M-component currents of first return strokes in natural negative cloud-to-ground lightning

[1] Records of M-component currents of first return strokes in natural negative cloud-to-ground lightning measured in a short instrumented tower are shown along with correlated video records, electric field changes, and relative luminosity. The synchronized fast-camera records clearly match the increase of luminosity of the faint lightning channel. The parameters of these M components are all consistent with those of triggered lightning. According to records of electric field change of lightning events near Morro do Cachimbo Station and of currents measured at the instrumented tower, the frequency of first return strokes exhibiting M components is within the range of tens of percent and not of a few percent, as usually accepted. This expectation of a few percent is based on a very low number of first return strokes expected to be followed by continuing currents. It was found that most of the analyzed first-return-stroke M components of natural lightning occur at the final phase of the return-stroke process and not during the flow of continuing currents. Apparently, this also holds true for most of the first M components of subsequent return strokes.