1. Nanosecond-scale structure of the electric field waveforms of atmospheric discharges A. Chilingarian, G. Khanikyanc, L. Kozliner, S. Soghomonyan

2. Fast electric field waveforms of atmospheric discharges detected at Aragats during September-December 2014 Nanosecond-scale pulses with periodic oscillations (not observed before) Narrow bipolar pulses (studied for about 30 years) Outline

3. HF emission and wideband fast electric field detection system MFJ-1022 active whip antenna is used for triggering. Frequency range : from 300 KHz to 200MHz The waveforms of fast wideband electric field change are recorded with flat plate antenna.

4. Wideband fast electric field measurement system Diameter D=0.52m, Area A=0.21 m 2, C A  80pF Passive integrator C=10nF, (C>>C A ), R=1 MΩ - input impedance of the oscilloscope The integrator is a high pass filter with cut-off frequency f=1/(2  RC)=16 Hz Useful frequency range of the measurement system: from 16 Hz to 50 MHz

5. Digital storage oscilloscope Picoscope 3206, two channels, 8 bit resolution Max sampling rate f max =200MS/s, sampling interval =5ns (For 2 channel operation f max =100 MS/s, sampling interval =10ns) Memory depth M=10 6 samples. At max sampling rate the maximum record length is T max =M/f max =5 ms, including 1 ms pre-trigger time Data storage: when the signal exceeds given threshold, the recording is triggered and data are stored to hard drive. Recording rate =1Hz.

6. Statistics of events recorded during September-December 2014 Total: 4045 events

7. Typical waveform of nanosecond-scale sine wave burst Salient feature: oscillatory behavior with period of 20-30ns.

8. Nanosecond-scale pulses Observed in about 35% of  1750 examined waveforms The waveforms can be grouped into three main categories 1) Isolated pulses within 5 ms record length (about 75%), total duration from 0.1 to 2  s 2) Trains of pulses (about 15%), total train duration from several microseconds to to 2.5ms, interpulse interval from 1 to 30  s, 3)Pulses followed by other signatures (about 10%)

9. Isolated pulse

10. Pulses embedded at the damped oscillations

11. Train of pulses interpulse period increases from 3  s in the beginning of the train to 30  s at the end. Total train duration is 2.5 ms

12. Nanosecond-scale pulse followed by positive microsecond pulse

13. Nanosecond-scale pulse followed by negative microsecond pulse

14. Nanosecond-scale pulse followed by a sequence of NBPs (November 12 2014 13:24:26 UT)

15. Nanosecond-scale pulse followed by a sequence of NBPs surprisingly similar waveforms! October 17, 2014, 08:00:42UT November 12 2014 13:24:26 UT

16. Nanosecond-scale pulse followed by negative NBP and other signatures ( detected also by 7 stations of the WWLLN network, 17.1 km )

17. Narrow Bipolar Pulses (NBP) A distinct class of isolated intracloud lightning discharges Also called Narrow Bipolar Events (NBE), or Compact Intracloud Discharges (CID) First reported by LeVine in 1980 The first waveforms analyzed by Willett et al. in 1989 Received their name, CID (Smith et al., 1999) due to their relatively small (hundreeds of meters ) spatial extent. “ Although CIDs have been studied for about three decades, they remain the most mysterious type of lightning” (A. Nag and V.Rakov, 2010)

18. Main features of Narrow Bipolar Pulses Sharp bipolar field change waveforms Main peak width 1.6-4  s, full duration 10-30  s, Temporal isolation from other thunderstorm radio emissions on time scales of at least a few milliseconds. The most radio-powerful intracloud lightning discharges Relatively dark (less luminous than ordinary lightning) Height range 3-18 km, (higher for negative NBPs) Small spatial extent of the discharge (  300-1000m )

19. Isolated NBPs detected at Aragats Positive NBP (24 events detected)Negative NBP (95 events detected) FWHM=1.4 ±0.1  s October 1, 2014, 15:05:26 UTSeptember 21, 2014, 08:28:40 UT “A positive polarity NBE exhibits a radiation field waveform that begins as a positive electric field change that results from a dipole discharge in which positive charge is located over negative charge, while a negative NBE has a negative starting radiation field change that results from an inverted dipole (negative charge over positive). “ [K.B. Eack 2004] + + - -

