Observing the Interstellar Medium Flow over the Past 6 Years with IBEX-Lo

Abstract:

The Interstellar Boundary Explorer (IBEX) has observed the interstellar neutral (ISN) gas flow over the past 6 years during the winter/spring when the Earth’s motion opposes the ISN flow direction. Since IBEX observes near the perihelion of the interstellar atom trajectories, we are able to use an analytical model (Lee et al. 2012) based upon orbital mechanics to determine characteristics of the interstellar flow. The interstellar inflow latitude, velocity, and temperature are coupled to the inflow longitude and are restricted by the IBEX observations to a narrow valley in this parameter space (McComas et al. 2012). In our original analysis we found that the small spacecraft spin axis pointing out of the ecliptic plane had a significant influence on the determination of the ISN inflow vector (Möbius et al. 2012; Bzowski et al. 2012). Introducing the spacecraft spin axis tilt into the analytical model has shown that IBEX observations with various spin axis tilt orientations can help restrict the natural degeneracy of the ISN flow parameters as a function of inflow longitude. The IBEX operations team was able to successfully point the spin axis to be in the ecliptic during the 2012 and 2013 seasons and about 5 degrees below the ecliptic during the 2014 season. With this robust data set we are able to further test and develop the analytical model while producing a more comprehensive analysis of the ISN Flow parameters. We found that in its current implementation the analytical model describes the ISN flow most precisely for the spin axis orientation exactly in the ecliptic. This analysis refines the derived ISN flow parameters with a possible reconciliation between velocity vectors found with IBEX and Ulysses, but at a substantially higher temperature than previously reported.