On the time and amplitude properties of electric fields near sources of lightning in the VLF, LF, and HF bands


  • Taketoshi Nakai


Observations of electric fields near sources of lightning, followed by analysis of the observed data statistically with an electronic computer, were made at the Imaichi Observatory in Japan during the summers of 1969, 1970, and 1971. The electric fields were measured simultaneously with four narrow-band receivers having the following central frequencies (and 3 dB bandwidths): 3 kHz (460 Hz), 50 kHz (480 Hz), 90 kHz (850 Hz), and 5 MHz (1 kHz). Particular emphasis was placed on analyzing the field data for which the ground discharges can be assumed to have outnumbered the cloud discharges. The characteristics of the electric fields near the sources in the VLF, LF, and HF bands are described in terms of (1) the envelope-level occurrence-time-interval distribution (OTID), and (2) the envelope-level crossing-rate distribution (CRD). The median curves of OTID and CRD are given for each of the four measurement frequencies. Relationships between the OTID and CRD from the same data sample are derived in an averaged form. The procedure used for describing the characteristics of the electric field make it possible to model the electric field near the source for each frequency by taking into account both the amplitude and the time characteristics. The behavior of the CRD in the VLF to HF range is observed over the same time interval during general variations in the lightning discharge conditions.