IBEX observations of heliospheric energetic neutral atoms: Current understanding and future directions

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 38,, doi: /2011gl048763, 2011 IBEX observations of heliospheric energetic neutral atoms: Current understanding and future directions D. J. McComas, 1,2 H. O. Funsten, 3 S. A. Fuselier, 4 W. S. Lewis, 1 E. Möbius, 5 and N. A. Schwadron 1,6 Received 12 July 2011; revised 18 August 2011; accepted 21 August 2011; published 16 September [1] The Interstellar Boundary Explorer (IBEX) has provided the first energy resolved all sky maps of energetic neutral atom (ENA) emissions from the heliosphere s boundary with the local interstellar medium (LISM). The IBEX maps reveal, superposed on a global ENA background, an enigmatic ribbon of enhanced ENA emission, a feature unpredicted by theory and numerical simulations and requiring a new paradigm for the heliosphere/lism interaction. The ribbon appears to be ordered by the interstellar magnetic field; it is up to 3 times brighter than the background emission and spectrally distinct from it. The ribbon s origin, whether inside or outside the heliopause or at more exotic locations in the LISM, is unknown. Here, we review the various hypotheses that have been proposed to explain the ribbon as well as what we have learned from the IBEX sky maps about the ENAs parent ion populations and about the structure, dynamics, and properties of the outer heliosphere and nearby interstellar medium. We conclude with a brief mention of new IBEX results on lunar and magnetospheric ENAs and a preview of a possible future mission that builds on the successes of IBEX as we continue to explore our home in the galaxy. Citation: McComas, D.J.,H.O.Funsten,S.A.Fuselier,W.S.Lewis,E.Möbius, and N. A. Schwadron (2011), IBEX observations of heliospheric energetic neutral atoms: Current understanding and future directions, Geophys. Res. Lett., 38,, doi: / 2011GL Introduction [2] Our solar system is located 30,000 light years from the galactic center, between the Milky Way s Sagittarius and Perseus spiral arms. It is immersed in a cloud of warm, partially ionized gas and dust, within which the supersonic outflow of plasma from the Sun (the solar wind) and its entrained interplanetary magnetic field (IMF) inflate a bubble called the heliosphere. The heliosphere s immediate environment (the Local Interstellar Medium, LISM; Table 1) 1 Southwest Research Institute, San Antonio, Texas, USA. 2 Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas, USA. 3 Los Alamos National Laboratory, Los Alamos, New Mexico, USA. 4 Lockheed Martin Advanced Technology Center, Palo Alto, California, USA. 5 Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire, USA. 6 Physics Department, University of New Hampshire, Durham, New Hampshire, USA. Copyright 2011 by the American Geophysical Union /11/2011GL will not be measured in situ until a Voyager spacecraft leaves the heliosphere, but its properties have been inferred from remote sensing data (e.g., absorption line and polarization observations toward nearby stars), measurements of interstellar He atoms inside the heliosphere [Witte, 2004], and modeling [e.g., see Slavin, 2009; Frisch et al., 2009]. The first direct measurements of interstellar H and O from IBEX [Möbius et al., 2009] are currently being folded into improved models of the LISM. [3] As the solar system moves through the LISM at 26.3 km s 1 [Witte, 2004], the interaction with the interstellar gas creates a complex structure at the heliospheric boundary (Figure 1), consisting of 1) the termination shock (TS), an inner discontinuity where the supersonic solar wind is slowed and heated; 2) the heliopause (HP), which separates the shocked solar wind from the ionized component of the LISM; and 3) a bow shock (BS) in the LISM (if it is supersonic) upstream of the heliosphere. The region of shocked solar wind between the TS and the HP is known as the inner heliosheath; the region of shocked LISM plasma between the HP and the BS is referred to as the outer heliosheath. [4] In the outer heliosheath, charge exchange between the shocked interstellar protons and inflowing interstellar neutral hydrogen leads to the formation of a wall of compressed and heated neutral hydrogen atoms [Baranov et al., 1991]. Inflowing interstellar neutrals that do not charge exchange with the shocked interstellar ions in the outer heliosheath pass through the HP and into the heliosphere, where some are ionized and picked up by the motional electric field of the solar wind. Like the core solar wind, these pickup ions (PUIs) undergo a shock transition at the TS and form a significant component of the shock processed ion population in the inner heliosheath. [5] The heliosphere/lism interaction has been the subject of both theoretical studies [Parker 1961] and increasingly sophisticated numerical simulations [e.g., Zank, 1999; Fahr et al., 2000; Zank et al., 2009; Izmodenov et al., 2009] for some 50 years. However, until Voyagers 1 and 2 crossed the TS in 2004 and 2007 [Stone et al., 2005; 2008], respectively, the only information available about the location, structure, and properties of the heliospheric boundary region was indirect: measurements of anomalous cosmic rays (ACRs), radio emissions from the outer heliosphere, and Lyman a absorption data. In situ data from the Voyagers have shown numerous surprising and puzzling results. The Voyager data are unique and extraordinarily valuable; however, because they are limited to two single point measurements, they cannot answer important outstanding questions about the global structure and dynamics of the heliosphere/lism 1of9

