Interstellar Gas Flow Vector and Temperature Determination over 5 Years of IBEX Observations

E Möbius,D J Mccomas,M Bzowski,S A Fuselier,M A Lee,D Heirtzler,P Wurz,T Leonard,H Kucharek,N Schwadron,J M Sokół,M A Kubiak

doi:10.1088/1742-6596/577/1/012019

Abstract

The Interstellar Boundary Explorer (IBEX) observes the interstellar neutral gas flow trajectories at their perihelion in Earth's orbit every year from December through early April, when the Earth's orbital motion is into the oncoming flow. These observations have defined a narrow region of possible, but very tightly coupled interstellar neutral flow parameters, with inflow speed, latitude, and temperature as well-defined functions of inflow longitude. The best- fit flow vector is different by ≈ 3° and lower by ≈ 3 km/s than obtained previously with Ulysses GAS, but the temperature is comparable. The possible coupled parameter space reaches to the previous flow vector, but only for a substantially higher temperature (by ≈ 2000 K). Along with recent pickup ion observations and including historical observations of the interstellar gas, these findings have led to a discussion, whether the interstellar gas flow into the solar system has been stable or variable over time. These intriguing possibilities call for more detailed analysis and a longer database. IBEX has accumulated observations over six interstellar flow seasons. We review key observations and refinements in the analysis, in particular, towards narrowing the uncertainties in the temperature determination. We also address ongoing attempts to optimize the flow vector determination through varying the IBEX spacecraft pointing and discuss related implications for the local interstellar cloud and its interaction with the heliosphere.

Highlights

The neutral gas component of the local interstellar cloud (LIC) that surrounds our solar system reaches the inner heliosphere as an interstellar neutral (ISN) wind due to the relative motion of the Sun and the interstellar material
As has been demonstrated in the previous sections and in accordance with earlier results in [17, 18, 20], the Interstellar Boundary Explorer (IBEX) ISN observations lead to a narrow tube of parameters for the LIC velocity vector and temperature, for which VISM∞, βISM∞, and TISM can be expressed as unique functions of λISM∞
IBEX observations of the ISN He flow during early winter each year provide a versatile tool to determine the physical parameters of the LIC, with the potential for continuing improvement with multiple data sets and varying observation strategies

Summary

Introduction

The neutral gas component of the local interstellar cloud (LIC) that surrounds our solar system reaches the inner heliosphere as an interstellar neutral (ISN) wind due to the relative motion of the Sun and the interstellar material. To illustrate the basis for the inflow direction in the IBEX analysis, the right panel in Figure 1 shows the peak location of simulated ISN flow distributions in latitude ΨPeak for the usable observation. In [19] the authors claim that an instrumental dead time τ ≈ 7 ms would allow reconciliation of the angular distributions observed with Ulysses and IBEX, [18] indicated that τ ≤ 5 ms is an upper limit and [20] reported that the effect on the width and the temperature was much smaller based on follow-up testing and simulation, as discussed in more detail below.

Other Consequences and Future Perspectives

Findings

Conclusions