Emission Heights of Energetic Lightning Pulses in Thunderstorm Clouds

Since 2004 the newly developed 3D lightning detection network LINET utilizes VLF/LF techniques for measurements of distinct pulses with waveforms and amplitudes that often resemble the ones typical for return strokes but occur inside thunder-clouds and do not connect to ground. For simplicity, these distinct signals are termed ‘IC-strokes’. The determined emission heights represent a new data source and call for further explanation. In a first attempt to understand the nature of these IC-strokes, 3D scans from polarimetric high-resolution radar of DLR (POLDIRAD) have been used for comparison, giving verification that the heights coincide with regions of graupel and ice aloft. A more specific IC-stroke analysis was carried out in 2010: a 7-sensor LINET system was set up at Kennedy Space Center (Florida) enabling a comparison with 3D lightning data from 4DLSS (also called LDAR2), the high-resolution VHF network operated by NASA. It turned out that the emission heights of LINET IC-strokes fit perfectly into channels defined by radio source points from 4DLSS, no matter whether these channels were located in lower (10 km) altitudes, and independent of whether the discharge was a ground- or a cloud flash. In fact, the accuracy of LINET heights was mostly better than ~10%. Furthermore, earlier observations [Betz et al., 2008] from comparisons with 2D VHF (Safir type) systems in Europe have been confirmed: in a substantial fraction of discharges the first measurable signal is an IC-stroke, whereby the VHF system does not record any prior electric activity