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

Abstract

Abstract:We report on a comprehensive signal processing procedure for very low signal levels for the measurement of neutral deuterium in the local interstellar medium from a spacecraft in Earth orbit. The deuterium measurements were performed with the IBEX-Lo camera on NASA's Interstellar Boundary Explorer (IBEX) satellite. Our analysis technique for these data consists of creating a mass relation in three-dimensional time of flight space to accurately determine the position of the predicted D events, to precisely model the tail of the H events in the region where the H tail events are near the expected D events, and then to separate the H tail from the observations to extract the very faint D signal. This interstellar D OPEN ACCESSEntropy 2014, 16 1135 signal, which is expected to be a few counts per year, 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. During the three years of the mission time when the deuterium signal was visible to IBEX, the observation geometry and orbit allowed for a total observation time of 115.3 days. Because of the spinning of the spacecraft and the stepping through eight energy channels the actual observing time of the interstellar wind was only 1.44 days. With the optimised data analysis we found three counts that could be attributed to interstellar deuterium. These results update our earlier work.

Science Opportunities for IMAP-Lo Observations of Interstellar Neutral Hydrogen and Deuterium during a Maximum of Solar Activity

“…The analysis of time-of-flight data from IBEX-Lo revealed three atoms of ISN D that accumulated over three observation seasons during the minimum of solar activity (Rodríguez Moreno et al 2013Moreno et al , 2014. A much larger number of D − ions was found in the data, but it was attributed to the terrestrialwater layer covering the conversion surface.…”

“…These latter authors suggested that the detection of just several individual ISN D atoms during one year of IBEX operation during low solar activity can be expected. Based on this insight, Rodríguez Moreno et al (2013) and Rodríguez Moreno et al (2014) found evidence for ISN D in the IBEX-Lo data, albeit based on fewer than 10 atoms, along with many more D atoms sputtered off the terrestrial-water layer covering the IBEX-Lo conversion surface. Clearly, acquiring a number of ISN D atoms on IMAP that would permit a statistical analysis with a reasonable confidence requires identifying the best suitable times and observation geometries during the mission.…”

“…The ratio c D,ISN /c D,terr is not only higher than for the IBEX-like geometry during solar minimum, but the time interval when this prerequisite is fulfilled is also longer. Certainly, however, the detection of ISN D will require a meticulous analysis performed using a method similar to that used by Rodríguez Moreno et al (2013) and Rodríguez Moreno et al (2014).…”

Measurements Beyond 1 au are Necessary for Exploration of the Outer Heliosphere and Local Interstellar Medium

“…Hydrogen Depletion Region ISN Hydrogen Density Radiation Pressure Predicted by models and measured by Cassini [12], Voyager [13], New Horizons (distance dependence, [14,15]), and IBEX-Lo (solar cycle dependence, [16]) New Horizons measures nH (r>20au) higher by ~40% than Ulysses at 5 au [14] IBEX-Lo measurements of ISN H at 1 au indicate a significant role of the solar radiation pressure in data interpretation [17,18] Ne/O and D/H Inside Heliosphere ENA at 1 au vs. 10 au Hydrogen Helioglows Interpretation of heavy element species of ISNs measured at 1 au is sensitive to details of ionization models [8,19,20]. Determination of the Ne/O and D/H ratios from 1 au is difficult and uncertain [20,21].…”

The Heliosphere and Local Interstellar Medium from Neutral Atom Observations at Energies Below 10 keV

“…Rigorous analysis of the individual counts measured with IBEX-Lo from 2009–2011 during low solar activity in energy bins from 10 eV to 100 eV (Rodríguez Moreno et al. 2013 , 2014 ) also allowed to identify likely interstellar D atoms. This in-situ method constrained the D/H ratio in the VLISM to , consistent with the D/H ratio of inferred by Hubble spectroscopy for a line of sight measurement of the Local Interstellar Cloud.…”