WARMER LOCAL INTERSTELLAR MEDIUM: A POSSIBLE RESOLUTION OF THE ULYSSES–IBEX ENIGMA

Our heliosphere—the region of space filled with the Sun's outflowing solar wind—carves out a cavity in the magnetized plasma of the surrounding local interstellar medium (LISM). In contrast, neutral atoms of the LISM are free to flow into the heliosphere, providing a direct sample of the interstellar medium that penetrates inside Earth's orbit. These low energy interstellar neutrals (ISN) have only been directly measured from two spacecrafts: Ulysses, with the GAS instrument (Witte et al. 1992; Witte 2004) and the Interstellar Boundary Explorer (IBEX; McComas et al. 2009a) with the IBEX-Lo instrument (Fuselier et al. 2009b; Möbius et al. 2009b).

IBEX's primary goal is to enable a global understanding of the heliosphere's interstellar interaction by making the first all-sky maps of energetic neutral atoms (ENAs) traveling in from the outer heliosphere. ENAs are formed by charge exchange between the solar wind (and interstellar pickup ions that become incorporated in it) and ISNs that penetrate into the heliosphere. Energy-resolved maps of these ENAs provide detailed information about the parent ion distributions that they came from. These observations led to the discovery of a nearly circular "ribbon" of enhanced ENA emissions apparently ordered by the external magnetic field in the LISM, and not predicted by any model or theory prior to the IBEX measurements (McComas et al. 2009b; Funsten et al. 2009; Fuselier et al. 2009a; Schwadron et al. 2009).

In addition to imaging the lower energy ENAs, the IBEX-Lo instrument (Fuselier et al. 2009b) also directly measures ISNs and has been collecting sets of these observations each winter/spring season since 2009. These observations have included the first direct measurements of interstellar H and O (Möbius et al. 2009a), Ne and the Ne/O ratio (Bochsler et al. 2012; Park et al. 2014), and deuterium (Rodriquez Moreno et al. 2013, 2014) in the LISM. The most detailed analyses of ISNs with IBEX have focused on determining the inflow characteristics of He (e.g., Möbius et al. 2009a, 2012; Bzowski et al. 2012; Lee et al. 2012; McComas et al. 2012), which was the only species previously measured by the Ulysses GAS instrument. Compared to H, radiation pressure is negligible for He because (1) the solar flux at 58.4 nm is very weak (at least two orders of magnitude less than Lyα), (2) the He mass (and Sun's gravitational force) is four times greater, and (3) He atoms that IBEX observes (those close to their perihelia) spend most of their time with radial speeds high enough to Doppler shift away from the narrow solar 58.4 nm line. Thus, interstellar He neutrals move along essentially hyperbolic orbits past the Sun.

Based on the first two seasons of IBEX data, a new range of ISN inflow parameters was explored with independent, complementary techniques of (1) an analytical model (Lee et al. 2012) that used the unique IBEX viewing geometry perpendicular to the solar direction to assess the interstellar He parameters (Möbius et al. 2012), and (2) a detailed forward model (the Warsaw Test Particle Model) that includes all of the important geometric and instrumental effects known (Bzowski et al. 2012; Kubiak et al. 2014).

A small discrepancy in the ISN He parameters obtained using these two methods (Möbius et al. 2012; Bzowski et al. 2012) was resolved by McComas et al. (2012) to give IBEX's best measurements based on its first two years of ISN observations. Equations (1)–(3) of that study provided the coupled equations that link the four observable interstellar parameters in the IBEX data. These parameters, the speed (V_ISM∞), ecliptic longitude (λ_ISM∞), ecliptic latitude (β_ISM∞), and temperature (T_He∞), define a narrow "tube" in their four-dimensional parameter space of possible combinations that are consistent within the errors of the IBEX data (we note that all latitudes and longitudes in this paper are given in ecliptic J2000 coordinates). These four parameters are so closely tied to each other because IBEX's observations are concentrated in the ecliptic plane. In contrast, the published values from Ulysses at that time were claimed to be uncoupled, indicating independent measurements of V_ISM∞ = 26.3 ± 0.4 km s⁻¹, λ_ISM∞ = 754 ± 05, β_ISM∞ = 52 ± 02, and T_He∞ = 6300 ± 340 K (Witte 2004). While subsets of Ulysses parameters could be found along the IBEX tube of allowable parameters, the combination of all four together could not, and at least one value always ended up significantly off of what was consistent with the IBEX data.

