Keywords

This study provides a deeper understanding of how the solar wind evolves with increasing distance from the Sun as it encounters an increasing amount of interstellar material. This work extends our prior work by (1) extending the solar wind proton data radial profiles for New Horizons (NH) out to nearly 43 au, (2) quantifying the observed amount of slowing in the solar wind in the outer heliosphere by performing a detailed comparison between the speeds at NH (21–43 au) with speeds at 1 au, and (3) resolving discrepancies between the measured amount of slowing and estimates of the amount of slowing determined from the measured amount of interstellar pickup present in the solar wind. We find that the solar wind density radial profile may decrease at nearly or slightly less than a spherical expansion density profile. However, the temperature profile is well above what would be expected for an adiabatic profile. By comparing outer and inner heliospheric solar wind observations, we find the solar wind speed is reduced by 5%–7% between 30 and 43 au. We find the solar wind polytropic index (γ_sw) steeply decreases toward zero in the outer heliosphere (21–43 au) with a slope of ∼0.031 au⁻¹. Using both this radial variation in γ_sw and the measured amount of interstellar pickup ions, we estimate the slowing in the solar wind and obtain excellent agreement with the observed slowing.

Interaction of the solar wind with interstellar matter involves, among other processes, charge exchange between interstellar neutral atoms and plasma, which results in the creation of a secondary population of interstellar neutral (ISN) atoms. The secondary population of interstellar He was detected by Interstellar Boundary Explorer (IBEX), but interpretation of these measurements was mostly based on an approximation that the primary interstellar neutral population and the secondary population were non-interacting homogeneous Maxwell–Boltzmann functions in the outer heliosheath. We simulate the distribution function in the outer heliosheath and inside the heliopause using the "method of characteristics" with statistical weights obtained from solutions of the production and loss equations for the secondary atoms due to charge-exchange collisions in the outer heliosheath. We show that the two-Maxwellian approximation for the distribution function of neutral He is not a good approximation within the outer heliosheath but a reasonable one inside the termination shock. This is due to a strong selection effect: the He atoms able to penetrate inside the termination shock are a small, peculiar subset of the entire secondary He population. Nevertheless, the two-Maxwellian approximation reproduces the density distribution of ISN He inside the termination shock well and enables a realistic reproduction of the orientation of the plane defined by the Sun's velocity vector through the local interstellar matter and the vector of the unperturbed interstellar magnetic field.

Interstellar neutral gas atoms penetrate the heliopause and reach 1 au, where they are detected by Interstellar Boundary Explorer (IBEX). The flow of neutral interstellar helium through the perturbed interstellar plasma in the outer heliosheath (OHS) results in the creation of a secondary population of interstellar He atoms, the so-called Warm Breeze, due to charge exchange with perturbed ions. The secondary population brings the imprint of the OHS conditions to the IBEX-Lo instrument. Based on a global simulation of the heliosphere with measurement-based parameters and detailed kinetic simulation of the filtration of He in the OHS, we find the number density of the interstellar He⁺ population to be (8.98 ± 0.12) × 10⁻³ cm⁻³. With this, we obtain the absolute density of interstellar H⁺ as 5.4 × 10⁻² cm⁻³ and that of electrons as 6.3 × 10⁻² cm⁻³, with ionization degrees of 0.26 for H and 0.37 for He. The results agree with estimates of the parameters of the Very Local Interstellar Matter obtained from fitting the observed spectra of diffuse interstellar EUV and the soft X-ray background.

We use observations from the Interstellar Boundary Explorer (IBEX) and Ulysses to explore the possibility that the interstellar neutral helium flowing through the inner solar system possesses an intrinsic non-Maxwellian velocity distribution. In fitting the IBEX and Ulysses data, we experiment with both a kappa distribution and a bi-Maxwellian, instead of the usual Maxwellian assumption. The kappa distribution does not improve the quality of fit to either the IBEX or Ulysses data, and we find lower limits to the kappa parameter of κ > 12.1 and κ > 6.0 from the IBEX and Ulysses analyses, respectively. In contrast, we do find evidence that a bi-Maxwellian improves fit quality. For IBEX, there is a clear preferred bi-Maxwellian solution with T_⊥/T_∥ = 0.62 ± 0.11 oriented about an axis direction with ecliptic coordinates (λ_axis, b_axis) = (572 ± 89, −16 ± 59). The Ulysses data provide support for this result, albeit with lower statistical significance. The axis direction is close to the interstellar medium (ISM) flow direction, in a heliocentric rest frame, and is therefore unlikely to be indicative of velocity distribution asymmetries intrinsic to the ISM. It is far more likely that these results indicate the presence of asymmetries induced by interactions in the outer heliosphere.

