1. Thundercloud electrodynamics and its influence on high-energy radiation enhancements and lightning initiation E.A. Mareev 1, D.I. Iudin 1, V.A. Rakov 1,2, A.Yu. Kostinskiy 1,3, V.S. Syssoev 1,4 1 Institute of Applied Physics RAS, Nizhny Novgorod, Russia 2 Department of Electrical and Computer Engineering, University of Florida, USA 3 National Research University Higher School of Economics, Moscow, Russia 4 High-Voltage Research Center of the Institute of Electrical Engineering, Istra, Moscow region, Russia

2. 2 Outline  Experimental data on the multi-layer charge structure of thunderstorm clouds.  The lower positive charge region (LPCR) and its possible effects on the development of CG and IC discharges and thunderstorm ground enhancements.  Fractal simulation code enabling to examine the occurrence of lightning flashes of different type as a function of the cloud charge structure.  Recent observations of electrical discharges in the artificial cloud of charged water droplets,  Description of a complex hierarchical network of interacting channels at different stages of development.  Some perspectives

3. 3 Sources of the secondary cosmic rays detected on the Earth's surface (Chilingarian, JASTP, 2014)

4. Cloud electrical environment 4 A vertical tripole representing the idealized gross charge structure of a thundercloud such as that shown in the left panel; the negative screening layer charges at the cloud top is ignored here. An isolated thundercloud in central New Mexico, with a rudimentary indication of how electric charge is thought to be distributed inside and around the thundercloud, as inferred from the remote and in situ observations. Adapted from Krehbiel (1986).

5. 5 Current description; ordinary clouds and MCS S. Davydenko, E.Mareev, T.Marshall, M.Stolzenburg, JGR, D5, 2004 Shepherd R.T., W.D. Rust, and T. C. Marshall, Electric fields and charges near 0 C in stratiform clouds, Monthly Weather Review, V.124, May 1996.

6. 6 Conceptual model of the electrical structure in mature, mid-latitude convection. Four main charge regions (with red + for positive charge, blue – for negative charge) are typically found in soundings through updrafts, while soundings outside updrafts have at least six charge regions in common. Representative electric field (E) and electrostatic potential (V) profiles in the nonupdraft (left) and updraft (right) of the convective region are also shown; the altitudes in these soundings do not correspond exactly to the conceptual model. Schematic representations of an intracloud flash (in green) and a cloud-to-ground flash (in purple) are shown as they might appear in lightning mapping data (Stolzenburg and Marshall, 2009)

7. Thunderstorm 1-2 June, 2015 7

8. 8 Electric field variations: thunderstorm of July 17, 2005 Klimenko et al., 2007

9. 9 Electric field variations: thunderstorm of July 18, 2006

10. 10 Transfer of electrical space charge from corona between ground and thundercloud: Measurements and modeling (Chauzy et al., JGR,1991; Soula, JGR, 1994)

11. We interpret the preliminary breakdown (PB) pulse train as being generated when a negatively-charged channel extends downward from the main negative charge region and encounters an appreciable lower positive charge region (LPCR). When the LPCR is small no PB pulse train may be produced. While the LPCR may serve to enhance the electric field at the bottom of the negative charge region and thereby facilitate the launching of a negatively-charged leader toward ground, presence of excessive LPCR may prevent the occurrence of negative CG flashes by ‘‘blocking’’ the progression of descending negative leader from reaching ground. We infer four conceptual lightning scenarios that may arise depending upon the magnitude of the LPCR. A. Nag and V.A. Rakov, Geophys. Res. Lett., doi:10.1029/2008GL036783, 2009 11 Role of Lower Positive Charge Region in facilitating Different Types of Lightning

12. Left panel is a schematic illustration (not to scale) of electric field enhancement and reduction effects of the lower positive charge region (+Q LP ) below the main negative charge region (-Q N ). The main positive charge region is not shown. Arrows indicate the direction of vertical components of electric field vectors. The total electric field is enhanced, (E N + E LP ), between the negative and positive charge regions and reduced, (E N – E LP ), below the positive charge region. Right panel shows (a) schematic representation of preliminary breakdown stepping process in negative ground flashes. Negatively-sloped arrow indicates the overall downward extension of negatively charged channel through the LPCR. Three steps giving rise to current (and light) pulses are shown. Each current pulse originates at the tip of downward-extending channel and propagates upward (positively-sloped arrows). (b) A sketch of expected electric field record of resultant wideband PB pulse train. 12

13. The left panels, A-D, schematically show four types of lightning that may arise depending upon the magnitude of the LPCR. The charge configuration in each of the scenarios represents only its vertical profile. Arrows indicate the direction of propagation of negative leader. The corresponding examples of expected electric field signatures are shown in the right panel. The field waveforms are from four different thunderstorms recorded at some tens of kilometers in Gainesville, Florida, using the same instrumentation with a decay time constant of 10 ms. PB = preliminary breakdown pulse train, RS = return-stroke waveform. 13

14. 14 Stolzenburg and Marshall, 2009 (1)

15. 15 Stolzenburg and Marshall, 2009 (2)

16. 16 An example of a horizontal lightning channel traveling during a period of preliminary breakdown within a potential well (S&M, 2009)

17. 17 Solid ellipses denote graupel gaining positive charge, dashed ellipses indicate negative charging of graupel. Significant charge regions are noted by the shaded rectangles (light shading for net negative, darker shading for net positive charge). For inverted polarity storms, simply reverse all signs (Mansell et al., JAS, 2010). SIMPLIFIED MODELS OF CHARGE STRUCTURES AND UNDERLYING NONINDUCTIVE CHARGE SEPARATION

18. 18 Electric potential (a) and z-component of electric eld (b) before the lightning discharge initiation

19. Parameters of three different cloud–charge configurations

20. Lightning Initiation Fractal Models References: [e.g., Niemeyer et al., IEEE Trans. Electr. Insul., 24, 309, 1989; Femia et al., J. Phys. D: Appl. Phys., 26, 619, 1993; Petrov and Petrova, Tech. Phys., 38, 287, 1993; Petrov and Petrova, Tech. Phys., 44, 472, 1999]

21. 21 Fractal models of lightning discharge Trakhtengerts and Iudin, 2000, 2003 Pasko, 2002; Pasko and Riousset, 2007; Krehbiel et al., 2008 Mansell et al., 2002; 2010

22. 22 Fractal dynamics of sprites D.Iudin, E.Mareev, V.Trakhtengerts, M.Hayakawa, Physics of Plasmas, 2007.