From the broad ranges of coupled He parameters encompassed by the four-dimensional tube from IBEX, McComas et al. (2012) examined the implications of values near the centers of their possible ranges where λ_ISM∞ = 79°, which then led to coupled values of V_ISM∞ = 23.2 km s⁻¹, β_ISM∞ = −498, and T_He∞ = 6300 K. In addition to being around the center of the allowable range from the 2009–2010 IBEX data, this selection also had a temperature identical to the published Ulysses value. Because of the relatively large interstellar magnetic field indicated by IBEX (McComas et al. 2009b; Schwadron et al. 2009) and by the difference between the Voyager 1 and Voyager 2 termination shock crossing distances (Stone et al. 2008; Opher et al. 2007), McComas et al. (2012) concluded that even a few km s⁻¹ slower inflow speed could lead to the heliosphere having no fast magnetosonic bow shock ahead of it. While that study examined the implications of interstellar He parameters near the middle of the allowed range, it is important to note that the full range of possible coupled parameters provided by McComas et al. (2012; see their Table 1) spanned coupled sets of parameters from (21.3 km s⁻¹, 820, −484, 5000 K) to (25.7 km s⁻¹, 755, −514, 8300 K) with 1σ uncertainties of ∼(±0.3 km s⁻¹, ±05, ±02, ±400 K) around any consistent set of the four parameters along the elongated four-dimensional parameter tube.

In a subsequent study, Frisch et al. (2013) examined the implications of taking the same central values for IBEX and combining them with the published Ulysses values and other, less direct measurements of the interstellar flow. These authors statistically analyzed the various published values and errors and concluded that together, they indicated that some change in the interstellar flow direction was "statistically likely." While a number of potential criticisms of the analysis techniques in that work were raised (Lallement & Bertaux 2014), these have been effectively rebutted on a variety of technical grounds (Frisch et al. 2014). We stress that Frisch et al. (2014) are not arguing that the interstellar flow direction has changed, but simply that the Frisch et al. (2013) analysis was not flawed and if one uses the published results and uncertainties, including the previously published central values from IBEX, that a directional change was likely on a statistical basis.

Both Ulysses and IBEX measurements have their advantages and disadvantages for measuring the ISN flow. On Ulysses, many different ISN trajectories around the Sun are sampled over different portions of the spacecraft's fast latitude scans. As Ulysses' orbit is almost perpendicular to the ISN flow, in principle it allows a precise determination of the flow direction and velocity (e.g., see Figure 1 in Witte et al. 1992). On the other hand, the GAS instrument's sensitivity is low, so observations are only made directly around the peak flux, and have significant statistical uncertainties. For example, the peak signal-to-noise ratio (S/N) for GAS is only ∼10 (Bzowski et al. 2014). In contrast, IBEX observations are made at 1 AU in the ecliptic plane and only during the winter/spring season, when the neutrals are gravitationally bent into the IBEX-Lo aperture. This is not as advantageous a geometry as Ulysses, but on the other hand, IBEX-Lo's sensitivity is very high, with peak S/N routinely >1000 (Bzowski et al. 2012; Möbius et al. 2012). This much greater sensitivity makes it possible to see far more of the particle distribution and enabled the discovery of an additional population of neutral He dubbed the "warm breeze" (Kubiak et al. 2014), which may be a secondary population of interstellar He created beyond the heliopause.

