J. M. Sokół

THE FIRST THREE YEARS OF IBEX OBSERVATIONS AND OUR EVOLVING HELIOSPHERE

This study provides, for the first time, complete and validated observations from the first three years (2009-2011) of the Interstellar Boundary Explorer (IBEX) mission. Energetic neutral atom (ENA) fluxes are corrected for both the time-variable cosmic ray background and for orbit-by-orbit variations in their probability of surviving en route from the outer heliosphere in to 1 AU where IBEX observes them. In addition to showing all six six-month maps, we introduce new annual ram and anti-ram maps, which can be produced without the need for algorithm-dependent Compton-Getting corrections. Together, the ENA maps, data, and supporting documentation presented here support the full release of these data to the broader scientific community and provide the citable reference for them. In addition, we show that heliospheric ENA emissions have been decreasing over the epoch from 2009 to 2011 with the IBEX Ribbon decreasing by the largest fraction and only the heliotail (which is offset from the down wind direction by the interstellar magnetic field) showing essentially no reduction and actually some increase. Finally, we show how the much more complete observations provided here strongly indicate a quite direct and latitude-dependent solar wind source of the Ribbon.

Warmer Local Interstellar Medium: A Possible Resolution of the Ulysses-IBEX Enigma

Interstellar Boundary Explorer (IBEX) measurements from 2009-2010 identified a set of possible solutions with very tight coupling between the interstellar He inflow longitude, latitude, speed, and temperature. The center of this allowable parameter space suggested that the heliosphere could be moving more slowly and in a slightly different direction with respect to the interstellar medium than indicated by earlier Ulysses observations. In this study we examine data from 2012-2014 and compare results from an analytic analysis and a detailed computer model. For observations where the IBEX spacecraft pointing is near the ecliptic plane, the latest measurements indicate a different portion of IBEXs four-dimensional tube of possible parameters—one that is more consistent with the Ulysses flow direction and speed, but with a much higher temperature. Together, the current combined IBEX/Ulysses values we obtain are V ISM∞ ~ 26 km s–1, λISM∞ ~ 75°, βISM∞ ~ –5°, and T He∞ ~ 7000-9500 K. These indicate that the heliosphere is in a substantially warmer region of the interstellar medium than thought from the earlier Ulysses observations alone, and that this warmer region may be roughly isothermal. However, measurements taken when IBEX was pointing ~5° south of the ecliptic are inconsistent with this solution and suggest a slower speed, lower temperature, and flow direction similar to IBEXs prior central values. IBEX measures much deeper into the tails of the distributions of the inflowing interstellar material than Ulysses did and these observations indicate that the heliospheres interstellar interaction is likely far more complex and interesting than previously appreciated.

Decades-Long Changes of the Interstellar Wind Through Our Solar System

Wind of Change The flow of interstellar gas and dust through the solar system was thought to be unvarying, but Frisch et al. (p. 1080) show that there has been a significant variation of the direction of the flow of interstellar helium through the solar system over the past 40 years. The data, collected by 10 different spacecraft over much of the space age, hint of changes rather than constancy in the solar systems galactic environment. Analysis of data collected by 10 different spacecraft indicates that our solar system’s local environment may be changing. The journey of the Sun through the dynamically active local interstellar medium creates an evolving heliosphere environment. This motion drives a wind of interstellar material through the heliosphere that has been measured with Earth-orbiting and interplanetary spacecraft for 40 years. Recent results obtained by NASAs Interstellar Boundary Explorer mission during 2009–2010 suggest that neutral interstellar atoms flow into the solar system from a different direction than found previously. These prior measurements represent data collected from Ulysses and other spacecraft during 1992–2002 and a variety of older measurements acquired during 1972–1978. Consideration of all data types and their published results and uncertainties, over the three epochs of observations, indicates that the trend for the interstellar flow ecliptic longitude to increase linearly with time is statistically significant.

LOCAL INTERSTELLAR MEDIUM: SIX YEARS OF DIRECT SAMPLING BY IBEX

The Interstellar Boundary Explorer (IBEX) has been directly observing neutral atoms from the local interstellar medium for the last six years (2009–2014). This paper ties together the 14 studies in this Astrophysical Journal Supplement Series Special Issue, which collectively describe the IBEX interstellar neutral results from this epoch and provide a number of other relevant theoretical and observational results. Interstellar neutrals interact with each other and with the ionized portion of the interstellar population in the “pristine” interstellar medium ahead of the heliosphere. Then, in the heliosphereʼs close vicinity, the interstellar medium begins to interact with escaping heliospheric neutrals. In this study, we compare the results from two major analysis approaches led by IBEX groups in New Hampshire and Warsaw. We also directly address the question of the distance upstream to the pristine interstellar medium and adjust both sets of results to a common distance of ~1000 AU. The two analysis approaches are quite different, but yield fully consistent measurements of the interstellar He flow properties, further validating our findings. While detailed error bars are given for both approaches, we recommend that for most purposes, the community use “working values” of ~25.4 km s⁻¹, ~75°7 ecliptic inflow longitude, ~−5°1 ecliptic inflow latitude, and ~7500 K temperature at ~1000 AU upstream. Finally, we briefly address future opportunities for even better interstellar neutral observations to be provided by the Interstellar Mapping and Acceleration Probe mission, which was recommended as the next major Heliophysics mission by the NRCʼs 2013 Decadal Survey.