23. Thundercloud structure and discharge initiation

24. Discharge development

25. Compact Intracloud Discharges (CIDs)

26. (a) The altitude of each new node that gives rise to more than two new links for charge configuration # 1. (b) The same for charge configuration # 2; (c) the same for charge configuration # 3

27. Istra instrumentaion Marx generator 6 MV The test site control room 27

28. Recent experiments: new diagnostic tools, including microwave absorption 28 1 – steam generator, 2 – high voltage power supply, 3 – charged aerosol cloud, 4 – microwave generator, 5 – horn antenna, 6 – lenses, 7 – microwave beam, 8 – microwave detector, 9 – oscilloscopes, 10 – measuring shunt, 11 – spark discharge cloud-ground.

29. 29 IR camera FLIR CS7700 M Diagnostic tools High-speed optical camera 4 Picos Crossbow bolt TenPoint TurboXLTII LeadSledDFT

30. Hot plasma structures (stalkers): IR images 30 Two successive frames (3a, the first and 3b, the second) taken by an IR camera, which records the upper (hidden by the aerosol) part of the upward discharge. Exposure time is 7 ms. The time between frames is 1.7 ms. The number of pixels in each frame is 640x520. All events recorded in the frame occurred inside the cloud. Only flares of scattered radiation are seen in the visible range in this experiment. 1 – the channel of the upward discharge, 2 - streamer corona, 3 - intracloud plasma structures.

31. The stalker as a space-stem-type structure 31

32. Observations in IR and optics (Geophys. Res. Lett.: doi:10.1002/2015GL065620) 32 Simultaneous IR and optic images of the same event. Exposure time of the FLIR 7700M camera is 8 ms. Shown in the upper left corner is a fragment of Figure depicting stalkers in the visible range. It can be seen that the contours of the brightest stalkers are similar in both images.

33. Hot plasma structures: + cloud (J. Geophys. Res, Atmos.: doi:10.1002/2015JD023827) 33 1 – the central plasma channel (stalker), negative leader – 2; branched stalker – 3, downward positive leader - 4, positive streamer corona – 5.

34. 34 Conclusions We have analyze multi-scale dynamics of thunderstorm electric structure as related to high-energy radiation enhancements and lightning initiation. A special attention was paid to the lower positive charge region and its possible effects on the development of CG and IC discharges and thunderstorm ground enhancements (TGEs). Based on the graph theory, we have developed a fractal simulation code to examine the occurrence of lightning flashes of different type as a function of the cloud charge structure. We show in particular that presence of relatively intense lower positive charge region prevents the occurrence of negative CG flashes by ”blocking” the progression of descending negative leader from reaching ground. Further, based on our recent observations of electrical discharges in the artificial cloud of charged water droplets, we present the description of a complex hierarchical network of interacting channels at different stages of development (some of which are hot and live for milliseconds), which can possibly be considered as a missing link in the still poorly understood lightning initiation process.

35. 35

36. 36 Positive & negative leaders. Streak photographs Bazelyan E., Raizer Yu., 1998

37. Negative leader steps. Space stem Stekolnikov I.S., Shkilev А.V., 1962 Les Renardiéres Group, 1977; Reess et al., 1995 Gorin B.N. et al., 1976

38. Профили электрического поля и заряда (индукционный механизм)

39. 39

40. Infrared images (negatives) obtained with 6.7-ms exposure that show the processes at different heights above the grounded plane (AGP) and different horizontal distances from the cloud axis: (a) The upward positive leader from the grounded sphere. No UPF is seen. (b) The upper part of the upward positive leader (lower right) and UPF (upper left), both inside the cloud. The two appear to be distinct discharge processes which interact, via their streamer zones. Relative to (a), the field of view of the IR camera was moved up and left (closer to the axis of the cloud). (c) The lower part of UPF (inside the cloud). The upward positive leader is outside the IR camera field of view, which was moved (relative to (b)) further up and left. The upward positive leader and the UPF apparently interact (outside of the field of view) via their streamer zones. (d) same as (c), but for the upper part of UPF near the central part of the cloud (see the dark formation on the left). Note that relatively faint UPF channels branch toward the axis of the cloud, but do not cross the axis. This direction of branching is opposite to that seen in (c), suggesting that UPFs extend bidirectionally.

41. 41 MCS soundings: electric field

42. 42

43. Electric field profiles (balloon measurements of May, 1991) and respective external current density Shepherd R.T., W.D. Rust, and T. C. Marshall, Electric fields and charges near 0 C in stratiform clouds, Monthly Weather Review, V.124, May 1996.

44. 44 Electric generators in the atmosphere: problems of parameterization Kalinin A.V. et al., 2011; Mareeva et al., 2011

45. Negative leader step nature 45 V. A. Rakov, The Physics of Lightning, Surveys in Geophysics, 2013 Biagi et al., JGR, 2010 Petersen and Beasley, JGR, 2013