We present maps of atomic carbon [C i](${}^{3}{{\rm{P}}}_{1}\to {{}^{3}{\rm{P}}}_{0}$) and [C i](${}^{3}{{\rm{P}}}_{2}\to {{}^{3}{\rm{P}}}_{1}$) emission (hereafter [C i] (1−0) and [C i] (2−1), respectively) at a linear resolution ∼1 kpc scale for a sample of one H ii, six LINER, three Seyfert, and five starburst galaxies observed with the Herschel Space Observatory. We compare spatial distributions of two [C i] lines with that of CO $J=1\to 0$ (hereafter CO (1−0)) emission, and find that both [C i] lines distribute similarly to CO (1−0) emission in most galaxies. We present luminosity ratio maps of ${L}_{[{\rm{C}}\,{\rm{I}}](1-0)}^{{\prime} }$/${L}_{\mathrm{CO}(1-0)}^{{\prime} }$, ${L}_{[{\rm{C}}\,{\rm{I}}](2-1)}^{{\prime} }$/${L}_{\mathrm{CO}(1-0)}^{{\prime} }$, ${L}_{[{\rm{C}}\,{\rm{I}}](2-1)}^{{\prime} }/{L}_{[{\rm{C}}\,{\rm{I}}](1-0)}^{{\prime} }$ (hereafter ${R}_{[{\rm{C}}{\rm{I}}]}$) and 70-to-160 μm far-infrared color of f₇₀/f₁₆₀. ${L}_{[{\rm{C}}\,{\rm{I}}](2-1)}^{{\prime} }$/${L}_{\mathrm{CO}(1-0)}^{{\prime} }$, ${R}_{[{\rm{C}}{\rm{I}}]}$ and ${f}_{70}/{f}_{160}$ are centrally peaked in starbursts; whereas they remain relatively constant in LINERs, indicating that star-forming activity can enhance carbon emission, especially for [C i] (2−1). We explore the correlations between the luminosities of CO (1−0) and [C i] lines, and find that ${L}_{\mathrm{CO}(1-0)}^{{\prime} }$ correlates tightly and almost linearly with both ${L}_{[{\rm{C}}\,{\rm{I}}](1-0)}^{{\prime} }$ and ${L}_{[{\rm{C}}\,{\rm{I}}](2-1)}^{{\prime} }$, suggesting that [C i] lines, similar to CO (1−0), can trace total molecular gas in H ii, LINER, Seyfert, and starburst galaxies on kpc scales. We investigate the dependence of ${L}_{[{\rm{C}}\,{\rm{I}}](1-0)}^{{\prime} }$/${L}_{\mathrm{CO}(1-0)}^{{\prime} }$, ${L}_{[{\rm{C}}\,{\rm{I}}](2-1)}^{{\prime} }$/${L}_{\mathrm{CO}(1-0)}^{{\prime} }$ and [C i] excitation temperature, T_ex, on dust temperature, T_dust, and find noncorrelation and a weak and modest correlation, respectively. The ratio of ${L}_{[{\rm{C}}\,{\rm{I}}](1-0)}^{{\prime} }$/${L}_{\mathrm{CO}(1-0)}^{{\prime} }$ stays a smooth distribution in most galaxies, indicating that the conversion factor of [C i] (1−0) luminosity to H₂ mass (${X}_{[\mathrm{CI}](1-0)}$) changes with CO (1−0) conversion factor (${\alpha }_{\mathrm{CO}}$) proportionally. Under optically thin and local thermodynamical equilibrium assumptions, we derive a galaxy-wide average carbon excitation temperature of ${T}_{\mathrm{ex}}\sim 19.7\pm 0.5\,{\rm{K}}$, and an average neutral carbon abundance of $X[\mathrm{CI}]/X[{{\rm{H}}}_{2}]\sim 2.5\pm 1.0\times {10}^{-5}$ in our resolved sample, which is comparable to the usually adopted value of 3 × 10⁻⁵, but ∼3 times lower than the carbon abundance in local (ultra)luminous infrared galaxies. We conclude that the carbon abundance varies in different galaxy types.

In this study, we analyze the directional distribution of the secondary interstellar neutral (ISN) O population observed by the IBEX-Lo neutral atom camera on the Interstellar Boundary EXplorer (IBEX) via the comparison with simulated ISN O intensity maps produced by an analytical model. In the analytical model, we assume that there are primary and secondary ISN populations at the heliopause. We further assume that each population is represented by a Maxwellian velocity distribution function with its own flow parameters. For the viewing directions of IBEX-Lo, we compute the incoming atom speeds at the heliopause with a Keplerian equation of motion in the solar gravity field. Then, we calculate analytically the distribution function to obtain the ISN intensities at Earth's orbit. We compare the simulated O intensity maps with the IBEX-Lo O sky map to determine the most likely flow parameters of the secondary ISN O population. Using this method, we find the most likely flow parameters of the secondary ISN O population: ${V}_{\mathrm{SecISNO}}=11\pm 2.2$ km s⁻¹, ${\lambda }_{\mathrm{SecISNO}}=67^\circ \pm 1\buildrel{\circ}\over{.} 5$, ${\beta }_{\mathrm{SecISNO}}=-12^\circ \pm 1\buildrel{\circ}\over{.} 6$, and ${T}_{\mathrm{SecISNO}}={\rm{10,000}}\pm 1500$ K. The results indicate that the secondary ISN O flow direction is deflected toward lower ecliptic longitude and higher negative ecliptic latitude from the ISN gas flow direction at the heliopause. The secondary ISN O flow direction is more deflected from the ISN gas flow direction than the secondary ISN He flow direction.

