high time-resolution sprite imaging: observations and implications

Leiden October 2007

High Time-Resolution Sprite Imaging: Observations and Implications

H. C. Stenbaek-NielsenGeophysical Institute

University of Alaska Fairbanks

M. G. McHargU.S. Air Force Academy

Leiden October 2007

This ppt version was prepared for conference web site posting.

There are no animations in this version. Animations havebeen replaced with a representative image, and some additionalexplanatory 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, 2007Stenbaek-Nielsen et al., GRL, 34, L11105, doi:10.1029/2007GL029881, 2007

For further info contact us by email:hnielsen@gi.alaska.edu

Leiden October 2007

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 ?????

Leiden October 2007

2 examples at 1000 fps

• 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)

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Sprite at 1000 fps

Sequence: 15 ms.

Selected frames from animationAnimation

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Sprite at 1000 fps

Sequence: 30 ms. Note: No halo with second sprite

Animation Selected frames from animation

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Rebrightening

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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.

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Effect of frame rate9 July 2005 06:06:46 UT

Streamers (almost) resolved at 50 μs (20,000 fps)

33 ms video 1 ms 50 s

10,000 fps makes a lot of difference…..

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Link to streamers in the lab

Exposure: 300 ns 50 ns 10 ns 2 nsCourtesy of Prof. Ute Ebert and T. Briels of TU Eindhoven, Netherlands

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Scaling of process time

• Time development scales as 1/n

• Density at ground level: 3 1019 /cc• Density at 80 km altitude:3 1014 /cc

• Scaling factor: 105

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Scaled to 80 km altitude

30 ms (~TV) 5 ms 1 ms 0.2 ms 33 fps 200 fps 1000 fps 5000 fps

Leiden October 2007

Scaled to 80 km altitude

30 ms (~TV) 5 ms 1 ms 0.2 ms 33 fps 200 fps 1000 fps 5000 fps

10,000 fps makes a lot of difference…..

Leiden October 2007

Streamer head formation

• 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

Animation

Leiden October 2007

Streamer head formation

9 July 2005 06:33:11 UT: 10,000 fps Gating 50 µs Duration 5.0 ms

Downward streamers first

Upwards streamers later and

- from lower altitude

- from existing structure

Streamer velocity up to 0.3 c

Animation

Leiden October 2007

Streamer head formation

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

Animation

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Downward Streamer

• 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

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Upward Streamer

9 July 200504:15:17 UT 10,000 fpsAnimation

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Upward Streamer

• 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

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Morphology

• C-sprite– Downward streamers– No upward streamers

• Carrot sprite– Downward streamers– Upward streamers

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Main Points

• 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

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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

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Size

• 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

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Size

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

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Streamer brightness presented is in the June 16 issue of GRL.

Reprints available here at meeting

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Implications

• Streamer heads very bright– Source of energy for local chemical processes

(talk Friday by D. Sentman) – Longer lasting effects?– Significant effects on the mesosphere?

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Streamer head splitting

9 July 200506:33:11 UT 10,000 fps

Animation

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Streamer head splitting

• 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

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Streamer head splitting

• 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

Animation

Leiden October 2007

Streamer head splitting

23 June 2007 04:22:49 UT10,000 fps, 50 μs exp.Field of view: 2.12x1.58 degAltitude 80 km at 600 kmVelocity: ~107 m/s; Size: ~0.1 to ~2 km

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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

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Afterglow

9 July 200504:38:00 UT 10,000 fps

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Afterglow

• 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

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Afterglow spectrum(300 fps)

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

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Streamer head spectrumSlitless spectroscpy

Star background. 0th and 1st order spectra clearly seen.

Wavelength coverage 400-900 nm

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Streamer head spectrum50 μs (20,000 fps)

Primarily N2 1P

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Atmospheric effects

Sprites were at 600-700 km so blue attenuated by ~x100

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Optical spectra

• 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

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Delayed Sprite

Video (30 fps). FOV: 21x16 degrees

Note: First one sprite then a large carrot sprite and finally some activity at lower altitude

1000 fps images covering the first sprite to the onset of the carrot sprite

(Left-right reversed. Sorry!)

AnimationAnimation

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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.