Properties of relatively long streamers initiated from an isolated hydrometeor

This paper reports a modeling study on the dynamics of relatively long streamers initiated from an isolated hydrometeor in subbreakdown (0.5E(k)) and overbreakdown (1.5E(k)) electric fields, respectively, where E-k is the conventional breakdown threshold field. The modeling results indicate that at 0.5E(k) field, the streamer shows a generally exponential growth feature, but its channel characteristics such as radius, electron density, and current density exhibit colocated fluctuations. On the other hand, at 1.5E(k) field, the streamer shows faster exponential growth, and no fluctuations of those parameters are observed. The exponential growth property allows one to use the modeling results to predict the characteristics of longer streamers, as well as to quantify the thunderstorm electrical conditions including the high-field region size and potential difference in order to accelerate the streamer to the high speed of fast positive breakdown that was observed recently in lightning initiation. At 0.5E(k), a potential difference of similar to 0.3-1.8MV is required to accelerate the streamer to the high speed of 1-5 x 10(7) m/s, while at 1.5E(k), similar to 0.07-0.4MV is required. The exponentially growing streamer current produces an exponentially increasing magnetic field, leading to electromagnetic (EM) field radiation. It is found that the growth rate of the magnetic field (1/tau(B phi)) radiated by a streamer can be obtained by a simple relation 1/(tau B phi) similar to 3/tau,where 1/tau is the growth rate of the streamer. Combining our modeling results with the Fourier analysis reported by Qin et al. (2012), we find that streamers at thundercloud altitudes radiate in the high frequency and very high frequency range of the EM spectrum.