High precision lightning measurements using coherent averaging of long-distance magnetic fields

Monday, 15 December 2014
Joel Lyle Weinert, Duke University, Durham, NC, United States and Steven A Cummer, Duke University, Electrical and Computer Engineering Department, Durham, NC, United States
Measurement of magnetic fields produced by lightning has many advantages over other methods of lightning characterization. Because low frequency magnetic fields produced by lightning decay slowly with distance, magnetic field measurements can be performed at large distances, often in the range of thousands of kilometers. As we have shown previously, coherent time-aligned averaging of similar lightning events can overcome many limiting factors associated with magnetic field measurements at large distances, such as sensitivity, as well as both environmental and sensor noise. Using such a method, it is possible to achieve as broadband noise level of tens of femtotesla, allowing for the detection of signals produced by current moments of a few hundred amp-kilometers. In this work, we present the results of investigation of lightning from four thunderstorms from summer 2013, each located several hundreds of kilometers from the measurement location. Cloud-to-ground (CG) events of both positive and negative polarities are compared between storms, allowing precise, quantitative measurement of flash processes with relatively small current moments, such as continuing currents and leader development. By comparing events from several storms, some conclusions about consistency of processes for both positive and negative CG flashes can be made.