Ionization Rate of Interstellar Neutral Helium from New Horizons/SWAP Observations

TL;DR

The paper tackles the discrepancy between modelled and actual helium photoionization rates in the heliosphere by deriving the ISN He ionization rate from New Horizons/SWAP helium PUIs observed at 22–54 au (2012–2022). It introduces a rotation-aware maximum-likelihood framework that fits core solar wind and PUI distributions to SWAP data, incorporating a rotation-averaged energy–angle response and a detailed instrument model. The analysis yields a helium-to-hydrogen efficiency of ζ ≈ 1.73 and a SWAP-derived helium ionization rate ≈ 1.43×β_{0,He,F10.7}, i.e., about 43% higher than TIMED/F10.7-based photoionization rates, supporting previous IBEX inferences. The work implies a stronger solar UV-driven ionization of ISN He and motivates revised heliospheric models that account for enhanced He ionization and its consequences for PUIs and heliospheric dynamics, with future cross-checks from IMAP/IMAP-Lo data.

Abstract

Interstellar neutral (ISN) atoms enable studies of the physical conditions in the local interstellar medium surrounding the heliosphere. ISN helium, which is the most abundant species at 1 au, is directly observed by space missions, such as Interstellar Boundary Explorer (IBEX). However, some of these atoms are ionized by solar ultraviolet radiation before reaching 1 au, producing pickup ions (PUIs). A recent analysis of IBEX data suggests that the helium photoionization rates predicted by models are underestimated by up to 40%. The Solar Wind Around Pluto (SWAP) instrument on board New Horizons enables the study of PUIs giving complementary insight into the other side of the ionization process. Our goal is to verify this increased helium ionization by determining the ionization rate of ISN helium in the heliosphere based on the SWAP observations of helium PUIs. For this purpose, we analyze SWAP data collected between 2012 and 2022, at distances 22 to 54 au from the Sun. We develop a new method for fitting model distribution functions to the observational data using the maximum likelihood method. Our approach accounts for the spacecraft's rotation and the SWAP response function, which depends on both energy and inflow direction. We estimate SWAP's efficiency for helium relative to that for hydrogen and determine the ISN helium ionization rate. We find that the photoionization rate obtained from the SWAP observations is 43% larger than the rates predicted by models, confirming the IBEX results.

Figures (6)

• Figure 1: Mapping of the angular distance from the center of the FOV $\eta$ and rotation angles $\psi + \omega$ onto the elevation angle $\theta$ (left panel) and azimuthal angle $\phi$ (right panel). During a full rotation of the spacecraft around its axis, a given angular distance from the SWAP FOV center, $\eta$, corresponds to different values of the $\theta$ and $\phi$ coordinates, which can then be averaged. The SWAP FOV (hatched in red) limits the regions of the sphere that are observed by SWAP.
• Figure 2: Left panel: the averaged energy-angle response function $R_{\mathrm{E,\theta}}(E/E_{\mathrm{step}}, \theta)$ obtained based on data from elliott_new_2016. Right panel: the energy-angle response function $R_{\mathrm{E,\eta}}(E/E_{\mathrm{step}}, \eta)$ for rotation-average FOV used in our analysis. Using this new energy–angle response function simplifies the numerical calculations by eliminating one integration dimension. Instead of integrating over the SWAP $\theta$ and $\phi$ coordinates, it is sufficient to integrate over $\eta$.
• Figure 3: Upper panel: Example of a SWAP daily averaged spectrum with Poisson uncertainties (black dots with error bars) with fit models (solid lines). The graph shows both the core solar wind ($\mathrm{H^+}$, $\mathrm{He^{2+}}$, $\mathrm{He^+}$) and PUIs ($\mathrm{H^+}$ PUI, $\mathrm{He^+}$ PUI). The core solar wind protons are shown in orange, alpha particles in green, $\mathrm{He^+}$ ions in red, $\mathrm{H^+}$ PUIs in purple, and $\mathrm{He^+}$ PUIs in brown. The data points used for the fit are marked in pink (cf. text). The gray dashed line is the background. The fit values of the most important parameters are listed in the figure. Lower panel: Residual signal between the total model and SWAP data normalized by the statistical uncertainty, with a thin black line marking zero. In the selected case, the low solar wind speed shifts the energy cutoff of the $\mathrm{He^+}$ PUI distribution close to the upper limit of the SWAP energy range.
• Figure 4: Left panel: The time variations of the alpha to proton density ratios from SWAP (orange) and from OMNI 2 database (green). The alpha to proton density ratios determined from the SWAP data are larger than those obtained from the OMNI 2 database by the $\zeta$ scaling factor. Right panel: The relation between the alpha to proton density ratio from SWAP and OMNI 2 database. The gray dashed line represents $\zeta$ = 1. The red line shows the fit of the scaling factor $\zeta$.
• Figure 5: Left panel: The time variations of the helium SWAP ionization rates (orange) and the photoionization rates derived from the model (green), and their ratio. The helium ionization rate ratio indicates that the SWAP ionization rates are higher than the photoionization rates from solar UV spectra and 10.7 radio flux. Right panel: The relation between helium SWAP ionization rates and photoionization rates with uncertainties. The gray dashed line represents a one-to-one relationship. The red solid line shows the model assuming proportionality of values, and the orange dashed line is the linear model. The model with only the scaling parameter fit shows that the helium ionization rates estimated from SWAP observations are 43% higher than the photoionization rates based on solar UV spectra and 10.7 radio flux.