Ultraviolet resonance radiation and the structure of the heliosphere.

Abstract

The solar system and heliosphere are embedded in a partially ionized medium flowing past the Sun at about 22 km s⁻¹. The Voyager and Pioneer 10 spacecraft are travelling upstream and downstream respectively, detecting Lyα radiation resonantly scattered from heliospheric hydrogen. None of the probes has encountered the solar wind termination shock, where the supersonic solar wind is believed to decelerate to subsonic speeds. Penetration of H atoms from the local interstellar flow is the principal source of heliospheric H. Solar gravitation, radiation pressure, and ionization processes largely control the H distribution. However, the presence of the solar wind termination shock is predicted to have two additional effects. H-p charge exchange reactions occurring in the hot, post-shock solar wind plasma should both reduce the number of penetrating H atoms and create a population of suprathermal H atoms. Therefore, heliospheric Lyα emission lines should be composed of narrow and wide components, which should be diagnostic of outer heliospheric structure. Previously unpublished Voyager Cruise Maneuver observations obtained between 15 and 40 AU reveal that upstream Lyα intensities fall as r⁽⁻⁰·⁷⁵ ⁺/⁻ ⁰·⁰⁵⁾. Beyond 15 AU downstream, Pioneer 10 Lyα falls as r⁽⁻¹·⁰⁷ ⁺/⁻ ⁰·¹⁾. These trends cannot be simultaneously reproduced using models which do not include the termination shock. The Voyager data suggest an additional source of Lyα in the upstream region beyond 40 AU. This may be due to suprathermal H gas and/or gradients in the H density, both predicted to be associated with the termination shock. A new method of estimating the heliospheric H density between the two Voyager spacecraft is introduced. The results are ambiguous and suffer due to the uncertainty in relative instrumental Lyα sensitivities.