A reexamination of important N₂ cross sections by electron impact with application to the dayglow: The Lyman‐Birge‐Hopfield Band System and N I (119.99 nm)

Abstract

The far ultraviolet emission spectrum (120 to 210 nm) of electron‐excited N2 has been obtained in a crossed‐beam laboratory experiment. The cross section of the Lyman‐Birge‐Hopfield (LBH) band system (a¹Πg → X¹Σg+) has been remeasured using experimental techniques we have previously developed for this metastable transition. The improved laboratory data set for the a¹Πg state allows a determination of the excitation, emission, and predissociation cross section from threshold to 200 eV for use in planetary atmosphere models of the dayglow and aurora. An analytic fit to the experimental cross section allows accurate estimates to arbitrarily high excitation energy. The close agreement in both energy dependence and absolute cross section values between the emission measurements, presented here, and published electron scattering results shows cascade is small (<5%). The total excitation cross section for the N2 a¹Πg state is estimated to be 6.22 ± 1.37×10−18 cm² at 100 eV. The absence of emission bands for υ′ ≥ 7 suggests the predissociation yield is unity. The excitation function of each vibrational level is found to have the same shape to within 5%. In the low‐energy region, ε < 20 eV, differences in excitation threshold lead to a significant departure of the relative vibrational cross sections from the Franck‐Condon distribution. Thus the relative LBH vibrational population distribution in a planetary dayglow or aurora is affected by the energy distribution of the electron flux; and we show that atmospheric models need to include this threshold effect. The N I (119.99 nm) cross section has also been remeasured and found to be 3.48 ± 0.77×10−18 cm² at 200 eV on the basis of a comparison with Lyman α emission from dissociative excitation of H2.

Solar extreme ultraviolet irradiance: Present, past, and future

“…Systematic uncertainty relates to the absolute radiance measured by the OI 135.6 nm and N 2 LBH channels, for which the principal systematic contributors are the GUVI absolute calibration (∼15%) and the photoelectron impact excitation cross sections for O (∼70%) and N 2 (∼25%). In both the limb and disk retrievals, the photoelectron impact excitation cross section used in the core algorithms is a scaled version of that measured by Ajello and Shemansky [1985]. A new measurement of this cross section by Young et al [2010] yields a value that is different in shape and magnitude (by nearly a factor of 2).…”

XUV Photometer System (XPS): Improved Solar Irradiance Algorithm Using CHIANTI Spectral Models

“…The XUV range is dominated by coronal emissions that include atomic (bound -bound) emission lines and bremsstrahlung (free -free) continuum, and these coronal emissions are enhanced through the dynamic magnetic activity on the Sun. Magnetic reconnection in the corona is thought to be the energy source for flare events (e.g., Aschwanden, 2004), and the magnetic-driven active regions, and their subsequent decay into active network, are the source for most of the longer term (days to years) variations (e.g., Woods et al, 2000). The XPS measurements clearly show these types of variations with a factor of 2 to even more than a factor of 100 at the shortest wavelengths seen during flare events and over the longer term.…”

“…For N 2 (a 1 g ), the parent state of the LBH bands, the total excitation cross section used is 4.3 × 10 −17 cm 2 at 18 eV. This cross section is based on the measurement of 3 × 10 −17 cm 2 by Ajello and Shemansky (1985) but adjusted to the current value by accounting for revised calibration standards and cascade from the a 1 u and w 1 u states of N 2 . Johnson et al (2005) have more recent results on the N 2 cross sections by electron impact, and those results are about a factor of 2 lower than the previous ones.…”

“…Flares are known to be multitemperature emissions (e.g., Aschwanden, 2007), but the single-temperature approach is a simple, reliable one for data processing because the GOES solar X-ray data are available near real time and because the GOES flare temperature relationship is well characterized (e.g., Garcia, 1994). For processing efficiency, the flare reference spectra, as well as the daily component reference spectra, are static (stored) data files.…”

Polar cap arcs from the magnetosphere to the ionosphere: kinetic modelling and observations by Cluster and TIMED

“…Emission cross-sections for process (d), i.e. N 2 LBH (0,3) emission at 135.4 nm, are initially taken from Ajello and Shemansky (1985) multiplied by 0.892 as recommended by Itikawa (2006).…”