Role of electronic excited N₂ in vibrational excitation of the N₂ ground state at high latitudes

Abstract

[1] Vibrationally excited N 2 is important in determining the ionospheric electron density and has also been proposed to play a role in the production of NO in disturbed atmospheres. We report here predictions of the absolute vibrational distributions in the ground electronic state of N 2 produced by electron impact excitation, at noon and midnight under quiet geomagnetic conditions and disturbed conditions corresponding to the aurora IBCII + and IBCIII + at 60°N latitude and 0°longitude, at altitudes between 130 and 350 km. These predictions were obtained from a model which includes thermal excitation and direct electron impact excitation of the vibrational levels of the N 2 ground state and its excited electronic states; radiative cascade from all excited electronic states to all vibrational levels of the ground electronic state; quenching by O, O 2 , and N 2 ; molecular and ambipolar diffusion; and the dominant chemical reactions. Results from this study show that for both aurora and daytime electron environments: (1) cascade from the higher electronic states of N 2 determines the population of the higher vibrational levels in the N 2 ground state and (2) the effective ground state vibrational temperature for levels greater than 4 in N 2 is predicted to be in the range 4000-13000 K for altitudes greater than 200 km. Correspondingly, the associated enhancement factor for the O + reaction with vibrationally excited N 2 to produce NO + is predicted to increase with increasing altitude (up to a maximum at a height which increases with auroral strength) for both aurora and daytime environments and to increase with increasing auroral strength. The contribution of the cascade from the excited electronic states was evaluated and found to be relatively minor compared to the direct excitation process.

Electron impact contribution to infrared NO emissions in auroral conditions

“…[9] Reaction rates used for most processes are as given in previous publications [Cartwright et al, 2000;Campbell et al, 2006Campbell et al, , 2007. Exceptions are included in Table 1, which summarises the rates for reactions involved in the production, loss, excitation and de-excitation of NO.…”

“…A moderate auroral flux of 0.5 erg.cm À2 s À1 is assumed, this being half of the value used in similar calculations [Barth, 1992;Bailey et al, 2002]. A ''statistical-equilibrium'' calculation [Campbell et al, 2006] is run to determine the densities of N 2 (A 3 S u + ) due to both photoelectron and auroral excitation. This calculation is necessary because the large range of rates involved in excitation and de-excitation makes a time-step calculation impractical.…”

“…Hence to calculate the infrared emissions in an aurora, the simulation is run again for 3 minutes in 0.001-s time steps, starting with the"background" densities, including those of NO and N( 4 S), determined in the 3-day calculation. The aurora produces much higher electron temperatures [Vallance Jones, 1974] than occur otherwise, so these are recalculated at 36-s intervals [Campbell et al, 2006].…”

“…Exceptions are included in Table 1, which summarises the rates for reactions involved in the production, loss, excitation and de-excitation of NO. Isotropic photoelectron and auroral fluxes [Campbell et al, 2006] of secondary electrons, plus a Maxwellian flux of thermal electrons, were folded with cross sections to determine ionisation and excitation rates.…”

“…Improvements to our computational model (e.g. time-dependent calculations, allowing modeling of solar input and molecular and eddy diffusion over a range of altitudes, and of a limited-duration aurora) [Campbell et al, 2006[Campbell et al, , 2007 allow predictions of both background and auroral-induced NO densities for the conditions of the measurements. Also, an updated rate for reaction (R2) has been determined [Duff et al, 2005].…”