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Problems in Lightning Physics: The Role of Polarity Asymmetry

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. Outline. Basic mechanism of the thundercloud

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Problems in Lightning Physics: The Role of Polarity Asymmetry

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  1. Problems in Lightning Physics:The Role of Polarity Asymmetry Earle WilliamsMIT XXVII International Conference on Phenomena in Ionized Gases (ICPIG) Veldhoven, The NetherlandsJuly 18-22, 2005

  2. 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

  3. Thundercloud Charge Separationby Ice Particle Collisions

  4. 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?

  5. All Lightning Flashes Initiated in the Cloud are Double-Ended ‘Trees’

  6. Vocabulary Issues Propagating discharges in air:

  7. Bi-Directional Lightning Discharge from an Airplane

  8. Polarity Asymmetry in Double-Ended Tree in a Laboratory Experiment Courtesy of Serge Larigaldie, ONERA

  9. Positive Discharge Negative Discharge

  10. 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

  11. Mobility Asymmetry of Charge Carriers(The Most Fundamental Asymmetry) Langevin Equation Mobility 1/Mass Positive Carriers Ions Negative Carriers (dominant) Electrons electron Massion > 104 ion Masselectron

  12. Bi-Directional Streamer-Leader(Polarity Asymmetry) DivergingElectrons ConvergingElectrons

  13. Critical Fields for Laboratory Scale Streamer Extension: A Key Polarity Asymmetry • Positive Streamers:                     Ec = 5 x 105 V/m      for  P = 1000 mb                        = 2 x 105 V/m     for P = 500 mb • Negative Streamers:                   Ec = 10 x 105 V/m   for P = 1000 mb Bazelyan and Raizer (2000)

  14. Bi-Directional Streamer-leader(Polarity Asymmetry)

  15. Asymmetry in Positive and Negative Leaders (Laboratory) Gallimberti et al (2002)

  16. Key Historical Developments in Discharge Asymmetry in Lightning

  17. VHF Radio Picture for a Normal Intracloud Flash Courtesy of Ron Thomas

  18. VHF Radio Picture for a Normal Cloud-to-Ground Flash Courtesy of Ron Thomas

  19. Polarity Contrast in Leader Behaviorin Laboratory Experiments Courtesy of Bazelyan and Raizer

  20. Polarity Asymmetry in Leader Velocity and Current Faster negative polarity requires more voltage to drive it  greater heating Reference: Les Renardieres

  21. 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.

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

  23. Polarity Asymmetry in Lightning Stroke Multiplicity Orville et. al. (1987)

  24. Arcs in Air (Negative Differential Resistance)

  25. VHF Radio Picture for a Complex Positive Cloud-to-Ground Lightning Courtesy of Ron Thomas

  26. Conventional Breakdown of Air Dielectric strength – plane-parallel electrodes Sea Level PressureEB = 3 x 106V/M Dielectric strength varies inversely with density. At one density scale height (7 km) EB = 3 x 106V/M = 1.1 x 106 V/M E

  27. 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

  28. Maximum Electric Field Magnitudes Measured in Thunderclouds

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

  30. Electric Fields in Thunderclouds(Measurements versus Theory) Gurevich and Zybin (2005)

  31. Positive Streamers in Air (Pressure and Water Vapor Dependence) Phelps and Griffiths (1976)

  32. Global Map of Terrestrial Gamma Flashes Courtesy of Space Sciences Laboratory University of California, Berkeley (2005)

  33. 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?

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

  35. Negative Flash Positive FlashLocations Locations Courtesy of W.A. Lyons

  36. Lightning Event Detection Over Seawater June 1 – Sept. 30, 2001 April 1 – Sept. 30, 2002 4664 Correlated Events June 1 – Sept. 30, 2001 1861 Correlated Events % Composition 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

  37. 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 conductiveseawater provides a fast ‘switch’ driving the final jump and initiation of the return strobe.

  38. 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.

  39. The End !!

  40. Initiation of a Sprite Timing based on visual images can be 10 ms off. Can models reproduce this slow sprite initiation? 3. 2. 1 Frame number indicates also time in ms. Courtesy of Dana Truffer-Moudra,GIUAF 11. 12. 10. 9.

  41. Positive LeaderStreak Camera Picture

  42. Negative 275

  43. Negative Plot 325

  44. Negative Plot 325

  45. Bileader DS

  46. Larigaldie Simulation (500)

  47. Larigaldie Simulation 1600

  48. Larigaldie Simulation 2200

  49. Larigaldie Simulation 3000

  50. Larigaldie Simulation 5000

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