Far‐Ultraviolet Echoes from the Frontier between the Solar Wind and the Local Interstellar Cloud

Abstract

We propose that first-order Fermi acceleration of Lyα photons should occur at the interface region between the solar and interstellar winds. In this process, photons are Doppler-shifted toward shorter wavelengths each time they are scattered by hydrogen neutrals that have been decelerated by charge exchange with protons in the transition region wrapping the heliopause. An additional component should therefore appear on the blue side of the line profile of the Lyα sky background emission. Its detection would not only reveal the existence of the heliopause interface but also trace its spatial distribution. We report here Hubble Space Telescope (HST) high-resolution Lyα line profiles of the sky background showing that the inner heliospheric hydrogen has a mean velocity of 23.2 ± 0.5 km s-1 in the upwind direction. This velocity corresponds to an averaged deceleration along the line of sight of the interstellar hydrogen of 2.8 ± 0.5 km s-1 after crossing the transition region. We show that an extra component, which we interpret to be the spectral signature of the Fermi process in the upwind direction, is present in the blue wing of the Lyα sky line profile. In addition, we interpret here the enigmatic excess in the Lyα sky brightness that was recently detected by Voyager deep in the inner heliosphere as being due to the additional Fermi emission. According to this interpretation, the distribution of this excess emission should also represent the spatial variation of the neutral velocity drop-off across the interface region. In this scope, we show that the region of the heliosphere's nose is ~12° away from the upwind axis—a good indication that the interstellar magnetic field deviates ~40° from the interstellar flow direction.