Payload Configuration, Nov. 2002

1. Lightning-driven electric and magnetic fields measured in the stratosphere during the Brazil 2002-03 Balloon Campaign Jeremy N. Thomas (U. of Washington, jnt@u.washington.edu), Robert H. Holzworth (U. of Washington, bobholz@ess.washington.edu), Michael P. McCarthy (U. of Washington, mccarthy@ess.washington.edu), Osmar Pinto Jr. (INPE, Brazil, osmar@dge.inpe.br) I. Abstract IV. Thunderstorms during Flight 1 V. Quasi-electrostatic fields due to nearby lightning compared to a numerical model During the Brazil Sprite Balloon Campaign 2002-2003, thousands of the lightning-driven electric and magnetic field changes were measured in the stratosphere above 30 km in altitude. Two-hundred of these lightning events occurred within 75 km horizontal distance of the balloon payload and drove quasi-electrostatic field (QSF) changes of up to 140 V/m measured at the payload. Although the sprite imaging cameras were blocked by clouds during these balloon flights, these QSF can be predicted at sprite altitudes using the in situ data along with a numerical QSF model. It is found that the electric fields at sprite altitudes (60-90 km) never surpass breakdown when the fields for each of these 200 nearby lightning events are propagated to the mesosphere. In addition to measuring nearby electric fields, the balloon payloads measured thousands of ELF to VLF electric and magnetic fields from distant lightning (>75 km). These ELF to VLF fields generally agree with ground-based measurements [Cummer et al., GRL, 25, 1281, 1998] and models [Pasko et al., GRL, 25, 3493, 1998] but are in disagreement with the previous balloon-borne measurements during the Sprites99 Campaign [Bering, Adv. Space Res., 34, 1782, 2004]. Comparison between model (red line) at 34 km horizontal distance from payload and 34 km in altitude and data (blue points) for two +CG strokes at 0:50 s and 0:64 s which occur at about 00:00:09 UT. top panel: vertical electric field. bottom panel: radial electric field. Eighty minutes of dc vertical electric field data during Flight 1. The two largest field changes measured occurred at about 00:00:09 and 00:16:03 UT, seen as large negative transients, and were correlated with +CG flashes measured by the Brazilian Lightning Network (BIN). GOES8 Satellite IR image, 23:45 Dec. 6, 2002 (courtesy of CPTEC, Brazil) • II. Motivations • Address the following questions regarding how thunderstorms affect the middle and upper atmosphere: • Are the magnitudes and relaxation times of nearby lightning-driven quasi-electrostatic fields (QSF) above thunderstorms sufficient for sprite production and growth? • Do lightning-driven ELF to VLF electromagnetic field changes measured in the stratosphere indicate that current is flowing in the mesosphere after CG strokes? • Do QSF and ELF/VLF models and other experiments agree with these measurements? Model prediction of the vertical electric field magnitude vs. altitude at R = 0 km using the best fit parameters to the measured electric field change at Z = 34 km, R = 34 km. The vertical electric field magnitude at three instants in time (1 ms before the 1st +CG stroke, 1 ms after the second +CG stroke, and 350 ms after the second +CG stroke) is compared to the various breakdown thresholds. Flight 1 Trajectory Flight 1 Trajectory and CG Lightning Quasi-electrostatic fields due to nearby -CG and IC lightning III. The Brazil Balloon Campaign 2002-2003 Overview VI. ELF to VLF field changes due to distant (>75km) lightning • Primary Measurement Objective: Measure nearby (<75 km) lightning-driven quasi-electrostatic fields. • Logistics: Balloon payloads launched from Cachoeira Paulista, Brazil • (about 200 km northeast of Sao Paulo) on Dec. 6, 2002 and March 6, 2003. • On-board Measurements: dc to VLF vector electric fields, VLF magnetic fields, x-rays, and optical power at a float altitude of 32-35 km. • Results: The electric and magnetic fields driven by thousands of lightning events were measured by the payloads, including 200 events with 75 km. IC Flash -CG Flash • VII. Conclusions • The amplitudes and relaxation times of the nearby lightning events generally agree with the numerical QSF model developed by the authors verifying that the QSF approach is valid. • Positive cloud-to-ground (+CG) lightning and in-cloud (IC) lightning generally produce larger nearby electric field changes compared to negative cloud-to-ground lightning (-CG), which agrees with the correlation between large +CG strokes and sprites. • It is found that the electric fields at sprite altitudes (60-90 km) never surpass conventional breakdown when the fields for each of these 200 nearby lightning events are propagated to the mesosphere. The relativistic runaway threshold is surpasses from some of these events but at too high of an altitude to allow for enough avalanche lengths (e-foldings of the electron population) to cause optical emissions. • The typical relaxation times of these predicted electric fields (70 ms at 70 km) are long enough to allow sprites to initiate and grow. • The Brazil payloads measured ELF to VLF field changes for every CG stroke at 75-600 km distance from the payload and rarely measured delayed ELF pulses after these CG sferics. This disagrees with the Sprites99 payloads which rarely measured the CG sferics but for 90% of CG strokes measured a delayed ELF pulse, which they have attributed to mesospheric current flow [Bering, Adv. Space Res., 34, 1782, 2004; Bhusal, Adv. Space Res., 34, 1811, 2004]. ELF to VLF fields were measured for every CG stroke detected by BIN. For 2% (15/750) of -CGs and 17% (31/184) of +CGs a CG delayed pulse was measured which may indicate mesospheric current. This bias to +CGs suggests that mesospheric currents are driven by large charge moment strokes and that the QSF from these strokes initiate the mesospheric breakdown. The example shown here is a +111 kA CG 328 km horizontal distance from the payload. Payload Configuration, Nov. 2002 Acknowledgments:Work supported by NSF grants ATM-0091825 and ATM-0355190.