Heavy coronal ions in the heliosphere. II. Expected fluxes of energetic neutral He atoms from the heliosheath

Aims. A model of heliosheath density and energy spectra of alpha-particles and He+ ions carried by the solar wind is developed. Neutralization of heliosheath He+ ions, mainly by charge exchange (CX) with neutral interstellar H and He atoms, gives rise to ~0.2 - ~100 keV fluxes of energetic neutral He atoms (He ENA). Such fluxes, if observed, would give information about plasmas in the heliosheath and heliospheric tail. Methods. Helium ions crossing the termination shock (TS) constitute suprathermal (test) particles convected by (locally also diffusing through) hydrodynamically calculated background plasma flows (three versions of flows are employed). The He ions proceed from the TS towards heliopause (HP) and finally to the heliospheric tail (HT). Calculations of the evolution of alpha- and He+ particle densities and energy spectra include binary interactions with background plasma and interstellar atoms, adiabatic heating (cooling) resulting from flow compression (rarefaction), and Coulomb scattering on background plasma. Results. Neutralization of suprathermal He ions leads to the emergence of He ENA fluxes with energy spectra modified by the Compton-Getting effect at emission and ENA loss during flight to the Sun. Energy-integrated He ENA intensities are in the range ~0.05 - ~50 cm^-2 s^-1 sr^-1 depending on spectra at the TS (assumed kappa-distributions), background plasma model, and look direction. The tail/apex intensity ratio varies between ~1.8 and ~800 depending on model assumptions. Energy spectra are broad with maxima in the ~0.2 - ~3 keV range depending on the look direction and model. Conclusions. Expected heliosheath He ENA fluxes may be measurable based on the capabilities of the IBEX spacecraft. Data could offer insight into the heliosheath structure and improve understanding of the post-TS solar wind plasmas. HT direction and extent could be assessed.Comment: 11 pages, 8 figures. Accepted Astronomy & Astrophysic

Solar wind He pickup ions as source of tens-of-keV/n neutral He atoms observed by the HSTOF/SOHO detector

Context. Since 1996, during periods of low solar activity, the HSTOF instrument onboard the SOHO satellite has been measuring weak fluxes of He atoms of 28-58 keV/n (helium energetic neutral atoms - He ENA). The probable source region is the inner heliosheath. Aims. We aim to understand the emission mechanism of He ENA based on knowledge of heliosheath spatial extent and plasma content resulting from Voyager 1 & 2 measurements in the period after termination shock crossings. Methods. He ENA are generated by charge-exchange neutralization of energetic helium ions on interstellar neutral H and He. Energy spectra of helium ions in the heliosheath are calculated by following the evolution of their velocity distribution functions when carried by and undergoing binary interactions with plasma constituents of a background flow whose particle populations are modeled to approximately render post-termination-shock Voyager data. Results. The observed HSTOF He ENA form a higher energy part of general heliospheric He ENA fluxes and can be explained by the proposed mechanism to within 2{\sigma} error. The main factor determining the level of emission (and its uncertainty) is the energy spectrum of He^+ pickup ions in post-termination shock plasmas.Comment: 6 pages, 2 figures, v2: version accepted for publication in Astronomy and Astrophysic

Spherical Harmonic Representation of Energetic Neutral Atom Flux Components Observed by IBEX

The Interstellar Boundary Explorer (IBEX) images the heliosphere by observing energetic neutral atoms (ENAs). The IBEX-Hi instrument onboard IBEX provides full-sky maps of ENA fluxes produced in the heliosphere and very local interstellar medium (VLISM) through charge exchange of suprathermal ions with interstellar neutral atoms. The first IBEX-Hi results showed that in addition to the anticipated globally distributed flux (GDF), a narrow and bright emission from a circular region in the sky, dubbed the IBEX ribbon, is visible in all energy steps. While the GDF is mainly produced in the inner heliosheath, ample evidence indicates that the ribbon forms outside the heliopause in the regions where the interstellar magnetic field is perpendicular to the lines of sight. The IBEX maps produced by the mission team distribute the observations into $6\deg\times6\deg$ rectangle pixels in ecliptic coordinates. The overlap of the GDF and ribbon components complicates qualitative analyses of each source. Here, we find the spherical harmonic representation of the IBEX maps, separating the GDF and ribbon components. This representation describes the ENA flux components in the sky without relying on any pixelization scheme. Using this separation, we discuss the temporal evolution of each component over the solar cycle. We find that the GDF is characterized by larger spatial scale structures than the ribbon. However, we identify two isolated, small-scale signals in the GDF region that require further study.Comment: 27 pages, 13 figures, v2 accepted for publication in ApJ

Distance to the IBEX Ribbon Source Inferred from Parallax

Maps of Energetic Neutral Atom (ENA) fluxes obtained from Interstellar Boundary Explorer (IBEX) observations revealed a bright structure extending over the sky, subsequently dubbed the IBEX ribbon. The ribbon had not been expected from the existing models and theories prior to IBEX, and a number of mechanisms have since been proposed to explain the observations. In these mechanisms, the observed ENAs emerge from source plasmas located at different distances from the Sun. Since each part of the sky is observed by IBEX twice during the year from opposite sides of the Sun, the apparent position of the ribbon as observed in the sky is shifted due to parallax. To determine the ribbon parallax, we found the precise location of the maximum signal of the ribbon observed in each orbital arc. The obtained apparent positions were subsequently corrected for the Compton-Getting effect, gravitational deflection, and radiation pressure. Finally, we selected a part of the ribbon where its position is similar between the IBEX energy passbands. We compared the apparent positions obtained from the viewing locations on the opposite sides of the Sun, and found that they are shifted by a parallax angle of $0.41^\circ\pm0.15^\circ$, which corresponds to a distance of $140^{+84}_{-38}$ AU. This finding supports models of the ribbon with the source located just outside the heliopause.Comment: 26 pages, 10 figures, 1 table, submitted to Ap

