A NASA sponsored study conducted at John Hopkins University Applied Physics Lab culminated in a community-inspired heliospheric mission concept called the Interstellar Probe (ISP). The ISP’s science goals include understanding our habitable astrosphere by investigating its interactions with the interstellar medium, and determining the structure, composition, and variability of its constituents. A suite of instruments were proposed to achieve these and other science objectives. The instruments include a Lyman-a spectrograph for velocity-resolved measurements of neutral H atoms. The capability to address key components of the ISP’s science objectives by utilizing high spectral resolution Lyman-a measurements are described in this presentation. These findings have been submitted as a community White Paper to the recent Heliophysics decadal survey.

Studying of solar ionizing environment for the interstellar plasma inside the heliosphere is an unavoidable aspect of the interpretation of the full solar cycle of IBEX observations. We present a recent revision of the observation-based model of the ionization rates inside the heliosphere discussed by Sokół et al. 2020 (ApJ 897:179). The solar wind (SW) and the extreme ultraviolet (EUV) radiation affect fluxes of interstellar atoms inside the heliosphere both in time and in space. We present a Sun–Heliosphere Observation-based Ionization Rates (SHOIR) model based on the SW and EUV data available in solar cycle 24. We revised the in-ecliptic variation of the SW parameters, the latitudinal structure of the SW speed and density, and the photoionization rates. The revision most affects the SW out of the ecliptic plane during solar maximum and the estimate of the photoionization rates, the latter because of a change of the reference data. The changes are not constant and vary in time and in latitude. Our study shows that the polar SW is slower and denser during the solar maximum of solar cycle 24, and that the current estimates of the total ionization rates are higher than the previous ones for H, O, and Ne, and lower for He. The changes for the in-ecliptic total ionization rates are less than 10% for H and He, up to 20% for O, and up to 35% for Ne compared to the previous estimates.

The Interstellar Boundary Explorer (IBEX) mission has shown that variations in the ENA flux from the outer heliosphere are associated with the solar cycle and longer-term variations in the solar wind. In particular, there is a good correlation between the dynamic pressure of the outbound solar wind and variations in the later-observed IBEX ENA flux. The time difference between observations of the outbound solar wind and the heliospheric ENAs with which they correlate ranges from approximately two to six years or more, depending on ENA energy and look direction. This time difference can be used as a means of “sounding” the heliosheath, that is, finding the average distance to the ENA source region in a particular direction. We apply this method to build a three-dimensional map of the heliosphere. We use IBEX ENA data collected over a complete solar cycle, from 2009 through 2019, corrected for survival probability to the inner heliosphere. We divide the data into 56 “macro-pixels” covering the entire sky, and as each point in the sky is sampled once every six months, this gives us a time series of 22 points per macro-pixel on which to time-correlate. Consistent with prior studies and heliospheric models, we find that the shortest distance to the heliopause dHP is slightly south of the nose direction (dHP ~ 110 – 120 au), with a flaring toward the flanks and poles (dHP ~ 160 – 180 au). The heliosphere extends at least ~350 au tailward, which is the distance limit of the technique.