M-component mode of charge transfer to ground in lightning discharges

The M-component mode of charge transfer to ground is examined using (1) mul tiple-station measurements of electric and magnetic fields at distances ran ging from 5 to similar to 500 in from triggered-lightning channels and (2) measured currents at the channel base. Data have been obtained in 1997, 199 9, and 2000 at the International Center for Lightning Research and Testing at Camp Blanding, Florida, for (1) "classical" M-components that occur duri ng the continuing currents following return strokes and (2) impulsive proce sses that occur during the initial stage of rocket-triggered lightning and are similar to the "classical" M components. All lightning events considere d here effectively transported negative charge to ground. For one triggered -lightning event the electric field 45 km from the lightning channel was me asured together with the current and close fields. The shapes and magnitude s of the measured close electric and magnetic fields are generally consiste nt with the guided-wave mechanism of the lightning M component. Specificall y, the M-component electric field peak exhibits logarithmic distance depend ence, ln(kr(-1)), which is indicative of a line charge density that is zero at ground and increases with height. Such a distribution of charge is dist inctly different from the more or less uniform charge density that is chara cteristic of the dart leaders in triggered lightning, as inferred from clos e electric field measurements. The M-component magnetic field peak decrease s as the inverse distance (i.e., r(-1)), which is generally consistent with a uniform current within the lowest kilometer or so of channel. The M-comp onent electric field at 45 km appeared as a bipolar, microsecond-scale puls e that started prior to the onset of the M-component current at the channel base. M-component-type processes can produce acoustic signals with peak pr essure values of the same order of magnitude as those from the leader/retur n stroke sequences in triggered lightning.