Since the publication of the earlier IBEX papers, there has been a substantial effort to reexamine the Ulysses observations. These studies included slightly improved pointing offsets for the Ulysses data, a first analysis of Ulysses' final (2006–2007) fast latitude, and a reanalysis of the prior GAS data. Wood et al. (2014) provided new He flow parameters of V_ISM∞ = 26.08 ± 0.21 km s⁻¹, λ_ISM∞ = 7554 ± 019, β_ISM∞ = −544 ± 024, and T_He∞ = 7260 ± 270 K, while Bzowski et al. (2014) used the Warsaw model, previously developed to analyze IBEX ISN data, and found their chi-squared minimum at (26 + 1/−1.5 km s⁻¹, 753 + 12/−11, −6° ± 1°, 7500 + 1500/−2000 K). Both the more recent Ulysses data and reanalysis of earlier data give inflow speeds and directions similar to earlier Ulysses results, but indicate significantly enhanced temperatures ∼1000 K more than previously published values. In the earlier evaluations of the GAS data (Witte 2004 and prior work), it turns out that the temperature determination was dependent on which count-rate level was chosen to separate the peak of the He distribution from background. Those authors assumed that there were two He components, a "cold" component (with a narrowly peaked distribution) riding on top of a "hot" component (with a much broader and flatter distribution), both on top of a flat omni-directional background. Therefore, only the top ∼80% of the He peak value (presumed to be the "cold" component) were included in the temperature determinations. As the limited statistical significance of the data prohibited any conclusive affirmation of this assumption, the earlier values of the temperature by Witte (2004) should now be regarded essentially as lower limits. The new Ulysses results are closer to the "hot/fast" end of the bounding range of the IBEX tube of possible observations with a center value of (25.7 km s⁻¹, 755, −514, 8300 K) from McComas et al. (2012). However, the most likely new Ulysses temperatures found are still significantly below those of the allowable IBEX values given the other parameters.

It is the purpose of this study to report the most recent IBEX results on the interstellar He inflow and try to resolve the apparent disagreement between the Ulysses and prior IBEX observations. For these purposes, we use a subset of IBEX data from 2012 to 2014 that includes measurements taken when the spacecraft spin axis pointing was in the ecliptic plane. This geometry is the simplest to analyze and enables us to continue to use two independent but parallel approaches for examining the IBEX data. In addition, data from 2013 and 2014 take advantage of the fact that we made an operational change to IBEX-Lo measurements starting in mid-2012 that allows all real events to be telemetered rather than just a statistical sampling of them. This change ensures the maximum statistical accuracy of IBEX's ISN data.

In the first of our two parallel approaches, we again use the analytic model of Lee et al. (2012), but this time explicitly include an analytic description of the drift of the IBEX spin axis away from Sun pointing over each orbit and include only data where the IBEX spacecraft pointing is within ±02 of the ecliptic plane (Leonard et al. 2014). As discussed in more detail below, this selection minimizes the effects of neglecting higher order terms in the angle of the trajectory out of the ecliptic plane, which appear to be more important than previously appreciated, and were not accounted for in the prior application of the analytic model. In this analytic approach, we also remove orbits where the warm breeze may significantly contribute to the observed interstellar He flow distribution and thus might influence the determination of the flow longitude at infinity.

As described by Möbius et al. (2012), the analytic method retrieves the ISN flow parameters in three consecutive steps, starting with the determination of the perihelion longitude of the ISN flow at 1 AU from the observed maximum of the integrated flux. After accounting for ionization and the fact that the maximum flux and not phase space density is observed, this longitude results in the unique relation between V_ISM∞ and λ_ISM∞ that is a key property of the narrow ISN parameter tube. Next, for each usable IBEX orbit, the latitude of the observed peak location is determined for the position of exact Sun pointing of the spin axis within the ecliptic plane from several time periods selected according to the criteria laid out in Möbius et al. (2012). The observed variation of the peak latitude with observer longitude is then used to find the combination of inflow longitude λ_ISM∞ and latitude β_ISM∞ that minimizes chi-squared in the analytic model. By this method, only the peak location of the ISN flow distribution in each orbit is used to determine the flow vector at infinity, which minimizes the sensitivity of this method to parts of the flow distribution outside the primary peak.

