Inferring lightning-channel geometry from polarimetry of VHF radio emissions

The “polarization state” of emitted radio waves sets important constraints on the geometry, coherence, and complexity of the radio-emitter structure. In the case of intracloud emitters, detailed optical imagery is usually impossible, so inferences based on radio techniques would be especially useful for better understanding the physics of lightning discharges. Early work [Shao and Jacobson, 2001; 2002] with the FORTE satellite [Jacobson et al., 1999] demonstrated the feasibility of measuring the polarization state of lightning-generated Very High Frequency (VHF) radio waves simultaneously recorded on two orthogonal antennas.

During the FORTE project it was observed that the most powerful intra-cloud radio emissions tend to be unpolarized, in comparison with the much weaker intracloud leader-step emissions [Jacobson and Light, 2003]. This suggestion relied on “eyeballing” data, rather than on applying the polarimetry method formally and systematically to a statistically meaningful dataset of intracloud-lightning VHF emissions. Thus trends could never be formally assessed. We have now addressed, and to some extent overcome, certain technical difficulties posed by “mass” application of VHF polarimetry. It is now possible for a large dataset of lightning VHF recordings by FORTE to be systematically analyzed for polarization state, making available further information pertinent to emission geometry. We will present preliminary findings on implications for emitter geometry, complexity, and coherence.

Acknowledgements: ARJ and RH are partially supported by the DARPA “Nimbus” program. X-MS participates under the auspices of the USDOE.

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