Prediction of photon yields for proton aurorae in an N₂ atmosphere

Abstract

Absolute photon yields for band emissions of the N2+ first‐negative and N2 second‐positive systems are predicted for proton aurorae in an N2 atmosphere. The predictions are made by using a proton auroral model based largely on collision cross‐section information, including the production of these emissions in the atmosphere by secondary electrons. The photon yields for Balmer‐alpha and Balmer‐beta emissions and the total secondary electron production during proton aurorae are also determined. The results are compared with other predictions and with a limited number of proton auroral observations.

Cross sections for electron capture and loss by H+ and H impact on O and O2

“…They both indicate that a mixed‐projectile flux moving through a purely O or O 2 atmosphere is almost completely dominated by the H‐atom flux component for projectile energies below 1 keV. This is in contrast with the data for N 2 targets [ Van Zyl et al , ], which reveal a broad maximum (with F 0 ≈ 0.83) at ~ 2.5 keV projectile energy, but then is followed by a slow decline (to F 0 ≈ 0.73) near 1 keV. Like those for N 2 , we expect the F 0 fractions for O and O 2 targets to slowly decrease at the higher projectile energies so that F 0 ≈ F 1 ~ 0.5 in the 20 –35 keV projectile‐energy range.…”

“…Here their cross sections can exhibit surprisingly detailed structures as a function of H-atom energy. We have found such striking behavior for H-atom impact on Ar [Van Zyl et al, 1977], N 2 and O 2 [Van Zyl et al, 1978], H 2 [Van Zyl et al, 1981], and other H atoms [Gealy and Van Zyl, 1987a], down to H energies of only 0.05 keV. But such structures persist down to even lower H energies, as nicely shown for rare-gas-atom targets by Aberle et al [1980].…”

“…As can be seen, the differences between the two data sets are very similar to the differences in the σ A /σ M ratios shown in Figure 4b. The σ 01 for both O and O 2 targets exhibits a rather smooth and rapid decrease with decreasing H-atom energy, so typical of such ionization-stripping reactions [Van Zyl et al, 1977;Van Zyl et al, 1978;Van Zyl et al, 1981;and Gealy and Van Zyl, 1987a].…”

“…Such projectile‐charge‐shuttling reactions can occur hundreds or even thousands of times, depending upon the initial H + energy, which can sometimes be up to 100 keV or more at the upper end of the primary H + energy distributions. Because of this, some proton‐auroral models [ Van Zyl et al , ] describe the penetrating‐particle flux as a composite of both H + and H, with each component reasonably approximated by the flux fractions (We will here ignore the small H − flux fraction because the electron affinity of H is only ~0.7 eV, so its “extra” electron is easily stripped off in almost any reaction.) While the projectile‐energy range examined here is near the lower end of typical primary‐proton‐auroral energies (commonly between about 3 and 100 keV [ Eather , ]), it should be remembered that all atmospherically incident H + /H particles must pass through this lower‐energy region on their way to eventual thermalization.…”

“…Such projectile-charge-shuttling reactions can occur hundreds or even thousands of times, depending upon the initial H + energy, which can sometimes be up to 100 keV or more at the upper end of the primary H + energy distributions. Because of this, some proton-auroral models [Van Zyl et al, 1984] describe the penetrating-particle flux as a composite of both H + and H, with each component reasonably approximated by the flux fractions…”