Venus exospheric structure: The role of solar radiation pressure

Abstract

The existence of a "hot" population of hydrogen atoms in the Venus exosphere is well known. In the outer coronal region where it is dominant (r [gsim] 2.0RV), hydrogen atoms are also subject to a relatively strong radiation pressure exerted by resonant scattering of solar Lyman-[alpha] photons. Collisionless models illustrating the consequent structure are discussed, with the nonthermal population mimicked by a dual Maxwellian exobase kinetic distribution. In these models, a considerable fraction of the "hot" atoms outside 2.0RV belongs to the quasi-satellite component, this fraction exceeding 1/2 for 4.0RV [lsim] r [lsim] 10.0RV. Quasi-satellites also raise the kinetic temperature near 2.0Rv by ~ 150 K. Solar ionization of bound atoms occurs mainly outside the ionopause, yielding a partial escape flux [gsim] 2 x 106cm-2s-1 over the day side exobase for assumed solar conditions. The inclusion of a cold exobase prescribed by Pioneer Venus observations has little influence on the outer region (in particular, the quasi-satellite component is unaltered) except that the transition to "hot" kinetic character occurs closer to the exobase on the nightside due to the colder main exobase temperatures there. Lastly, a "tail" of bound atoms is formed as in the terrestrial situation.