Assessment of detectability of neutral interstellar deuterium by IBEX observations

TL;DR

This study evaluates the potential to detect neutral interstellar deuterium with IBEX, considering interstellar flux, background contamination, and solar activity, to inform future observations and data interpretation.

Contribution

It provides a detailed simulation of interstellar D detection prospects with IBEX, including background analysis and optimal observation conditions, which is novel.

Findings

Most D counts are from water layer sputtering, overshadowing interstellar D.

Detection of interstellar D is possible during low solar activity, especially in March-April.

Approximately 2 interstellar D atoms are expected to be detected in the first three years.

Abstract

The abundance of deuterium in the interstellar gas in front of the Sun gives insight into the processes of filtration of neutral interstellar species through the heliospheric interface and potentially into the chemical evolution of the Galactic gas. We investigate the possibility of detection of neutral interstellar deuterium at 1 AU from the Sun by direct sampling by the Interstellar Boundary Explorer (IBEX). We simulate the flux of neutral interstellar D at IBEX for the actual measurement conditions. We assess the number of interstellar D atom counts expected during the first three years of IBEX operation. We also simulate observations expected during an epoch of high solar activity. In addition, we calculate the expected counts of D atoms from the thin terrestrial water layer, sputtered from the IBEX-Lo conversion surface by neutral interstellar He atoms. Most D counts registered by IBEX-Lo are expected to originate from the water layer, exceeding the interstellar signal by 2 orders of magnitude. However, the sputtering should stop once the Earth leaves the portion of orbit traversed by interstellar He atoms. We identify seasons during the year when mostly the genuine interstellar D atoms are expected in the signal. During the first 3 years of IBEX operations about 2 detectable interstellar D atoms are expected. This number is comparable with the expected number of sputtered D atoms registered during the same time intervals. The most favorable conditions for the detection occur during low solar activity, in an interval including March and April each year. The detection chances could be improved by extending the instrument duty cycle, e.g., by making observations in the special deuterium mode of IBEX-Lo.