the Earth carry a net negative charge? Why do the majority of cloud-to-ground lightning discharges transfer negative charge to Earth? Why are sprites associated almost exclusively with positive cloud-to-ground lightning?

Veldhoven -5 ERW July 2005 All Lightning Flashes Initiated in the Cloud are Double-Ended ‘Trees’

Veldhoven -6 ERW July 2005 Vocabulary Issues NAMERANGE OF LENGTHS RANGE OF CURRENTS PHYSICAL NATURE STREAMERSMillimeters to meters Microamperes to milliiamperes Non-thermal plasma LEADERSTens of meters to tens of kilometers Amperes to kiloamperes Thermalized plasma Propagating discharges in air:

Veldhoven -7 ERW July 2005 Bi-Directional Lightning Discharge from an Airplane

Veldhoven -8 ERW July 2005 Polarity Asymmetry in Double-Ended Tree in a Laboratory Experiment Courtesy of Serge Larigaldie, ONERA

Veldhoven -9 ERW July 2005 Positive Discharge Negative Discharge

Veldhoven -10 ERW July 2005 Growth of Carrot Sprites with 1 msec Resolution Note formation of long-lasting “stationary” (non moving) features, best visible after perhaps 5 ms - “puffs” above branches – - multiple forms in center of sprite - beads, columns (note how the center effectively restructured itself)  Challenge for models to produce these stationary features Hans Stenback-Nielson University of Alaska

Veldhoven -11 ERW July 2005 Mobility Asymmetry of Charge Carriers (The Most Fundamental Asymmetry) Langevin Equation Mobility 1 / Mass Positive Carriers Ions Negative Carriers (dominant) Electrons electron Mass ion ion Mass electron > 10 4

Veldhoven -12 ERW July 2005 Bi-Directional Streamer-Leader (Polarity Asymmetry) Diverging Electrons Converging Electrons

Veldhoven -13 ERW July 2005 Critical Fields for Laboratory Scale Streamer Extension: A Key Polarity Asymmetry  Positive Streamers: E c = 5 x 10 5 V/m for P = 1000 mb = 2 x 10 5 V/m for P = 500 mb  Negative Streamers: E c = 10 x 10 5 V/m for P = 1000 mb Bazelyan and Raizer (2000)

Veldhoven -14 ERW July 2005 Bi-Directional Streamer-leader (Polarity Asymmetry)

Veldhoven -15 ERW July 2005 Asymmetry in Positive and Negative Leaders (Laboratory) Gallimberti et al (2002)

Veldhoven -16 ERW July 2005 Key Historical Developments in Discharge Asymmetry in Lightning 1938 B. Schonland identifies negative stepped leaders in streak camera observations 1960 Bidirectional streamer-leader concept (H. Kasemir) 1962 Russian work on long air gaps (Stekelnikov and Shkilev) Stepping behavior for negative leaders in the laboratory 1970’s French work at "Les Renardieres" on 10 m air gaps Image intensifier cameras document bi-directional development on negative leader tips 1989 Application of bi-directional lightning development to aircraft lightning strikes (V. Mazur) 2000 Application of lab results to lightning (No direct observations of bi-directional development on negative end of lightning tree) (Gallimberti and Bondiou; Bazelyan and Raizer) 2001 Pronounced asymmetry of VHF radiation from the lightning 'tree' with new lightning mapping systems (R. Thomas)

Veldhoven -17 ERW July 2005 VHF Radio Picture for a Normal Intracloud Flash Courtesy of Ron Thomas

Veldhoven -18 ERW July 2005 VHF Radio Picture for a Normal Cloud-to-Ground Flash Courtesy of Ron Thomas

Veldhoven -19 ERW July 2005 Polarity Contrast in Leader Behavior in Laboratory Experiments Courtesy of Bazelyan and Raizer

Veldhoven -20 ERW July 2005 Polarity Asymmetry in Leader Velocity and Current Reference: Les Renardieres Faster negative polarity requires more voltage to drive it  greater heating LEADER POLARITY GAP LENGTHRECORDED VELOCITY RANGE PEAK CURRENT Positive m x 10 4 m/s~ 1A Negative5 - 7 m10 5 m/s~ 100A

Veldhoven -21 ERW July 2005 Behavior of Rocket-Triggered Lightning  Need a stronger field to initiate a negative leader from the ground (in comparison with the more common positive leader).  Once initiated, the negative leader (and ensuing discharge) is more energetic and carries more current than the positive polarity case.

Veldhoven -22 ERW July 2005 The Polarity Asymmetry of Cloud-to-Ground Lightning Why do positive ground flashes tend to be single-stroke with a continuing current?

