Initiation of Sprite Streamers from Natural Mesospheric Structures
Abstract:Sprites are large, luminous electrical discharges in the upper atmosphere caused by intense cloud-to-ground lightning flashes. They manifest a possible, impulsive coupling mechanism between low atmospheric regions and the upper atmosphere. Their dynamics are governed by filamentary plasma discharges, of tens to hundreds of meter wide, known as streamers. The propagation properties of sprite streamers have been well studied by past work [e.g., Liu, et al., JGR, 114, A00E02, 2009; Liu et al, JGR, 114, A00E03, 2009; Luque and Ebert, Nat. Geosci., 2, 757, 2009; Liu, GRL, 37, L04102, 2010; Luque and Ebert, GRL, 37, L06806, 2010]. However, how sprite streamers are initiated is not well understood. Recent high-speed images show that mesospheric/lower ionospheric structures are frequently involved in initiation of sprite streamers [e.g., Stenbaek-Nielsen et al., Surv. Geophys., 34, 769, 2013; Qin et al., Nat. Comm., 5, 2014]. Although earlier theoretical and numerical studies routinely used strong plasma inhomogeneities to initiate streamers, it is only recently that inhomogeneities are concluded to be required for sprite streamer initiation [e.g., Qin et al., JGR, 116, A06305, 2011; Liu et al., PRL, 109, 025002, 2012; Kosar et al., JGR, 117, A08328, 2012; Kosar et al, GRL, 40, 6282, 2013]. However, the inhomogeneities used in various models are rather ad hoc and often unrealistic.
In this talk, we present numerical simulations to show naturally-existing mesospheric structures, such as those produced by gravity waves via instability and breaking [e.g., Fritts and Alexander, Rev. Geophys., 41, 1003, 2003], can initiate sprite streamers under the influence of the measurement-inferred lightning field. Evidence from high-speed video observations supporting this theory is discussed. This mechanism naturally explains many aspects of observed sprite streamer initiation including variability in the delay of sprite initiation, sprites caused by weak lightning, optical signatures of streamer initiation, initiation of isolated streamers from different sites, etc. In addition, the dependence of the initiation process on the mesospheric structures can potentially be utilized to study those structures at sprite initiation altitudes, which at present are nearly impossible to be observed by other means.