Modeling sprite streamer initiation from an unstable sprite halo front

High-speed videos have shown that sprite streamers either form out of dark background in the lower ionosphere or appear to be initiated from inhomogeneities at the bottom of a sprite halo [e.g., Cummer et al., GRL, 33, L04104, 2006; McHarg et al., GRL, 34, L06804, 2007; Stenbaek-Nielsen et al., GRL, 34, L11105, 2007]. On the other hand, modeling studies have concluded that inhomogeneities in the lower ionosphere are required for sprite streamer initiation at both overbreakdown [e.g., Qin et al., JGR, 116, A06305, 2011] and subbreakdown conditions [e.g., Liu et al., PRL, 109, 025002, 2012; Kosar et al., JGR, 117, A08328, 2012].