Simultaneous remote electric and magnetic field measurements of lightning continuing currents

[1] Remote detection and measurement of the slowly varying continuing current that follows some lightning return strokes is difficult. Slow changes in quasi-static electric fields have been most frequently used to remotely measure continuing current. Because of the rapid attenuation of dipole electric fields with distance, though, the range over which the current will produce a measurable signature in the field is limited. Here we report the analysis of continuing current signatures observed in both the quasi-static electric and magnetic fields from four lightning strokes that were recorded from a range of 52 to 96 km at a measurement site near Fort Collins, Colorado on 15 July 2005. Electroand magnetostatic analyses show that the distinct magnetic and electric field signatures are produced by the continuing current and the associated net charge transfer, respectively. The measurements confirm that the quasi-static magnetic fields that follow some return strokes are signatures of continuing current. Using realistic noise estimates based on our measurements, we also show that modest to large continuing currents can be reliably detected and measured from long ranges (more than 1000 km for 1 kA) using quasi-static magnetic field signatures.