As seen in our previous analyses (Bzowski et al. 2012; Möbius et al. 2012), the angle of the spin axis (ε_z) out of the ecliptic plane changes the observed latitude peak of the ISN flow as a function of observer longitude and thus must be included in the modeling. We subsequently found that varying this spin axis orientation provides additional leverage for determining λ_ISM∞ (Möbius et al. 2014) and larger off-pointing angles were included in the IBEX operations for the 2014 ISN flow season. When analyzing these data with the analytic model, the chi-squared minimization returned optimum λ_ISM∞ values between ≈79° (for a spin axis tilt of ε_z = 07 above the ecliptic plane as used in 2009 and 2010), ≈75° (for orbits in 2013 and 2014 where ε_z = 0°), and ≈70° (for the orbits where ε_z = −49 in 2014; Leonard et al. 2014). Obviously, this variation is an artifact of the model behavior for a spin axis orientation out of the ecliptic because the interspersed orbits with ε_z ≈ −49 and ε_z ≈ 0° in 2014 indicated different longitudes. This trend of the analytic model was verified by applying the same method to simulated data sets that were provided from the Warsaw Test Particle Model. Most importantly, the resulting ISN flow parameter set agrees between the two models for the IBEX spin axis exactly in the ecliptic plane, i.e., ε_z = 0°. As a consequence, Leonard et al. (2014) used IBEX data from 2012–2014 restricted to spin axis pointing close to the ecliptic plane and for |ε_z| < 007 found λ_ISM∞ = 745 ± 16. In subsequent steps of our analytic process, the observed peak position in longitude and latitude are used as proxies for the first moment of the ISN flow distribution, i.e., the flow vector, which minimizes the influence of the distribution's wings on these bulk flow parameters. In the final step, a temperature is derived from the latitudinal width of the distribution. This process yields coupled parameters of β_ISM∞ = −521 ± 024, V_ISM∞ = 26.7 ± 0.5 km s⁻¹, and T_He∞ = 9070 ± 480 K for the derived center value of λ_ISM∞.

In our second approach, we use the Warsaw Test Particle Model (Bzowski et al. 2012) with the time-dependent ionization rate adopted from Bzowski et al. (2013) and again analyze the times from 2013 and 2014 when the spacecraft spin axis is essentially in the ecliptic plane. Here we used data from the same orbits as in the analytical analysis, which were selected to be as free from background as possible. We also make a preliminary analysis of a subset of the 2014 observations when the pointing was −49 from the ecliptic. This pointing out of the ecliptic samples trajectories from a different plane and thus may sample a somewhat different portion of the upstream region.

For this analysis, we assume that the neutral helium in front of the heliosphere is composed of two distinct and collisionless Maxwell–Boltzmann populations, one with known flow parameters and the other one with the parameters being sought. Kubiak et al. (2014), who discovered the warm breeze mentioned above, were quantifying this lower density population, assuming the parameters of the interstellar helium gas from Bzowski et al. (2012). We now turn this approach around and adopt as known the parameters of the warm breeze, searching for improved parameters of the higher density interstellar gas. Because the warm breeze is significantly smaller than the primary ISN flow, its inclusion has a relatively small impact on the derived parameters and, while ultimately a simultaneous optimization for both set of parameters will be performed, the current approach should be very close for the purposes of defining the primary component. Modeling presented by Bzowski et al. (2012) indicates that the signal from interstellar He should be approximately Gaussian as a function of IBEX spin angle. Therefore, we retained only those 6° pixels in the selected orbits whose residuals were within 2σ of Gaussian. This left us with about six data points per orbit with minimum to maximum value ratios from ∼0.3% to ∼2%. The parameter search was carried out by numerical minimization of chi-squared, as done by Kubiak et al. (2014) and Bzowski et al. (2012). This method requires the model to reproduce all data points (simultaneously for all 6° pixels and all orbit arcs) with accuracy consistent with the uncertainties of the measurements. Therefore, the optimization for the flow vector in this method is sensitive to the entire distribution, including its width as a function of observer longitude and its outlying portions. Figure 1 shows the chi-squared minimization curves for the 2013 season when the spacecraft pointing was in the ecliptic to within ±04 and from 2014 separately for orbits where the pointing was within ±02 of the ecliptic and below the ecliptic at −49.

