1. Lightning-Driven Electric Fields in the Stratosphere: Comparisons Between In-Situ Measurements and Quasi-Electrostatic Field Model Jeremy N. Thomas, Robert H. Holzworth, Michael P. McCarthy, Nimisha Ghosh Roy, Natalia N. Solorzano, Osmar Pinto, Jr., and Mitsuteru Sato UW – USU – INPE collaboration Supported by US: NSF and Brazil: FAPESP ELF Data supplied by Tohoku University Japan

2. Outline I.The Data Set: In-Situ Balloon Measurements During the Brazil Balloon Campaign 2002-2003 II.Case Study: A quasi-electrostatic field (QSF) model to simulate a measured lightning-driven electric field perturbation (+CG) III.Prediction: Use the QSF model to predict the lightning-driven electric field at sprite altitudes IV.Sprite Production: How does this predicted electric field compare to the magnitude and duration needed to produce sprites?

3. Data Set: In-Situ Balloon Measurements 38 electric field changes greater than 10 V/m were measured above 30km in alt. Location of strokes: Brazilian Integrated Ground Based Lightning Network (BIN) Sprites not ruled out, although none were confirmed optically The balloon payload also measured the conductivity Flight 1 Trajectory and BIN CGs

4. Two positive cloud-to-ground (+CG) strokes 150ms apart 34 km hor. distance from the balloon payload (alt=34km) Charge moment: 436 C-km estimated from remote ELF (extremely low frequency) magnetic field measurements (M. Sato) Case Study: A Large +CG Event ELF Data from Syowa, Antarctica

5. Case Study: Simulating A Large +CG Event An axi-symmetric stroke centered numerical simulation of the quasi-static electric field change after a +CG based on the work of Pasko et al., JGR, 102, 4529, 1997 Important input parameters: charge moment, cloud charge distribution, discharge time, and atmospheric conductivity profile From Pasko et al. 1997

6. Model assumptions: No horizontal currents: The cylindrical symmetry prevents this. The atm. conductivity is not affected by the lightning stroke No magnetic field perturbations Only the change in electric field due to +CG is modeled, not the background field before and after the +CG Case Study: Simulating A Large +CG Event Equations Solved Numerically:

7. Vertical Electric Field Pulse for +CG data model 2 sec

8. Radial Electric Field Pulse For +CG model data 2 sec

9. Predicting Electric Fields at Sprite Altitudes The parameters that best fit the quasi-static field model to the balloon data are used to predict the electric field perturbation at sprite altitudes (50- 80km) These electric field pulses are compared to the electrical breakdown thresholds (conventional and relativistic) The duration of the pulse is compared to the duration of observed sprites

10. Model Output: Predicted lightning-driven electric fields at sprite altitudes (Z=60km) 220 ms

11. Model Output: Predicted lightning-driven electric fields at sprite altitudes (Z=70km) 22 ms

12. Comparison to breakdown thresholds

13. Conclusions For the +CG event studied, the electric field never surpasses the conventional electrical breakdown threshold at sprite altitudes but does surpass the relativistic breakdown threshold. The duration of the electric field pulse at sprite altitudes (22 ms at 70km) is comparable to the time duration of sprites. Better electron conductivity profiles (dependent on location, weather, and solar activity) are needed to more accurately model these electric field pulses at sprite altitudes

14. Contact info: E-mail: jnt@u.washington.edu Webpage: http://www.ess.washington.edu/students/jnt/