Voyager 1/UVS Lyman $\alpha$ measurements at the distant heliosphere (90-130AU): unknown source of additional emission

TL;DR

Voyager 1's Lyman alpha measurements from 2003-2014 reveal an unexpected emission component in the outer heliosphere, suggesting an unknown source possibly near the heliopause or external galactic/extragalactic origins.

Contribution

This study provides the first analysis of Voyager 1/UVS Lyman alpha data at 90-130 AU, identifying an unexplained emission component not predicted by existing models.

Findings

Observed intensity remains constant despite model predictions of decrease.

A dense hydrogen layer near the heliopause could explain additional emission.

External galactic or extragalactic Lyman alpha emission may contribute to observed data.

Abstract

In this work, we present for the first time the Lyman $\alpha$ intensities measured by Voyager 1/UVS in 2003-2014 (at 90-130 AU from the Sun). During this period Voyager 1 measured the Lyman $\alpha$ emission in the outer heliosphere at an almost fixed direction close to the upwind (that is towards the interstellar flow). The data show an unexpected behavior in 2003-2009: the ratio of observed intensity to the solar Lyman $\alpha$ flux is almost constant. Numerical modelling of these data is performed in the frame of a state-of-art self-consistent kinetic-MHD model of the heliospheric interface (Izmodenov & Alexashov, 2015). The model results, for various interstellar parameters, predict a monotonic decrease of intensity not seen in the data. We propose two possible scenarios that explain the data qualitatively. The first is the formation of a dense layer of hydrogen atoms near the heliopause. Such a layer would provide an additional backscattered Doppler shifted Lyman $\alpha$ emission, which is not absorbed inside the heliosphere and may be observed by Voyager. About 35 R of intensity from the layer is needed. The second scenario is an external non-heliospheric Lyman $\alpha$ component, which could be galactic or extragalactic. Our parametric study shows that $\sim$25 R of additional emission leads to a good qualitative agreement between the Voyager 1 data and the model results.