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The inflow longitude from the 2013 and in-ecliptic pointing orbits in 2014 agree with each other within 1σ, with those obtained from the analytic method discussed above, and with Ulysses results. However, the chi-squared value for 2013 exceeds the acceptability limit, which suggests that either the error estimates are too optimistic or the model assumptions are not exactly fulfilled. The 2014 results with ε_z = −49 agree with the values from IBEX observations obtained by Bzowski et al. (2012) and Möbius et al. (2012) from 2009 and 2010 observations, but disagree by more than 2σ with the 2013 result. Table 1 provides the resulting full sets of parameters for the three data sets examined. It should be noted that the various parameter combinations for different data selections all vary along IBEX's previously identified four-dimensional parameter tube (Möbius et al. 2012; McComas et al. 2012).

Table 1. Warsaw Test Particle Model Results for 2013 and 2014 Data

Data (Degrees of Freedom)	λISM∞	βISM∞	VISM∞	THe∞	Reduced χ2
2013, |εz| < 004 (63)	74.7 ± 1.2	5.06 ± 0.23	26.9 ± 0.15	8550 ± 195	1.862
2014, |εz| < 002 (22)	76.2 ± 1.7	5.09 ± 0.24	25.6 ± 0.17	7210 ± 165	0.764
2014, εz ∼ −49 (30)	78.3 ± 1.2	4.96 ± 0.23	24.05 ± 0.15	6330 ± 160	0.922

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As yet another check of the resulting parameters, for this study we also developed a new hybrid between the analytic model (Lee et al. 2012) and a numerical scheme for integrating over the instrument response function. This approach avoids the approximations needed to make the analytic model tractable and is a natural extension of the interstellar hydrogen model developed by Schwadron et al. (2013). Analytic solutions produce He atom trajectories through the heliosphere, including ionization of He atoms assuming that the ionization rate falls off as the radial distance squared. The model then performs three primary numerical integrations; each solved using a fourth-order scheme. For each orbit segment, the spin axis direction is known and the collimator boresight is a function of this vector and the spacecraft spin phase. Given an incident direction of neutral atoms into the collimator, integration of the differential flux over energy produces the total incident flux of He atoms. This integrated flux is then used to estimate the contributions from sputtered components inside the IBEX-Lo sensor in each of the lower ESA steps for the specified collimator orientation. The model next integrates over the point-spread function of the collimator (Fuselier et al. 2009b; Schwadron et al. 2009, 2013). This second integration takes into account all directions from which the collimator admits neutral atoms for a specific spin phase. The final integration is performed over spin phase for the specific 6° spin bin in which observations were taken.

Results from our new hybrid model agree quite well with the analytic results for ε_z = 0° and we find the same trend for negative and positive ε_z values as discussed above when comparing the analytic model with this new model. Close to the peak longitude of the ISN distribution, the agreement is within 0.3% in the in-ecliptic flow angle, 2% in the out-of-ecliptic flow angle, and 4% in temperature. The small differences between the hybrid model and the analytic calculation likely reflect differences in the integration over the instrument response, which can only be approximated in the analytic technique. The hybrid model provides a way to account for the response of the instrument to changes in pointing and to include more complicated distributions of the incident ISN atoms. In the future, this model will be utilized heavily to test and refine our understanding of interstellar parameters and the incident interstellar distribution function.

