Resonance scattering by auroral N₂⁺: steady state theory and observations from Svalbard

Studies of auroral energy input at high latitudes often depend on observations of emissions from the first negative band of ionised nitrogen. However, these emissions are affected by solar resonance scattering, which makes photometric and spectrographic measurements difficult to interpret. This work is a statistical study from Longyearbyen, Svalbard, Norway, during the solar minimum between January and March 2007, providing a good coverage in shadow height position and precipitation conditions. The High Throughput Imaging Echelle Spectrograph (HiTIES) measured three bands of N2+ 1N (0,1), (1,2) and (2,3), and one N2 2P band (0,3) in the magnetic zenith. The brightness ratios of the N2+ bands are compared with a theoretical treatment with excellent results. Balance equations for all important vibrational levels of the three lowest electronic states of the N2+ molecule are solved for steady-state, and the results combined with ion chemistry modelling. Brightnesses of the (0,1), (1,2) and (2,3) bands of N2+ 1N are calculated for a range of auroral electron energies, and different values of shadow heights. It is shown that in sunlit aurora, the brightness of the (0,1) band is enhanced, with the scattered contribution increasing with decreasing energy of precipitation (10-fold enhancements for energies of 100 eV). The higher vibrational bands are enhanced even more significantly. In sunlit aurora the observed 1N (1,2)/(0,1) and (2,3)/(0,1) ratios increase as a function of decreasing precipitation energy, as predicted by theory. In non-sunlit aurora the N2+ species have a constant proportionality to neutral N2. The ratio of 2P(0,3)/1N(0,1) in the morning hours shows a pronounced decrease, indicating enhancement of N2+ 1N emission. Finally we study the relationship of all emissions and their ratios to rotational temperatures. A clear effect is observed on rotational development of the bands. It is possible that greatly enhanced rotational temperatures may be a signature of ion upflows

Modelling N21P contamination in auroral O+ emissions

Modelling of N21P (5,3) contamination at the O+ doublet emissions in the region of 732 nm is presented. The method is derived from a known relationship between emission from the N21P (5,3) band and emissions from the N21P (5,2), (4,1) and (3,0) bands. A synthetic molecular spectrum is used to quantify a temperature-dependent emission ratio of these band systems as a function of filter characteristics and emission altitude. Five optical observations of high-energy auroral periods on 9 January 2008 are compared with results from the synthetic spectrum. Two cameras from a high sensitivity, high frame rate (20 Hz) ground based imager in combination with a co-located high resolution spectrograph are used to identify events which are dominated by molecular nitrogen emissions. There is good agreement between the observed and modelled ratios. The temperatures associated with these ratios agree well with temperature profiles extracted from fitting the synthetic spectra to the spectrograph data. A synthetic spectrum is important for future work when the removal of N21P (5,3) contamination from O+ (2P) doublet emissions is required at high temporal resolution

Modelling of N21P emission rates in aurora using various cross sections for excitation

Measurements of N₂1P auroral emissions from the (4,1) and (5,2) bands have been made at high temporal and spatial resolution in the region of the magnetic zenith. The instrument used was the auroral imager ASK, situated at Ramfjordmoen, Norway (69.6 N, 19.2 E) on 22 October 2006. Measurements from the European Incoherent Scatter Radar (EISCAT) have been combined with the optical measurements, and incorporated into an ionospheric model to obtain height profiles of electron density and emission rates of the N₂1P bands. The radar data provide essential verification that the energy flux used in the model is correct. One of the most important inputs to the model is the cross section for excitation to the B³&Pi;_<I>g</I> electronic state, as well as the cross sections to higher states from which cascading into the B state occurs. The balance equations for production and loss of the populations of all levels in each state are solved in order to find the cascade contributions. Several sets of cross sections have been considered, and selected cross sections have been used to construct "emission" cross sections for the observed bands. The resulting brightnesses are compared with those measured by ASK. The importance of specific contributions from cascading is found, with more than 50% of the total brightness resulting from cascading. The cross sections used are found to produce a range of brightnesses well within the uncertainty of both the modelled and measured values