Heliospheric Lyman-α Absorption Toward Voyager 2

Abstract

Abstract Motivated by recent Voyager 2 (V2) measurements of surprisingly high plasma temperatures outside the heliopause, we study the heliospheric hydrogen Lyman-a absorption observed by the Hubble Space Telescope for the closest available line of sight to the V2 direction, 5 Pav, which is 9° from V2. The absorption is well reproduced by a typical global heliospheric model that predicts outer heliosheath temperatures of T«20, 000 K. Arbitrarily increasing these temperatures by a factor of two to be more consistent with the initial T=30, 000-50, 000 K values measured by V2 leads to significant overprediction of absorption. Thus, the high plasma temperatures first encountered by V2 near the heliopause cannot be present throughout the outer heliosheath.