Figure 2 shows the combined results from the analyses and data taken for IBEX spin pointing near the ecliptic plane (implications of "off pointing" results are discussed below). For the analytic analysis (red), the center values reflect the weighted average between the results from the two data selections (|ε_z| < 007 and |ε_z| < 02). For the Warsaw model (blue), we calculated the centers and weighted averages for the combination of the 2013 season and near-ecliptic pointing orbits from 2014. The lines indicate the centroids of the four-dimensional tubes of possible parameters from the IBEX data. For both, 1σ values for errors (across the tubes) and ranges of possible coupled parameters (along the tubes) are shown by the shaded regions. Clearly, the two new independent analyses give very similar results for similar selections of data. In addition, both sets of results are consistent with the coupled IBEX parameters given previously by McComas et al. (2012; black line).

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A consistent picture for the local interstellar flow emerges in Figure 2 that brings together (1) new analyses of the Ulysses observations showing significantly higher temperatures than previously thought (Wood et al. 2014; Bzowski et al. 2014), (2) a solution consistent with IBEX's previously published four-dimensional tube of coupled parameters (McComas et al. 2012), and (3) two new independent analyses of more recent IBEX data for orbit segments with the simplest (in ecliptic) spacecraft pointing. Together, all of these indicate a preferred portion of the IBEX tube at slightly higher speeds and significantly higher temperatures than the central values from our earlier studies (Bzowski et al. 2012; Möbius et al. 2012; McComas et al. 2012). While the precise value ranges and error estimates still vary, we recommend using a combined IBEX/Ulysses solution for now of V_ISM∞ ∼ 26 km s⁻¹, λ_ISM∞ ∼ 75°, β_ISM∞ ∼ −5°, and T_He∞ ∼ 7000–9500 K.

Increasing both the temperature and relative speed of the interstellar cloud surrounding the heliosphere has a number of significant implications for our galactic environment. Velocity and temperature serve as the primary basis for relating the local ISNs with the broader interstellar medium observed toward nearby stars. First, a speed of ∼26 km s⁻¹, is back squarely between the speeds of the local interstellar cloud (LIC; ∼24 km s⁻¹) and the G-Cloud (nearly 30 km s⁻¹) as given by Redfield & Linsky (2008). The 25.5 km s⁻¹ LIC velocity found by Gry & Jenkins (2014) also agrees within errors with the combined IBEX/Ulysses velocity and flow direction. However, neither the Gry & Jenkins model, nor the IBEX/Ulysses data, are consistent with the temperature or velocity of the interstellar gas in front of the nearest useful star for understanding the upwind LIC, 36 Oph. The single interstellar cloud in front of 36 Oph—the G-cloud (Redfield & Linsky 2008) is cooler (6900 ± 500 K) and faster (−27.9 + −0.2 km s⁻¹) (Wood et al. 2000) than the IBEX/Ulysses measurements suggest. For an LIC neutral H density of 0.2 cm⁻³, the Wood et al. (2000) results indicate that the LIC edge is within 0.1 pc, suggesting that the Sun should transition out of the LIC within the next 3500 yr. In fact, Redfield & Linsky (2008) suggested that perhaps the heliosphere is already in a boundary region between the two clouds. A boundary layer with an intermediate speed at the interface certainly raises questions about the interaction of these two regions of the interstellar medium and about the nature of the decelerating cluster of LICs found in our very local galactic environment (Frisch et al. 2011). Such an intermediate speed might also be indicative of turbulence in the interstellar flow and the possibility that the heliosphere currently exists in some local "eddy" at or near the boundary.

