Precise Measurement of the Interstellar Flow with IMAP, Informed by IBEX Observations

The Sun's motion relative to the surrounding interstellar medium leads to an interstellar neutral (ISN) wind through the heliosphere. For several species, including He, this wind is moderately depleted by ionization and can be analyzed in-situ with pickup ions and direct neutral atom imaging. Since 2009, observations of the wind at 1 AU with the Interstellar Boundary Explorer (IBEX) have returned a precise 4-dimensional parameter tube for the flow vector (speed V_ISN, longitude λ_ISN, and latitude β_ISN) and temperature T_ISN of interstellar He in the local cloud, which organizes V_ISN, β_ISN, and T_ISN as a function of l_ISN, and the local flow Mach number (V_ThISN/V_ISN). Due to the limitation of the ISN flow observations to nearly perpendicular to the Earth-Sun line and thus over a very limited range in ecliptic longitude, the uncertainties along this functional dependence are substantially larger than across. Over the past two years, IBEX operations were modified to let the spin axis pointing of IBEX drift to the maximum offset (7^o) west of the Sun, which is the limit for the IBEX spacecraft. This expansion of the IBEX viewing results in an additional constraint that helps breaking the degeneracy of the ISN parameters along the 4D tube. This constraint is part of the complement of drivers that are effective in the full Chi-square-minimization to obtain the ISN parameters by comparison with a detailed model of the ISN flow. The next generation IBEX-Lo sensor on IMAP will be mounted on a pivot platform, which enables IMAP-Lo to follow the ISN flow over almost the entire spacecraft orbit around the Sun. In this way, a continuous set of 4D parameter tubes will be observed for He, as well as for O, Ne and H that cross at varying angles in the full parameter space. The crossing at varying angles substantially reduces the flow parameter uncertainties for these species and allows mitigation of systematic uncertainties, such as from ionization and the presence of secondary components. The pointing strategy possible with the pivot platform will also provide tomographic views of the secondary ISN flow created in the outer heliosheath. As a result, IMAP will probe the interstellar neutral flow in detail and also the 3-dimensional flow pattern of the interstellar plasma around the heliopause.