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The Electric Potential of a Giant Positive Jet Suggested by Simultaneous Sprite Emissions

The Electric Potential of a Giant Positive Jet Suggested by Simultaneous Sprite Emissions. Torsten Neubert and Olivier Chanrion National Space Institute Technical University of Denmark. December 12, 2009.

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The Electric Potential of a Giant Positive Jet Suggested by Simultaneous Sprite Emissions

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  1. The Electric Potential of a Giant Positive Jet Suggested by Simultaneous Sprite Emissions Torsten Neubert and Olivier Chanrion National Space Institute Technical University of Denmark

  2. December 12, 2009 Fully developed jet jet stem jet stem expanding crown sprite and re-brigthning jet

  3. Key Question • the occurrence of the crown jet in the last frame can tell us about the potential structure of the giant jet

  4. Approach • Our method • To model the source electric field of the jet with a positive line charge • To model the QE field driving the sprite from a +CG discharge • To simulate the response of the atmosphere-ionosphere to the two driving fields • Best fit with data represent estimates of the electric potential and currents established during the event • Other aspects that we think we understand • That giant jets in general seem to have have two parts: the slower forming stem and the fast and short-lived upper part • Expansion of the giant jet stem

  5. Data availabe • EM waves at many locations • Optical from Italy • Meteosat data on clouds • LINET data on lightning

  6. Duke University Charge and Current moments

  7. Some numbers • zc=6 km altitude [van der Velde et al., 2010] • If the ionosphere is at 80 km altitude: dl =74 km • the peak currents are then: • P1: Io ~3.8 kA • P2: Io ~3.5 kA • With the duration of the two current pulses, P1 and P2, of ~100 ms, • the average current during these pulses is Ia ~1.6 kA. • Qdl ~1.2 x 104 Ckm • net positive charge of Qo ~160 C carried to the ionosphere during P1 and P2.

  8. The electric conductivity • The conductivity is affected by the electric field by: • Attachment • Ionisation • First we explore the driving electric fields without perturbations to the conductivity

  9. The electric field from a cloud discharge • Positive layer discharges:

  10. The electric field from a +CG Qc = 250 C td = 17 ms t = 10 ms

  11. Line charge of a leader • q = 1-32 x 10-3 Cm-1 • [Rakov and Uman, 2003, p. 125-126] • Joseph E. Borovsky, Lightning energetics: Estimates of energy dissipation in channels, channel radii, and channel-heating risetimes, J. Geophys. Res., 103, D10, 11,537-11,553, 1998 • Many ways to estimate it

  12. The electric field from a line charge • The field off axis: Er(r,z) = (sinb2-sinb1)(qo/4peo)/r Ez(r,z) = (cosb2-cosb1)(qo/4peo)/r • The field on axis – above line charge: Ez(r,z) = ~(qoL/4peo)/[(z-zo)(z-(zo+L))]

  13. The electric field from a line charge t = 24 ms

  14. 101021 LU 101021 LU The response of the atmosphere

  15. The response of the atmosphere 50 x 20 ms cloud discharge pulses E/Ek ~1.4 E/Ek ~0.9

  16. Radial Expansion – 50 km altitude Atmosphere energy density: P = 72 Jm-3 Deposited energy: PW~E.I.dt E ~ 2.25 kVm-1 (breakdown field) I ~1.6 kA dt ~50 ms PW = 1.8 x 105 Jm-3 Radial expansion: rj2/ro2 = PW/P, or rj = 50 ro rj ~7 km Radius of a leader in the troposphere: ro(z=0 km) ~ 10 cm Dependence of neutral gas density n: ro(z) ~ 1/n Radius at 50 km: ro(z=50km) ~ 140 m

  17. Leader ”stem” - streamer ”canopy” • The E-field at a tip of aline-charge falls off-slower than that of a spherical charge • Esp/Eline ~ 1 – L/(z-zo) • When L becomes large relative to the system size it can create the condition that E allows for stramer propagation all the way to the ionosphere • The field is cancelled shortly after because of the rapid response of the ionosphere

  18. Discussion • Conclusions • The horizontal displacement of sprites agrees with observations • The vertical displacement perhaps not • What have we forgotten: • The bacground conductivity is perturbed by the first jet (pulse one) • The sprite is displaced from the jet and is further away

  19. THE END

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