Radio frequency radiation beam pattern of lightning return strokes: Inferred from FORTE satellite observations

The fast propagating return stroke is expected to produce a radiation pattern differing from a dipole pattern, with a “correction” of (1 −vcos θ/c)−1or the so‐calledFfactor. The FORTE satellite measures lightning Very High Frequency (VHF) radiation at different angles from the up space and offers the first opportunity of examining theFfactor. In this report, we studied a group of FORTE‐detected lightning events that were also observed by the National Lightning Detection Network (NLDN). NLDN provided the discharges' geolocations and helped to identify the discharge types. During the summers of 1998 and 1999, 25,721 coincident events were found. Among these, 2092 were found to be narrow (<100 ns), VHF‐intense and highly polarized bursts and were found to be associated with the beginning of return strokes. Through careful statistical analysis regarding the distributions of the event occurrence, we found that the overall ensemble of events can be considered to have an isotropic pattern in the upper half‐space. The subset of the narrow bursts displays a beam pattern that agrees with theFfactor at a propagating speed ofv= 0.75c. The latter is inferred by comparing FORTE observations to a free‐space transmission line (TL) model. The analysis shows that the ground does not affect the narrow‐burst beam pattern observed from the upper half‐space; the source for the narrow burst needs to be a few tens of meters above the surface of the Earth and is apparently associated with the junction point of the attachment process. The physical size of the corresponding discharge is estimated to be less than 20 m. The analysis also suggests that a single upward current is responsible for the observations, rather than a bidirectional current as suggested by others for the attachment process. Similarly, the traveling current source (TCS) model that consists of a simultaneous downward current is found less suitable for the initiation of return strokes.