Letter to the Editor

Detectability of neutral interstellar deuterium by a forthcoming SMEX mission IBEX

Received: 16 February 2008
Accepted: 7 April 2008

Abstract

Context. We study the feasibility of detection of neutral interstellar deuterium by the forthcoming NASA SMEX mission IBEX.

Aims. Using numerical simulations, we study the absolute density and flux in Earth orbit of neutral interstellar deuterium and check its detectability using IBEX.

Methods. Our simulations were performed using the Warsaw 3D time-dependent test-particle model of neutral interstellar gas in the inner heliosphere, which was specially adapted to the case of deuterium, and state-of-the-art models of the ionization field and radiation pressure. The modeling predicted the density, bulk velocity, and flux of interstellar D at different positions of the Earth during the solar cycle. We paid particular attention to the time interval in which IBEX observations will be performed.

Results. Using our simulations, we predict a large enhancement of deuterium abundance in Earth orbit with respect to the abundance at the termination shock. The energy of the D atoms at IBEX will be within the energetic sensitivity band of its Lo instrument, apart from during a short time interval between September and November each year. Because of the specific observing geometry of IBEX, there will be one opportunity each year to search for I/S D, when Earth is close to ecliptic longitude , i.e. in March. Assuming that the TS abundance of D is identical as in the Local Cloud, which is equal to , and that the density of H at TS is 0.11 cm^-2 s^-1, we estimate the expected relative flux to be approximately 0.015 cm^-2 s^-1, which corresponds to the local absolute flux of about 0.007 cm^-2 s^-1. The dependence of the expected flux on the phase of solar cycle is relatively weak. The flux scales proportionally to the density of deuterium at the termination shock and depends only weakly on the bulk velocity and temperature of the gas in this region.