Numerical Simulation of the chemical effects of sprite phenomena in the mesosphere

We conducted numerical simulation of the chemical effects of sprite halos in the mesosphere. Our target is to clarify how much O(¹D) is produced by dissociation of O₂ in a single sprite halo event. Monte Carlo simulation is performed to examine a quasi-static condition of the electron energy distribution functions (EEDFs) and to obtain the rate coefficients of electron impacts (especially in the processes related to ionization of N₂ and O₂, dissociation and dissociative attachment of O₂) for a non-thermal case of interest. Numerical simulation showed that ambient electrons are heated by intense lightning-induced electric field, up to an average energy of 2-7 eV, which is sufficient to dissociate O₂. The EEDFs are characterized by a non-Maxwellian distribution, but are in a quasi-static condition. The consistency of cross section data is evaluated, and the rate constants defined by the EEDFs as functions of E/N are compared with the previous results of other researchers. Thus, the effect of the uncertainty of the rate constants used in a quasi-electrostatic model in the estimation of the O(¹D) production will be discussed in this paper. Solving the continuity equations for O(¹D, ³P), electron, O^-, coupled with a 2-D quasi-electrostatic model (using the Poisson's equation and the equation of charge transfer), we estimated the total O(¹D) production by a single sprite halo. Initial results indicated that the total O(¹D) production is 10³-10⁴ cm^-3 at 70-90 km. We also examined the dependence of thundercloud charge moment and initial conditions (e.g., assuming density profiles in the daytime or nighttime) on the O(¹D) production and the temporal variation of O(¹D) during occurrence of a sprite halo. These results imply that the sprite halos become an important source of O(¹D) as giving rise to chemical potential changes of the mesosphere because O(¹D) is to initiate the oxidation of a wide variety of atmospheric long-lived tracer constituents (e.g., water vapor). We are improving our model for investigating the impact of sprite phenomena on the mesospheric HOx chemistry at their chemical equilibrium state. The effect on the HOx chemistry obtained from the improved model will be discussed.