Energy Relaxation of Helium Atoms in Astrophysical Gases

Abstract

We report accurate parameters describing energy relaxation of He atoms in atomic gases, important for astrophysics and atmospheric science. Collisional energy exchange between helium atoms and atomic constituents of the interstellar gas, heliosphere, and upper planetary atmosphere has been investigated. Energy transfer rates, number of collisions required for thermalization, energy distributions of recoil atoms, and other major parameters of energy relaxation for fast He atoms in thermal H, He, and O gases have been computed in a broad interval of energies from 10 meV to 10 keV. This energy interval is important for astrophysical applications involving the energy deposition of energetic atoms and ions into atmospheres of planets and exoplanets, atmospheric evolution, and analysis of non-equilibrium processes in the interstellar gas and heliosphere. Angular-and energy-dependent cross sections, required for an accurate description of the momentum-energy transfer, are obtained using ab initio interaction potentials and quantum mechanical calculations for scattering processes. Calculation methods used include partial wave analysis for collisional energies below 2 keV and the eikonal approximation at energies higher than 100 eV, keeping a significant energy region of overlap, 0.1-2 keV, between these two methods for their mutual verification. The partial wave method and the eikonal approximation excellently match results obtained with each other as well as experimental data, providing reliable cross sections in the astrophysically important interval of energies from 10 meV to 10 keV. Analytical formulae, interpolating obtained energy-and angular-dependent cross sections, are presented to simplify potential applications of the reported database. Thermalization of fast He atoms in the interstellar gas and energy relaxation of hot He and O atoms in the upper atmosphere of Mars are considered as illustrative examples of potential applications of the new database.

Propagation of Torsional Alfvén Waves from the Photosphere to the Corona: Reflection, Transmission, and Heating in Expanding Flux Tubes

“…The strength of the coupling is determined by the value of the friction coefficients, a bb¢ . We note that, in the case of the ionized fluid, the friction coefficients account for the collisions of all the ionized species with neutrals and are computed as Lewkow et al 2012;Vranjes & Krstic 2013). The collision frequency n bb¢ is computed from the friction coefficient as…”

Scattering of Particles and Radiation in Astrophysical Environments

“…In the extreme case of the 5 keV collision detected with the minimal detection angle of 0.12 • the integrated total cross section would 25% of the actual value as 75% of the scattering angle probability occurs at angles smaller than 0.12 • [10].…”

“…Utilizing ab initio interaction potentials and quantum mechanical methods, accurate energy-angular cross sections have been calculated for collisions between atoms of major astrophysical gases H+H, He+H, He+He, and He+O from temperatures ∼100 K, to 10 keV [10]. Additionally, empirical scaling cross sections have been developed for predictions of complicated atom-molecule and molecule-molecule collisions which are currently unrealistic for ab initio calculation at such high energies [11].…”

“…SW ions, which in turn produce ENAs [10]. Details of the calculated cross sections, comparisons with laboratory data, and development of analytic fitting formulas follow.…”

“…where the constants σ 0 and α were fit over several different energy intervals and the constant E 0 was equal to 1 keV for all energies [10]. Tables 2.1 to 2.6 give the values for the fitting parameters σ 0 and α over the entire energy interval.…”