科研之友
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摘要
A full time-dependent kinetic study is presented for the main microscopic collisional and radiative processes underlying the optical flashes associated with an impulsive (tau = 5 mu s) discharge in the form of a single sprite streamer passing through an air region of the mesosphere at three different altitudes (63, 68 and 78 km). The kinetic formalism developed includes the coupling of the rate equations of each of the different species considered (electrons, ions, atoms and molecules) with the Boltzmann transport equation so that, in this way, all the kinetics is self-consistent, although, in the present approach, the electrodynamics (no Poisson equation is considered) is not coupled. The chemical model set up for air plasmas includes more than 75 species and almost 500 reactions. In addition, a complete set of reactions (more than 110) has been considered to take into account the possible impact of including H(2)O (humid chemistry) in the generated air plasmas. This study also considers the vibrational kinetics of N(2) and CO(2) and explicitly evaluates the optical emissions associated with a number of excited states of N(2), O(2), O in the visible, CO(2) in the infrared (IR) and ultraviolet (UV) emissions of sprite streamers due to the N(2) Lyman-Birge-Hopfield (LBH) and the NO-gamma band systems. All the calculations are conducted for midnight conditions in mid-Latitude regions (+38 degrees N) and . longitude, using as initial values for the neutral species those provided by the latest version of the Whole Atmosphere Community Climate Model (WACCM). According to our calculations, the impact of 4 ppm of H(2)O is only slightly visible in O(3)(-) at 68 and 78 km while it strongly affects the behaviour of the anion CO(4)(-) at all the altitudes investigated. The local enhancement of NO(x) predicted by the present model varies with the altitude. At 68 km, the concentrations of NO and NO(2) increase by about one order of magnitude while that of NO(3) exhibits a remarkable growth of up to almost three orders of magnitude. The variation of the O(3) density predicted by the model in the sprite streamer head is negligible in all the altitudes investigated. The analysis of the time dependence of the electron distribution EDF) in the sprite plasma during the pulse reveals that the EDF transient is quite fast, reaching its 'steady' values during the pulse in less than 100 ns (much shorter than streamer head lifetimes). In addition, the calculated EDF during the pulse and in the afterglow is far from being Maxwellian, especially for energetic electrons (with epsilon > 30 eV). Finally, the evaluation of the mid-latitude nighttime electrical conductivity of air plasmas under the influence of a single sprite event reveals an increase of up to four orders of magnitude (at 68 km) above its measured background level of 10(-11) similar to cm(-1) at an altitude of similar to 70 km. This sudden increase in the electrical conductivity lasts for 100 ms (at 68 km), being shorter (similar to 1 ms) and longer (1 s) at 63 km and 78 km, respectively. The total power delivered by the streamer head of a single sprite event has been estimated to be approximately 1677 W (at 78 km), 230 kW (at 68 km) and 78 MW (at 63 km).
- 出版日期2008-12-7
