Energetic oxygen atoms in the polar geocorona

摘要

The role of the auroral sources induced by the electron and proton precipitation in the formation of the hot oxygen corona in the polar upper atmosphere is studied. It is found that both electron precipitation through exothermic chemistry and proton precipitation through atmospheric sputtering significantly contribute to the population of the hot oxygen geocorona. It is also found that only atmospheric sputtering results in the formation of the escape flux of energetic oxygen atoms, providing an important source of heavy atoms for the magnetosphere. The exothermic chemistry induced by the electron precipitation and/or by the absorption of the solar UV radiation is operating continuously in the polar upper atmosphere and results in a steady population of the very near-Earth environment by suprathermal oxygen atoms with energies below a few eV. By contrast, atmospheric sputtering by magnetospheric protons provides a more variable contribution, strongly coupled with the cusp region. It produces the more energetic oxygen atoms that populate the external regions of the hot oxygen geocorona. The results of calculations are in a good agreement with the analysis of the low-latitude perigee Low Energy Neutral Atom (LENA) images showing that the instrument signal consists of low to medium energy (5–30 eV) oxygen atoms produced in and near the cusp region. The more energetic (>30 eV) fraction of energetic oxygen atoms produced by the ion-induced atmospheric sputtering could be responsible for the energetic neutrals observed by the instrument far away from the cusp or oval regions. The total escape flux of oxygen atoms associated with atmospheric sputtering by protons is found about 8 × 1023 s-1; therefore this mechanism may provide a substantial contribution to the magnetospheric oxygen population.