Veldhoven -23 ERW July 2005 Polarity Asymmetry in Lightning Stroke Multiplicity Orville et. al. (1987)

Veldhoven -24 ERW July 2005 Arcs in Air (Negative Differential Resistance)

Veldhoven -25 ERW July 2005 VHF Radio Picture for a Complex Positive Cloud-to-Ground Lightning Courtesy of Ron Thomas

Veldhoven -26 ERW July 2005 Conventional Breakdown of Air Dielectric strength – plane-parallel electrodes Sea Level Pressure E B = 3 x 10 6 V / M Dielectric strength varies inversely with density. At one density scale height (7 km) E B = 3 x 10 6 V / M = 1.1 x 10 6 V / M E

Veldhoven -27 ERW July 2005 Electric Fields in Thunderclouds and the Origin of Gamma Radiation  Why are electric fields in thunderclouds so much smaller than the dielectric strength of air?  What physical processes are producing X-rays and gamma-rays in thunderclouds

Veldhoven -28 ERW July 2005 Maximum Electric Field Magnitudes Measured in Thunderclouds ReferenceSounding Type Maximum Electric Field, Vm -1 Gunn (1948)Aircraft3.4 x 10 5 Imyanitov et al. (1971)Aircraft2.8 x 10 5 Winn et al.Rockets4 x 10 5 Winn et al.Balloons1.4 x 10 5 Weber et al.Balloons1.1 x 10 5 Byrne et al.Balloons1.3 x 10 5 Fitzgerald (1984)Aircraft1.2 x 10 5 Marshall and Rust (1991)Balloons1.5 x 10 5 Kasemir (as reported by MacGorman and Rust 1998) Aircraft3 x 10 5

Veldhoven -29 ERW July 2005 Possible Explanation for Low Electric Field Magnitudes in Thunderclouds 1. Heterogeneities in the medium (hydrometeors) lower the dielectric strength (Latham and Craib, Baker) 2. The threshold for breakdown is the critical field for positive streamer propagation (Phelps and Griffiths, 1976) 3. The threshold for breakdown is the critical field for runaway electrons (Gurevich and Zybin, 2005)

Veldhoven -30 ERW July 2005 Electric Fields in Thunderclouds (Measurements versus Theory) Gurevich and Zybin (2005)

Veldhoven -31 ERW July 2005 Positive Streamers in Air (Pressure and Water Vapor Dependence) Phelps and Griffiths (1976)

Veldhoven -32 ERW July 2005 Global Map of Terrestrial Gamma Flashes Courtesy of Space Sciences Laboratory University of California, Berkeley (2005)

Veldhoven -33 ERW July 2005 Outstanding Questions Pertaining to X-Ray and Gamma-Ray Emission from Thunderclouds Are runaway electrons a fundamental aspect of lightning initiation? OR Are runaway electrons a product of one stage of lightning-- the leader process? OR Are gamma-rays produced without any connection with lightning?

Veldhoven -34 ERW July 2005 Polarity Asymmetry in Lightning Flashes to Seawater Why do negative flashes to seawater exhibit shorter time scales and larger peak currents than positive flashes?

Veldhoven -35 ERW July 2005 Negative Flash Positive Flash Locations Locations Courtesy of W.A. Lyons

Veldhoven -36 ERW July 2005 Lightning Event Detection Over Seawater June 1 – Sept. 30, Correlated Events % Composition June 1 – Sept. 30, 2001 April 1 – Sept. 30, Correlated Events Figure 4. Histogram showing the percent occurrence of each lightning type (with polarity) for the 4664 events simultaneously observed by both GPS and LASA. D. Suszcynsky, LANL

Veldhoven -37 ERW July 2005 Hot and Cold Ends of Lightning  The negative end of lightning is the 'hot' end because the local bi-directional development provides for thermalization and hence low conductivity of air plasma  The positive end of lightning is the 'cold' end because only unidirectional streamer activity is present Working Hypothesis: The hot end in proximity to conductive seawater provides a fast ‘switch’ driving the final jump and initiation of the return strobe.

Veldhoven -38 ERW July 2005 Conclusions Polarity asymmetry is lurking in a wide variety of unsolved problems in lightning research. A more comprehensive understanding of lightning requires greater interaction between laboratory researchers and thunderstorm researchers.

Veldhoven -39 ERW July 2005 The End !!

Veldhoven -40 ERW July 2005 Initiation of a Sprite Timing based on visual images can be 10 ms off. Can models reproduce this slow sprite initiation? Frame number indicates also time in ms Courtesy of Dana Truffer-Moudra, GIUAF

Veldhoven -41 ERW July 2005 Positive Leader Streak Camera Picture

Veldhoven -42 ERW July 2005 Negative 275

Veldhoven -43 ERW July 2005 Negative Plot 325

Veldhoven -44 ERW July 2005 Negative Plot 325

Veldhoven -45 ERW July 2005 Bileader DS

Veldhoven -46 ERW July 2005 Larigaldie Simulation (500)

Veldhoven -47 ERW July 2005 Larigaldie Simulation 1600

Veldhoven -48 ERW July 2005 Larigaldie Simulation 2200

Veldhoven -49 ERW July 2005 Larigaldie Simulation 3000

Veldhoven -50 ERW July 2005 Larigaldie Simulation 5000

Veldhoven -51 ERW July 2005 DS Propagation

Veldhoven -52 ERW July 2005 Fig Elec Neg #1

Veldhoven -53 ERW July 2005 Fig Elec Neg #2

Veldhoven -54 ERW July 2005 Positive Surface Discharge

Veldhoven -55 ERW July 2005 Negative Strikes to Seawater (Detections by VLF Radiation) Jacobson and Shao (2002)