This paper reports a modeling study on sprite streamer formation at subbreakdown conditions, i.e., the initiation of sprite streamers in a lightning field below the breakdown threshold field Ek. Successful formation of streamers is observed for a variety of electric fields with the lowest field of 0.3Ekwhen an appropriate ionization column is introduced. The simulation results also indicate that the initial ionization patch from which a streamer is initiated may become very bright following the streamer formation, and its brightness persists as the streamer continues its propagation. The overall luminous structure of the streamer and the ionization patch is very similar to the appearance of initiation of a single sprite streamer on high‐speed images. The brightness of the patch depends on its initial density, and the patch with smaller density is brighter than the denser one. An analytical formula describing the temporal and spatial variation of the electric field in the streamer channel is derived to explain the brightening of the ionization column as well as the luminous streamer trail. Finally, comparisons between the streamers from the ionization patch and those forming in the vicinity of a conducting sphere in an electric field belowEk show that the exponential growth rates associated with streamer characteristics are similar.