We present analytical expressions for direct evaluation of ℓ-mixing rate coefficients in proton-excited hydrogen atom collisions and describe a software package for efficient numerical evaluation of the collisional rate coefficients. Comparisons between rate coefficients calculated with various levels of approximation are discussed, highlighting their range of validity. These rate coefficients are benchmarked for radio recombination lines for hydrogen, evaluating the corresponding departure coefficients from local thermal equilibrium.

In 2009, the Interstellar Boundary Explorer (IBEX) discovered the existence of a narrow "ribbon" of intense energetic neutral atom emission projecting approximately a circle in the sky. It is believed that the ribbon originates from outside of the heliopause in radial directions (${\boldsymbol{r}}$) perpendicular to the local interstellar magnetic field (ISMF), ${\boldsymbol{B}}$, i.e., ${\boldsymbol{B}}\cdot {\boldsymbol{r}}=0$. Swaczyna et al. estimated the distance to the IBEX ribbon via the parallax method comparing the ribbon position observed from opposite sides of the Sun. They found a parallax angle of 041 ± 015, yielding a distance of ${140}_{-38}^{+84}$ au to a portion of the ribbon at high ecliptic latitudes. In this study, we demonstrate how the apparent shift of the ribbon in the sky, and thus the apparent distance to the ribbon's source found via the parallax, depends on the transport effects of energetic ions outside the heliopause. We find that the apparent shift of the ribbon based on the "spatial retention" model with ion enhancement near ${\boldsymbol{B}}\cdot {\boldsymbol{r}}=0$, as proposed by Schwadron & McComas, agrees with the parallax of the source region. Parallax is also accurate for a homogeneously distributed emission source. However, if there is weak pitch-angle scattering and ions propagate freely along the ISMF, the apparent shift is significantly smaller than the expected parallax because of the highly anisotropic source. In light of the results from Swaczyna et al., our results indicate that the IBEX ribbon source is spatially confined.

The ribbon of enhanced energetic neutral atom flux, discovered by the Interstellar Boundary Explorer (IBEX) in 2009, has redefined our understanding of the heliosphere's interaction with the local interstellar medium (LISM). Yet, its origin continues to be a topic of scientific debate. The ribbon is circular and traces the region where the putative LISM magnetic field (B_LISM) is perpendicular to the radial direction from the Sun. Using nine years of IBEX-Hi observations, we investigate the ribbon circularity and location as functions of time and energy. We provide updated locations of the ribbon center at five energy passbands (centered at 0.7, 1.1, 1.7, 2.7, and 4.3 keV) in ecliptic coordinates [longitude, latitude]: [21741 ± 095, 4436 ± 093], [21972 ± 095, 4150 ± 087], [22051 ± 119, 3996 ± 100], [21808 ± 166, 3844 ± 124], and [21468 ± 148, 3413 ± 119] respectively. The weighted mean center location over all energies and all years is [21833 ± 068, 4038 ± 088] and its radius is 7481 ± 065. As viewed by IBEX at 1 au, we find that (1) the ribbon is stable over time, with distinct centers at each energy; (2) ribbon centers exhibit small temporal variations, likely caused by the solar wind (SW) speed and density variations; and (3) ribbon location in the sky appears to be driven by (i) the inherent alignment of the ribbon centers along the plane connecting the presumed B_LISM and the heliospheric upwind direction, and (ii) the variable SW structure along the heliographic meridian, further emphasizing that the ribbon source is outside the heliosphere.

We study the distribution of the interstellar neutral (ISN) gas density and the pickup ion (PUI) density of hydrogen, helium, neon, and oxygen in the heliosphere for heliocentric distances from inside 1 au up to the solar wind termination shock (TS), both in and out of the ecliptic plane. We discuss similarities and differences in the large-scale structures of the ISN gas and PUIs formed in the heliosphere between various species. We discuss the distribution of ISN gas and PUI densities for two extreme phases of the solar activity cycle, it is the solar minimum and the solar maximum. We identify the location of the ISN gas density cavity of various species. We study the relative abundance ratios of Ne/O, H/He, Ne/He, and O/He for ISN gas and PUIs densities and their variation with location in the heliosphere. We also discuss the modulation of relative abundance ratios of ISN gas and PUIs along the TS. We conclude that the preferable locations for detection of He⁺ and Ne⁺ PUIs are in the downwind hemisphere within 1 au, whereas for H⁺ and O⁺ PUIs the preferable locations for detection are for distances from Jupiter to Pluto orbits.