Modulation of neutral interstellar He, Ne, O in the heliosphere. Survival probabilities and abundances at IBEX

Direct sampling of neutral interstellar (NIS) atoms by the Interstellar Boundary Explorer (IBEX) can potentially provide a complementary method for studying element abundances in the Local Interstellar Cloud and processes in the heliosphere interface.}{We set the stage for abundance-aimed in-depth analysis of measurements of NIS He, Ne, and O by IBEX and determine systematic differences between abundances derived from various calculation methods and their uncertainties.}{Using a model of ionization rates of the NIS species in the heliosphere, based on independent measurements of the solar wind and solar EUV radiation, we develop a time-dependent method of calculating the survival probabilities of NIS atoms from the termination shock (TS) of the solar wind to IBEX. With them, we calculate densities of these species along the Earths orbit and simulate the fluxes of NIS species as observed by IBEX. We study pairwise ratios of survival probabilities, densities and fluxes of NIS species at IBEX to calculate correction factors for inferring the abundances at TS.}{The analytic method to calculate the survival probabilities gives acceptable results only for He and Ne during low solar activity. For the remaining portions of the solar cycle, and at all times for O, a fully time dependent model should be used. Electron impact ionization is surprisingly important for NIS O. Interpreting the IBEX observations using the time dependent model yields the LIC Ne/O abundance of

WARM BREEZE FROM THE STARBOARD BOW: A NEW POPULATION OF NEUTRAL HELIUM IN THE HELIOSPHERE

0.16\pm40%

Neutral interstellar He parameters in front of the heliosphere 1994-2007

. The uncertainty is mostly due to uncertainties in the ionization rates and in the NIS gas flow vector.}{The Ne/He, O/He and Ne/O ratios for survival probabilities, local densities, and fluxes scaled to TS systematically differ and thus an analysis based only on survival probabilities or densities is not recommended, except the Ne/O abundance for observations at low solar activity.

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

We investigate the signals from neutral helium atoms observed in situ from Earth orbit in 2010 by the Interstellar Boundary Explorer (IBEX). The full helium 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 helium that we call the Warm Breeze. The Warm Breeze is approximately 2 times 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 ~19° from the inflow direction of interstellar gas. The Warm Breeze seems to be a long-term, perhaps permanent feature of the heliospheric environment. It has not been detected earlier because it is strongly ionized inside the heliosphere. This effect 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. We discuss possible sources for the Warm Breeze, including (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 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. Whatever the nature of the Warm Breeze, its discovery exposes a critical new feature of our heliospheric environment.

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

Analysis of IBEX measurements of neutral interstellar He flux brought the inflow velocity vector different from the results of earlier analysis of observations from GAS/Ulysses. Recapitulation of results on the helium inflow direction from the past ~40 years suggested that the inflow direction may be changing with time. We reanalyze the old Ulysses data and reprocess them to increase the accuracy of the instrument pointing to investigate if the GAS observations support the hypothesis that the interstellar helium inflow direction is changing. We employ a similar analysis method as in the analysis of the IBEX data. We seek a parameter set that minimizes reduced chi-squared, using the Warsaw Test Particle Model for the interstellar He flux at Ulysses with a state of the art model of neutral He ionization in the heliosphere, and precisely reproducing the observation conditions. We also propose a supplementary method of constraining the parameters based on cross-correlations of parameters obtained from analysis of carefully selected subsets of data. We find that the ecliptic longitude and speed of interstellar He are in a very good agreement with the values reported in the original GAS analysis. We find, however, that the temperature is markedly higher. The 3-seasons optimum parameter set is lambda = 255.3, beta = 6, v = 26.0 km/s, T = 7500 K. We find no evidence that it is varying with time, but the uncertainty range is larger than originally reported. The originally-derived parameters of interstellar He from GAS are in good agreement with presently derived, except for the temperature, which seems to be appreciably higher, in good agreement with interstellar absorption line results. While the results of the present analysis are in marginal agreement with the earlier reported results from IBEX, the most likely values from the two analyses differ for reasons that are still not understood.

Solar Parameters for Modeling the Interplanetary Background

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