Evidence of direct detection of interstellar deuterium in the local interstellar medium by IBEX

¹ Physics Institute, University of Bern, Sidlerstrasse 5, 3012 Bern, Switzerland
e-mail: diego.rodriguez@space.unibe.ch
² Space Research Centre, Polish Academy of Sciences, Bartycka 18A, 00–716 Warsaw, Poland
³ Department of Astronomy and Astrophysics, University of Chicago, 5640 S. Ellis Avenue, Chicago, IL 60637, USA
⁴ Southwest Research Institute, San Antonio, TX 78228–0510, USA and University of Texas at San Antonio, San Antonio, TX 78249, USA
⁵ Space Science Center and Department of Physics, University of New Hampshire, 39 College Road, Durham, NH 03824, USA

Received: 6 March 2013
Accepted: 16 July 2013

Abstract

We report the first in situ measurements of neutral deuterium originating in the local interstellar medium (LISM) in Earth’s orbit. These measurements were performed with the IBEX-Lo camera on NASA’s interstellar boundary explorer (IBEX) satellite. All data from the spring observation periods of 2009 through 2011 have been analysed. In the three years of the IBEX mission time, the observation geometry and orbit allowed for a total observation time of 115.3 days for the LISM. However, the effects of the spinning spacecraft and the stepping through 8 energy channels mean that we are only observing the interstellar wind for a total time of 1.44 days, in which 2 counts for interstellar deuterium were collected. We report here a conservative number, because a possibility of systematic error or additional noise, though eliminated in our analysis to the best of our knowledge, only supports detection at a 1-sigma level. From these observations, we derive a ratio D/H = (5.8 ± 4.4) × 10^-4 at 1 AU. After modelling the transport and loss of D and H from the termination shock to Earth’s orbit, we find that our result of D/H_LISM = (1.6 ± 1.2) × 10^-5 agrees with D/H_LIC = (1.6 ± 0.4) × 10^-5 for the local interstellar cloud. This weak interstellar signal is extracted from a strong terrestrial background signal consisting of sputter products from the sensor’s conversion surface. As reference, we accurately measure the terrestrial D/H ratio in these sputtered products and then discriminate this terrestrial background source. Because of the diminishing D and H signal at Earth’s orbit during the rising solar activity due to photoionisation losses and increased photon pressure, our result demonstrates that in situ measurements of interstellar deuterium in the inner heliosphere are only possible during solar minimum conditions.