High Time-Resolution Sprite Imaging: Observations and Implications

1. High Time-Resolution Sprite Imaging: Observations and Implications H. C. Stenbaek-Nielsen Geophysical Institute University of Alaska Fairbanks M. G. McHarg U.S. Air Force Academy

2. This ppt version was prepared for conference web site posting. There are no animations in this version. Animations have been replaced with a representative image, and some additional explanatory text has been added. I will be preparing compressed version of animations. (Most are too large for email) Some are available with our publications through AGU’s journal website: McHarg et al. GRL, 34, L06804, doi:10.1020/2006GL027854, 2007 Stenbaek-Nielsen et al., GRL, 34, L11105, doi:10.1029/2007GL029881, 2007 For further info contact us by email: hnielsen@gi.alaska.edu

3. Trans Luminous Events (TLE) Elve: 1 ms E&M pulse Halo: 1-5 ms: glow discharge Sprite: 1-10 ms; streamer discharge Afterglow: 1-200 ms chem. processes Blue jets, beads, crawlers, ambers etc.: 5-1000ms process ?????

4. Sprite dynamics Images show features well known from TV image sequences with better detail Halo, tendrils, branches, and beads Images colorized for emphasis Altitude scale derived assuming sprite to be at lightning strike (NLDN) 2 examples at 1000 fps

5. Sprite at 1000 fps Animation Selected frames from animation Sequence: 15 ms.

6. Sprite at 1000 fps Animation Selected frames from animation Sequence: 30 ms. Note: No halo with second sprite

7. Rebrightening

8. Features to be addressed Time scales ← Branches < 1 ms (not resolved at ms) ← Beads/afterglow > 1 ms ←Tendrils < 1 ms (not resolved at ms) Image: 18 Aug 1999, WIRO, Wy.

9. Effect of frame rate9 July 2005 06:06:46 UT 10,000 fps makes a lot of difference….. 1 ms 50 ms 33 ms video Streamers (almost) resolved at 50 μs (20,000 fps)

10. Link to streamers in the lab Exposure: 300 ns 50 ns 10 ns 2 ns Courtesy of Prof. Ute Ebert and T. Briels of TU Eindhoven, Netherlands

11. Time development scales as 1/n Density at ground level: 3 1019 /cc Density at 80 km altitude: 3 1014 /cc Scaling factor: 105 Scaling of process time

12. Scaled to 80 km altitude 30 ms (~TV) 5 ms 1 ms 0.2 ms 33 fps 200 fps 1000 fps 5000 fps

13. Scaled to 80 km altitude 10,000 fps makes a lot of difference….. 30 ms (~TV) 5 ms 1 ms 0.2 ms 33 fps 200 fps 1000 fps 5000 fps

14. Streamer head formation Animation • First downward, later upwards moving streamer heads • Upward starts from • Lower altitude • Existing luminous sprite structures (animation: GRL, 34, 11, 2007) 9 ms image sequence

15. Streamer head formation Animation Downward streamers first Upwards streamers later and - from lower altitude - from existing structure Streamer velocity up to 0.3 c 9 July 2005 06:33:11 UT: 10,000 fps Gating 50 µs Duration 5.0 ms

16. Streamer head formation Animation Downward streamers first Upwards streamers later and - from lower altitude - from existing structure Streamer velocity up to 0.3 c 7 July 2005 08:31:20 UT: 5,000 fps Gating 100 µs Duration 5.0 ms

17. Summary of downward streamers: Streamer head starts 70 - 90 km altitude Halo may or may not be present Streamer head brightens as it moves down Direction largely straight down Velocity up to 6 107 m/s Both increase and decrease in speed observed Downward Streamer

18. Upward Streamer 9 July 2005 04:15:17 UT 10,000 fps Animation

19. Summary of upward streamers: Not present in all events Starts later than downward streamers Starts at lower altitude than downward streamers Starts from bead structures Ends with a “puff” and upward motion stops Velocities similar to downward streamers Significant horizontal velocity component Upward Streamer

20. C-sprite Downward streamers No upward streamers Carrot sprite Downward streamers Upward streamers Morphology

21. All sprites start with downward moving streamer heads Streamer heads are small. Appear in images just like background stars Individual streamer heads move in one direction only No example of double headed streamers No evidence of geomagnetic field effects Main Points

22. Streamer head brightness • Streamer heads: • Gaussian profile • Similar to stars  Smaller than spatial resolution • Point sources • Stellar mag -6 Emission rates: 5 1021 to 3 1024 phot/s

23. Smaller than our ~150 m resolution Telescope obs: 10-200 m (Gerken et al.) Models ~25 m (Liu and Pasko) Assuming 25 m size: Average brightness: 1 – 100 GR Size

24. Size Assuming 8 1011 #/cm3/s (Sentman et al.): Size from 30 – 300 m

25. Streamer brightness presented is in the June 16 issue of GRL. Reprints available here at meeting

26. Streamer heads very bright Source of energy for local chemical processes (talk Friday by D. Sentman) Longer lasting effects? Significant effects on the mesosphere? Implications

27. Streamer head splitting Animation 9 July 2005 06:33:11 UT 10,000 fps

28. Splitting on the run 2005 data show splitting a ‘slow’ process: Individual streamer heads propagate ~30 km, but only a few splitting events observed Only 1 or 2 new streamer heads formed in each splitting No ‘slow down’ while splitting But 2007 telescopic images show many and multiple streamer heads forming Streamer head splitting

29. Streamer head splitting Animation • 23 June 2007 07:01:01 UT, Langmuir, NM • 10,000 fps, 50 μs exposures • Field of view: 2.12x1.58 degrees • Altitude 80 km at 600 km range • Velocity: 3 107 m/s; Size: ~0.1 to ~2 km

30. Streamer head splitting 23 June 2007 04:22:49 UT 10,000 fps, 50 μs exp. Field of view: 2.12x1.58 deg Altitude 80 km at 600 km Velocity: ~107 m/s; Size: ~0.1 to ~2 km

31. Streamer head splitting • Preliminary analysis • Difficult to match with earlier larger FOV images • More splits • More streamer heads • Maybe the sequences are from the central part of the sprite Animation

32. Afterglow 9 July 2005 04:38:00 UT 10,000 fps

33. Very little spatial motion Vertical structure and brightness not as expected from streamer head brightness Duration of afterglow vary between events Energy beyond streamer heads alone Can last from a few ms to several 100 ms Total optical power may be larger than for streamer heads Afterglow

34. Afterglow spectrum(300 fps) N2 1P spectra. Some altitude differences indicating additional local processes (Kanmae et al., GRL, 2007).

35. Streamer head spectrumSlitless spectroscpy Star background. 0th and 1st order spectra clearly seen. Wavelength coverage 400-900 nm

36. Streamer head spectrum50 μs (20,000 fps) Primarily N2 1P

37. Atmospheric effects Sprites were at 600-700 km so blue attenuated by ~x100

38. Primarily N2 1P band emissions in both streamer heads and afterglow Spectral altitude differences in afterglow No N2 ion emissions detected (yet) Would expect differences between streamer heads and afterglow (not proven yet – we are working on it) Chemical processes and their consequences Optical spectra

39. Delayed Sprite Animation Animation 1000 fps images covering the first sprite to the onset of the carrot sprite (Left-right reversed. Sorry!) Video (30 fps). FOV: 21x16 degrees Note: First one sprite then a large carrot sprite and finally some activity at lower altitude

40. Lightning Lightning at USAFA 66,000 fps (15 μs) Distance? (very close!!) Field of view: 8x8 deg Pix size: ~0.2 m (200 m range) Thanks for your attention.