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

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.

Signal Processing for the Measurement of the Deuterium/Hydrogen Ratio in the Local Interstellar Medium

“…The closest star representing the LIC is Sirius, where the D/H ratio of 16 ± 4 ppm is typical of the Local Bubble interior [7]. In our initial analysis, the D/H measurements by direct sampling on IBEX were found to be consistent with the Sirius measurements [8]. Here we present the details of the analytical technique and an updated D/H value for the Local Interstellar Medium (LISM).…”

“…The mass equations allow an analytic interpolation to find the TOF position of species not investigated during instrument calibration nor are directly observed in the flight data. The comparison between the locus of deuterium derived by this method and estimates based on instrument simulation [8] were highly favourably.…”

“…In the present section, we show the successive development of the analysis method used to obtain the position of a species in the three-dimensional TOF space, and the subsequent counting of the D events at this location [8]. In this description also included are unsuccessful attempts, because they gave us direction how to continue our analysis, so therefore they are an important part of the method as such.…”

“…For reference, we show the expected location (from Table 2) and magnitude of the D TOF peak in yellow lines. All time intervals are defined as a combination of six successive orbits that share approximately the same observation times in each year, same ecliptic longitude [8]. This combination of six orbits is repeated for the time intervals 1 through 4.…”

“…In one dimension the H peak is very clearly identified [8], obtaining its σ H values is a straightforward task from the fit parameters, then the range for counting events of deuterium is determined from the sigma σ H taking the hydrogen peak as a Gaussian distribution near its top. From the analysis of each TOF channel we obtain σ H = 0.6609 ns, 1.2969 ns, and 1.155 ns, for TOF0, TOF1, and TOF2, respectively.…”