Interstellar neutral helium in the heliosphere from IBEX observations. III. Mach number of the flow, velocity vector, and temperature from the first six years of measurements

We analyzed observations of interstellar neutral helium (ISN~He) obtained from the Interstellar Boundary Explorer (IBEX) satellite during its first six years of operation. We used a refined version of the ISN~He simulation model, presented in the companion paper by Sokol_et al. 2015, and a sophisticated data correlation and uncertainty system and parameter fitting method, described in the companion paper by Swaczyna et al 2015. We analyzed the entire data set together and the yearly subsets, and found the temperature and velocity vector of ISN~He in front of the heliosphere. As seen in the previous studies, the allowable parameters are highly correlated and form a four-dimensional tube in the parameter space. The inflow longitudes obtained from the yearly data subsets show a spread of ~6 degree, with the other parameters varying accordingly along the parameter tube, and the minimum chi-square value is larger than expected. We found, however, that the Mach number of the ISN~He flow shows very little scatter and is thus very tightly constrained. It is in excellent agreement with the original analysis of ISN~He observations from IBEX and recent reanalyses of observations from Ulysses. We identify a possible inaccuracy in the Warm Breeze parameters as the likely cause of the scatter in the ISN~He parameters obtained from the yearly subsets, and we suppose that another component may exist in the signal, or a process that is not accounted for in the current physical model of ISN~He in front of the heliosphere. From our analysis, the inflow velocity vector, temperature, and Mach number of the flow are equal to lambda_ISNHe = 255.8 +/- 0.5 degree, beta_ISNHe = 5.16 +/- 0.10 degree, T_ISNHe = 7440 +/- 260 K, v_ISNHe = 25.8 +/- 0.4$ km/s, and M_ISNHe = 5.079 +/- 0.028, with uncertainties strongly correlated along the parameter tube.Comment: Updated reference

Warm Breeze from the starboard bow: a new population of neutral helium in the heliosphere

We investigate the signals from neutral He atoms observed from Earth orbit in 2010 by IBEX. The full He signal observed during the 2010 observation season can be explained as a superposition of pristine neutral interstellar He gas and an additional population of neutral He that we call the Warm Breeze. The Warm Breeze is approximately two-fold slower and 2.5 times warmer than the primary interstellar He population, and its density in front of the heliosphere is ~7% that of the neutral interstellar helium. The inflow direction of the Warm Breeze differs by ~19deg from the inflow direction of interstellar gas. The Warm Breeze seems a long-term feature of the heliospheric environment. It has not been detected earlier because it is strongly ionized inside the heliosphere, which brings it below the threshold of detection via pickup ion and heliospheric backscatter glow observations, as well as by the direct sampling of GAS/Ulysses. Possible sources for the Warm Breeze include (1) the secondary population of interstellar helium, created via charge exchange and perhaps elastic scattering of neutral interstellar He atoms on interstellar He+ ions in the outer heliosheath, or (2) a gust of interstellar He originating from a hypothetic wave train in the Local Interstellar Cloud. A secondary population is expected from models, but the characteristics of the Warm Breeze do not fully conform to modeling results. If, nevertheless, this is the explanation, IBEX-Lo observations of the Warm Breeze provide key insights into the physical state of plasma in the outer heliosheath. If the second hypothesis is true, the source is likely to be located within a few thousand of AU from the Sun, which is the propagation range of possible gusts of interstellar neutral helium with the Warm Breeze characteristics against dissipation via elastic scattering in the Local Cloud.Comment: submitted to ApJ

The Interstellar Neutral He haze in the heliosphere: what can we learn?

Neutral interstellar helium has been observed by the Interstellar Boundary Explorer (IBEX) since 2009 with a signal-to-noise ratio well above 1000. Because of the geometry of the observations, the signal observed from January to March each year is the easiest to identify. However, as we show via simulations, the portion of the signal in the range of intensities from 10^{-3} to 10^{-2} of the peak value, previously mostly left out from the analysis, may bring important information about the details of the distribution function of interstellar He gas in front of the heliosphere. In particular, these observations may inform us about possible departures of the parent interstellar He population from equilibrium. We compare the expected distribution of the signal for the canonical assumption of a single Maxwell-Boltzmann population with the distributions for a superposition of the Maxwell-Boltzmann primary population and the recently discovered Warm Breeze, and for a single primary population given by a kappa function. We identify the regions on the sky where the differences between those cases are expected to be the most visible against the background. We discuss the diagnostic potential of the fall peak of the interstellar signal, reduced by a factor of 50 due to the Compton-Getting effect but still above the detection limit of IBEX. We point out the strong energy dependence of the fall signal and suggest that searching for this signal in the data could bring an independent assessment of the low-energy measurement threshold of the IBEX-Lo sensor.Comment: This paper is a part of the special issue of Astrophysical Journal Supplement Series on interstellar neutrals measured by IBE