20. Sequence of two and three NBPs

21. Definition of NBP parameters FWHM Rise time (10-90% ) Overall duration( 10-10%) Overshoot amplitudeInitial peak amplitude

22. Main temporal characteristics of 119 isolated NBPs observed at Aragats Mean±Std.Dev PositiveNegative Rise time (10-90%), µs 1.4±0.91.9±1.1 FWHM of initial peak, µs 1.4 ±0.1 Overall duration, µs 24.7±10.422.7±7.3 Initial peak/ overshoot peak3.9±1.44.0±2.7 The electric field waveforms for all 119 isolated NBPs were clearly dominated by the radiation field component, so we conclude that they were produced at large distances (more than tens of kilometers).

23. Near and distant station waveforms for four positive NBE events with observed electrostatic field changes 9.7 km 8.9km 9.9 km 14.5km  200 km

24. Three components of electric field Three terms of equation are referred to as the electrostatic (1/R 3 ), induction (1/R 2 ), and radiation (1/R) terms, respectively. Electric field from a transmission of an arbitrary current along a vertically- oriented finite antenna above a conducting plane (Uman et al., 1975, “The electromagnetic radiation from a finite antenna”). M is the dipole moment, [M.A. Uman, Ligtning, 1969]

25. The event of October 4, 2014, 14:13:32 UT

26. The event of October 4, 2014, 14:13:32 UT Fast electric field waveform Precursor N1Precursor N2 80  s view very short spike

27. The event of October 4, 2014, 14:13:32 UT Fast electric field Precursor N1: bipolar pulse, 100ns between the peaks of opposite polarity

28. The event of October 4, 2014, 14:13:32 UT Fast electric field: Precursor N2 :nanosecond-scale pulses,  20 ns oscillation period

29. The event of October 4, 2014, 14:13:32 UT Fast electric field 5  s view  500 ns, very short fall time to zero

30. Observation by V. Cooray et al. (2010)

31. “We agree that these fine pulses were probably caused by rapid changes in the current and velocity as suggested by Willet and Krider (1979). However until a thorough study of these small pulses has been completed, we speculate that during the process there might be many micro scale breakdowns due to enormous amount of electrons stimulated by cosmic ray showers”

32. Laboratory spark: capacitor discharge C=10  F, U=300V Period of damped oscillations T 1  35  s fast oscillations T  10ns C=20  F, U=300V Period of damped oscillations T 2  48  s expected T 2 =35 ·1.414  49  s

33. Laboratory spark 10  F capacitor discharge Oscillation period 10 ns

34. Laboratory spark Switching on a 100 W incandescent lamp Oscillation period 20 ns

35. Plasma oscillations Hypothesis: nanosecond-scale pulses may be associated with ion oscillations in the plasma of an atmospheric discharge. The characteristic frequency f pi of ion oscillations in plasma is determined by the density of the ions n i and their mass M i. [Tonks and Langmuir 1929] where e is the elementary charge, and  0 the vacuum permittivity. For a typical ion density of 10 13 cm -3 the characteristic frequency f pi of ion oscillations is about 10 8 Hz, while the frequency of electron oscillations is about 10 10 Hz Oscillations of N +4 ions in the plasma of an atmospheric discharge can produce radiation signals of  50 MHz oscillation frequency, if the ion density is  0.35  10 13 cm -3. This ion density can be achieved at the altitude of  7-8 km. f pi =(n i e 2 /4  2  0 M i ) 1/2

36. Summary Electric field pulses with oscillation period of 20-30 ns are detected. They are observed as isolated pulses, train of pulses, or pulses followed by other signatures These pulses are not observed with isolated NBPs Physical processes which produce these pulses are not well understood yet. The pulses may be associated with some small-scale discharges in the initial breakdown processes. NBPs of both polarity are detected, with negative NBPs being detected four times more frequently than positive NBPs All detected NBPs are produced at large distances (more than tens of kilometers). This follows from the fact, that their waveforms are clearly dominated by the radiation field component.