Photodetachment of electrons at sprite altitudes by visible and UV emissions of molecular nitrogen as a mechanism of initiation of sprite streamers

Modeling [Qin et al., JGR, 116, A06305, 2011; GRL, 40, 4777, 2013; Nat. Comm., 5, 3740, 2014] and experimental [Lang et al., JGR, 116, A10306, 2011] findings indicate that the presence of plasma irregularities is a necessary condition for the initiation of sprite streamers. Sprite streamers are often preceded by a relatively low intensity diffuse glow discharge referred to as sprite halo [Barrington-Leigh et al., JGR, 106, 1741, 2001]. Janalizadeh and Pasko [Abstracts AE21A-08; AE21B-3132; presented at 2018 Fall Meeting, AGU, Washington DC, 10-14 Dec., 2018] have recently introduced a modeling framework allowing investigation of halo induced electron impact ionization and photoionization of small traces of metallic species existing at sprite altitudes due to meteoric ablation as a source of plasma seeds required for initiation of sprite streamers. In the present work we extend this modeling framework to studies of photodetachment of electrons from O^-, O₂^-, OH^- and NO₃^- ions due to halo emissions. O₂^-, O^- and OH^- are the primary negative ions of the ambient D-region ionosphere [e.g., Pavlov, Surv. Geophys., 35, 259, 2014], and O^-, O₂^- and NO₃^- are abundantly produced by streamer discharges in sprites [Sentman et al., JGR, 113, D11112, 2008; Gordillo-Vazquez, J. Phys. D: Appl. Phys., 41, 234016, 2008] and may provide relatively long-lasting seeds for sequential initiation of sprite streamers. Our analysis indicates that levels of these ions required for initiation of first sprite streamers can be produced by over-dense meteor trails with initial electron line densities > 10¹⁶ m^-1 [e.g., Hocking et al., Ann. Geophys., 34, 1119, 2016]. The electron affinities of O^-, O₂^-, OH^- and NO₃^- are approximately 1.46, 0.45, 1.83, and 3.94 eV, respectively [e.g., Phelps, Can. J. Chem., 47, 1783,1969; Pavlov, 2014]. The first positive N₂, second positive N₂, first negative N₂⁺, and Lyman-Birge-Hopfield (LBH) N₂ band systems that have been documented in sprites [e.g., Liu et al., GRL, 33, L01101, 2006, and references therein], all contribute to photodetachment of electrons from O^-, O₂^-, and OH^- ions. Due to the relatively high affinity of NO₃^- (3.94 eV), only second positive N₂ and N₂ LBH band systems contribute to photodetachment of electrons from NO₃^-.