A speed of ∼26 km s⁻¹ also reopens the question of whether there is a bow shock ahead of the heliosphere in the LISM. While Zieger et al. (2013) showed that a spatially limited slow magnetosonic shock might be possible, here we focus on the question of a more traditional fast magnetosonic bow shock. Recent measurements from Voyager 1 indicate a value of ∼4.6 μG just outside the heliopause (Burlaga & Ness 2014). Such a strong magnetic field could mean that there is still no fast magnetosonic shock ahead of the heliosphere. On the other hand, Scherer & Fichtner (2014) argue for a fast bow shock, based on inclusion of LISM He⁺ in their model, which reduces the Alfvén and fast magnetosonic speeds. Another key factor is that charge exchange and the resulting ENAs play a major role in transferring plasma properties over hundreds of AU and thus act to mediate the bow wave/shock interaction in the upstream medium. Zank et al. (2013) examined such a mediation process and found more smoothly varying plasma parameters and a bow wave, even for parameters that would normally be expected to create a bow shock; these authors also showed that such an interaction can account for observations of the hydrogen wall as well as traditional bow shock models can. Thus, the debate continues as models evolve, become more sophisticated, and try to keep up with our ever growing understanding of the upstream conditions and heliospheric interaction.

The temperature of the LIC is a vital diagnostic of the physical processes that affect this warm, low density, cloud where both hydrogen and helium are significantly ionized. An interstellar medium of 7000–9500 K is much warmer than previously suggested by observations inside the heliosphere. However, it is consistent with other, remote sensing observations of a warmer medium (Redfield & Linsky 2004; Frisch et al. 2014). It is also consistent with the independently derived interstellar oxygen/neon temperature (see gold curve/region in Figure 2) obtained from IBEX (Möbius et al. 2014). In contrast to the historic Ulysses He temperature of 6300 K, this suggests that the interstellar medium around the heliosphere may be roughly isothermal—all constituents having the same temperature—even though the scale length for coupling of He to the other interstellar species is larger than the heliosphere itself (Kubiak et al. 2014). In future studies we will further refine both the He and O/Ne temperatures to determine if they are truly in equilibrium or if a small difference exists; any such difference would be important and could be related either to the interaction of the heliosphere with the LISM or something inherent to the interstellar material the Sun is traveling through.

Temperature is an important diagnostic of the thermal equilibrium of the LIC, with consequences for the equipartition of magnetic and thermal energies determined from modeling of the LIC. The rapid cooling time of a low density recombining plasma at a temperature of 6300 K gives hydrogen ionization fractions well above the 20% level of the LIC (Slavin 2009). The primary source of LIC heating (∼65%) is provided by photons from the conductive interface between the LIC and the surrounding hot plasma, according to photoionization equilibrium LIC models (Slavin & Frisch 2008). The conductive interface also supplies the energetic radiation required to maintain the large relative ionization of He (∼39%) compared to that of hydrogen (∼22%) observed in the LIC. Maintaining the LIC equilibrium at a higher temperature requires either additional heating from the cloud interface or reduced cooling rates.

As discussed in Slavin (2014), the requirement that the total LIC pressure, including magnetic pressure, be equal to the Local Bubble pressure sets limits on the LIC ionization and temperature that depend on the interface properties. For pressure equilibrium between the thermal gas and the magnetic field, an increase in gas temperature will require a larger magnetic field strength. For the same angle between the magnetic field and cloud surface, an increase in the magnetic field strength also increases the ionizing flux from the interface. For a magnetic field direction somewhat parallel to the cloud surface this larger magnetic field strength will suppress the cloud evaporation, raise the thermal temperature in the evaporative boundary, and change the cloud heating. The calculated interface fluxes are also highly sensitive to the soft X-ray emissivity of the local bubble hot plasma through the still-uncertain contamination of the hot bubble X-ray emissivity by foreground emission from charge-exchange processes inside of the heliosphere (Slavin 2014). Evaporation of the LIC into the surrounding hot plasma depends on the interface temperature (Slavin & Frisch 2002). The LIC temperature then becomes a fundamental diagnostic of the role of a conductive interface in setting the fluxes of EUV ionizing radiation, the poorly understood Local Hot Bubble plasma, and the evaporation of the LIC itself.

