The ENA Ribbon and the ISN Flow as Key Tools for the ISM-Heliosphere Interaction – Open Questions, the Need for Future Observations with IBEX and IMAP

Abstract:

The unexpected ribbon in the IBEX energetic neutral atom (ENA) maps is still far from understood. According to most models, the interstellar magnetic field (B_ISM) controls its location and shape, with the direction in agreement with the termination shock (TS) asymmetry found by the Voyagers, the deflection of the interstellar neutral (ISN) flow, and the high energy cosmic ray anisotropy. With direct ISN flow velocity vector V_ISM and temperature observations, along with secondary neutrals, most likely from the outer heliosheath, IBEX also probes the conditions and interaction outside the heliospheric boundary. Precise knowledge of the ISN flow direction is key, because small differences have substantial leverage on the V_ISM-B_ISM plane, which controls the large-scale heliosphere structure.

For quantitative tools, the ribbon formation must be understood and the ISN flow parameters must be further refined. IBEX maps show that the latitudinal ribbon structure carries the imprint of fast and slow solar wind (SW). These results support models that involve charge exchange with the SW, currently in two renditions: secondary ENAs from neutral SW reaching into the outer heliosheath and reflection of SW at the TS. In the TS model, the ribbon distance maps the TS, and reactions to changing SW at 1 AU follow within 1 – 2 years. In the secondary ENA model, ribbon ENAs provide an energy-dependent spatio-temporal probe of the outer heliosheath over several years after SW changes at 1 AU. Therefore, observations over a full solar cycle with IBEX, probing the ribbon depth with SW modulation, are key to its understanding. Likewise, expanding the successful variation of the IBEX pointing strategy over times with varying ionization rates will refine the ISN flow vector. The capabilities of the Interstellar Mapping and Acceleration Probe (IMAP), which has highest priority in the recent NRC Heliophysics Decadal Survey, are needed to probe the spatio-temporal fine-structure of the ribbon, extend observations to higher energy with better resolution, and provide precision observations of the ISN flow and secondary neutrals from different vantage points. To that end, IMAP will provide a combination of increased collecting power, angular, and energy resolution, the capability to scan the ISN flow, and a dedicated pickup ion instrument.