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

OutlineI. The Data Set: In-Situ Balloon Measurements

During the Brazil Balloon Campaign 2002-2003II. 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?

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

• 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

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 profileFrom Pasko et al. 1997

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:

Vertical Electric Field Pulse for +CG

data

model

2 sec

Radial Electric Field Pulse For +CG

model

data

2 sec

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

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

220 ms

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

22 ms

Comparison to breakdown thresholds

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

Contact info:

E-mail: jnt@u.washington.edu

Webpage: http://www.ess.washington.edu/students/jnt/