A precise determination of the He inflow vector is critical for the orientation of the B_ISM∞–V_ISM∞ (B–V) plane, which has a key influence on the global heliospheric structure. In particular, theoretical arguments and simulations indicate that species strongly affected by the heliospheric interface (e.g., H and O) should be deflected in this B–V plane (Lallement et al. 2005; Izmodenov et al. 2005; Opher et al. 2006; Pogorelov et al. 2009). Even quite small changes of a few degrees in the inflow direction have a substantial effect on the orientation of this plane (Bzowski et al. 2012; Möbius et al. 2014). With an ecliptic longitude back close to the original Ulysses value, the observed hydrogen deflection plane matches well with the B–V plane, using the center of the IBEX ribbon as the best measurement of the external magnetic field (McComas et al. 2009b; Schwadron et al. 2009). However, the fact that the ribbon center varies with energy (Funsten et al. 2013) may indicate that the ribbon is effectively sampling greater distances beyond the heliopause with the highest energies reflecting the most distant regions. As noted by Möbius et al. (2014), while the low energy ribbon center matches the hydrogen deflection plane with the Ulysses values well, higher energies are more consistent with greater longitudes as suggested by some of the IBEX data. On the other hand, we continue to discover more complexity and asymmetries in the heliosphere's global interaction with the LISM. Most recently, McComas & Schwadron (2014) suggested that in the inner heliosheath, the plasma flows away from a ridge of maximum pressure ∼20° south of the heliosphere's nose. Such asymmetric flows may be driven by suprathermal particle dynamics that are not in current models. Future models, including more of the fundamental physics of the plasmas and their interactions will be needed to match the complex structure of our heliosphere.

We now return to the discrepancy between the ε_z = 0° and ε_z = −49 subsets of the 2014 data in the Warsaw model analysis. The source of the ∼2σ difference between these is unknown, and while it is possible that this could just be a statistical coincidence, it is unlikely. More likely is that either we are missing some unknown but significant observational aspect, or this aspect is missing in our modeling. It would certainly be an exciting resolution if some important additional physical process was operating in the LISM or at the heliospheric interface, not accounted for in the current model. One possible source of such a discrepancy may have to do with the warm breeze population, which is included in the model, but only based on our original analyses. Analysis of the 2014 and 2013 simultaneously for the main interstellar inflow and the warm breeze is currently underway. Furthermore, the interstellar particle distributions may be highly non-Maxwellian. For example, Livadiotis & McComas (2011) showed that the addition of pickup ions (such as those that mediate the heliosphere's interaction) actually add order to plasmas and drive distributions toward anti-equilibrium—the furthest state away from equilibrium and Maxwell distributions (see Livadiotis & McComas 2013, and references therein).

Thus, for significantly higher interstellar temperatures than previously found, the Ulysses/GAS observations approach the four-dimensional tube of possible interstellar parameters allowed by IBEX observations. In addition, new IBEX observations for spacecraft pointing near the ecliptic plane through two independent approaches also lead to preferred solutions around the same region of the allowable parameter space tube. We conclude that the combined IBEX/Ulysses data are consistent with a single set of interstellar parameters with V_ISM∞ ∼ 26 km s⁻¹, λ_ISM∞ ∼ 75°, β_ISM∞ ∼ −5°, and T_He∞ ∼ 7000–9500 K. These results have important implications for many aspects of the heliosphere's interstellar interaction as discussed above. Finally, the current values are surely not the last word from IBEX and for the 2015 season, we are collecting data from a new spin axis direction ∼5° on the other side (north) of the ecliptic. These and other future observations and analysis of even more high sensitivity ISN observations from IBEX will surely lead to exciting new discoveries of the detailed shape of the ISN distribution and open the possibility of further discovery science and possibly even new populations of neutrals from the LISM and/or its heliospheric interaction.

We thank all of the outstanding men and women who made the IBEX mission possible. This work was carried out under the IBEX mission which is part of NASA's Explorer Program. Support was also provided by the Polish National Science Center grant 2012-06-M-ST9-00455.