Transport‐theoretic model for the electron‐proton‐hydrogen atom aurora: 2. Model results

Abstract

In the companion paper (Basu et al., this issue), a self‐consistent transport‐theoretic model for the combined electron‐proton‐hydrogen atom aurora was described. In this paper, numerical results based on the model are presented. This is done for the pure electron aurora, the pure proton‐hydrogen atom aurora, and finally for the combined aurora. Adopting commonly used types of energy distributions for the incident particle (electron and proton) fluxes, we give numerical solutions for the precipitating electron, proton, and hydrogen atom differential number fluxes. Results are also given for ionization yields and emission yields of the following features: N2+ first negative group (3914 Å), N2 second positive group (3371 Å), selected N2 Lyman‐Birge‐Hopfield bands (1325 Å, 1354 Å, 1383 Å, 1493 Å, and all bands between 1700 and 1800 Å), O I (1356 Å), Lα (1216 Å), Hβ (4861 Å), and Hα (6563 Å). The yield at 1493 Å also contains a contribution from N I (1493 Å), which in fact dominates LBH emission. A major new result of this study is that the secondary electron flux produced by the proton‐hydrogen atom aurora is much softer than that produced by the electron aurora. This increased softness is due to the fact that (for energies of auroral interest) cross sections for secondary electron production by proton and hydrogen atom impact decrease exponentially with increasing secondary electron energy, whereas the cross sections for electron impact decrease as an inverse power law with increasing secondary energy. In our study of the pure electron aurora (no primary protons or hydrogen atoms present) and the pure proton‐hydrogen atom aurora (no primary electrons present), two important results obtain. First, certain emission features (for example, 3371 Å) are excited in completely different ways for the two kinds of aurora. Second, the "eV per electron‐ion pair" as a function of the characteristic energy E0 is nearly constant for the pure electron aurora, with a value of about 34, but varies by about 20% for the pure proton‐hydrogen atom aurora. In our study of the combined electron‐proton‐hydrogen atom aurora, two additional results obtain. Since the proton‐hydrogen atom contribution to the total incident energy flux in the midnight sector is, on the average, about 20 to 25% of that of the electrons, we find that when it is neglected, the ionization yield as well as the yields of many emission features will be underestimated, on the average, by about the same percentage. We also find that in the morning sector of the combined aurora, a double bump in the altitude profile of the E region electron density is possible for certain auroral conditions.