2 Table 1. Properties of the LISM a Property Value n(h ) cm 3 n(e) 0.06 cm 3 T 6300 K H + /(H + H + ) 0.24 He + /(He + He + ) 0.40 B 2.7 mg a From Frisch et al. [2009]. interaction; those answers require some way to globally image the entire interaction. 2. The IBEX Mission [6] Charge exchange between inflowing interstellar neutrals and shocked solar wind ions and PUIs at the heliospheric boundary creates energetic neutral atoms (ENAs) with essentially the same energy and velocity as the parent ions. The motion of the ENAs is not controlled by the magnetic field, and they receive the instantaneous velocity vectors of their parent ions at the moment of charge exchange. ENAs travel on ballistic trajectories, some of which are directed back inward, toward the Sun. These ENAs can be detected at 1 AU by extremely sensitive instruments, allowing construction of global images of the heliospheric interaction region (Figure 2) and derivation of the energy spectra of the parent ions [Gruntman, 1992, 1997; Gruntman et al., 2001]. [7] The potential of ENA imaging as a means of studying the heliosphere/lism interaction from 1 AU has been realized for the first time and with remarkable success by the Interstellar Boundary Explorer (IBEX), a Small Explorer (SMEX) mission launched 19 October 2008 [McComas et al., 2009a]. IBEX is instrumented with two highly sensitive, large aperture, single pixel ENA cameras that provide overlapping coverage of energies kev. IBEX Hi [Funsten et al., 2009a] detects ENAs from kev in six energy steps, while IBEX Lo [Fuselier et al., 2009a] covers from kev in eight energy steps. IBEX spins at 4 rpm, with its spin axis pointed roughly toward the Sun and cameras viewing perpendicular to the spin axis. With each spin, the two cameras view a 7 wide (FWHM) swath of the sky, sampling the same swath over their respective energy ranges throughout each 7.5 day orbit. During each perigee pass, the spacecraft spin axis is repointed, allowing observations of the adjacent 7 wide swath, so that over 6 months the entire sky is mapped in successive swaths. [8] The goal of the IBEX mission is to discover the global interaction between the solar wind and the interstellar medium [McComas et al., 2004, 2009a, p. 17]. During its over two and a half years of science operations to date, IBEX has collected five complete sets of energy resolved ENA sky maps and continues to accumulate new data. In this frontier article, we review what IBEX has revealed so far about the heliospheric frontier, discoveries that challenge our pre IBEX theories and models of this complex and dynamic region and open the way to a deeper understanding of our heliosphere and other astrospheres as well. 3. IBEX Results [9] The first set of IBEX sky maps revealed a remarkable feature a narrow ribbon of intense ENA emissions (Figure 3a) that was completely unanticipated by any theory or model prior to the IBEX observations [McComas et al., 2009b; Schwadron et al., 2009a]. This ribbon is superposed on diffuse, globally distributed emissions that span all directions in the sky [McComas et al., 2009b]. Determining the properties and source(s) of the overall emissions, and especially of this surprising ribbon feature, and interpreting the clues that they hold about the nature of the heliosphere/ LISM interaction have been major focuses of the IBEX team since launch. The results discussed here are based on the first two sky maps. Analysis of maps 3 5 is still ongoing Ribbon Morphology [10] The ribbon forms a circular arc that extends more than 300 across the sky and is centered on an ecliptic longitude (l) of and latitude (b) of 39 [Funsten et al., 2009b]. It is clearly identifiable at energies >0.2 kev and is most pronounced in the 1.1 kev sky maps [Fuselier et al., 2009b], where it appears as a bright narrow crescent that curves from the high latitude northern hemisphere through mid latitudes in the southern hemisphere and back toward the equator (Figure 3a). The width of the ribbon varies from <15 to >25, averaging 20 over the energy range 0.7 kev to 2.7 kev [Fuselier et al., 2009b]. Figure 1. 2D hydrodynamic simulation of the heliosphere/ LISM interaction [Zank and Müller, 2003], labeled to show the principal heliospheric regions and the structures and processes involved in the interaction. The color scheme indicates temperature, which ranges from K (dark blue) in the LISM to 10 6 K (dark orange) in the inner heliosheath. (Reprinted with permission from McComas et al. [2004]. Copyright 2004, American Institute of Physics.) 2of9

3 Figure 2. Extremes of differential ENA fluxes from kev predicted for (a) a strong gas dynamical TS and (b) a TS weakened by a large pickup ion pressure. The calculated ENA fluxes are taken from Gruntman et al. [2001] and have been replotted as IBEX sky maps for comparison with the actual observations. The maps use solar ecliptic coordinates and are centered on the heliospheric nose. V1 and V2 mark the positions of the Voyager TS crossings. [11] ENA fluxes vary along the length of the ribbon and at 1 kev in the brightest regions are 2 3 times greater than the globally distributed flux [Fuselier et al., 2009b]. A particularly bright localized emission knot can be seen, especially at higher energies (e.g., at >2 kev), between 30 and 12 longitude and 38 and 72 latitude [McComas et al., 2009b; Funsten et al., 2009b]. Livadiotis et al. [2011] recently identified several possible additional knots, consistent with the suggestion that the apparently continuous ribbon may consist of a string of more localized, overlapping knots [McComas et al., 2009b, p. 962]. In addition to these large scale features, the IBEX sky maps also reveal the presence of fine scale structure in the ribbon (Figure 3b) [(McComas et al., 2009b]. [12] The ribbon maps to locations on the sky where the IBEX line of sight vector (r) is perpendicular to the modeled interstellar magnetic field (B) that is draped around the heliosphere (B r = 0) [Schwadron et al., 2009b] (Figure 4), suggesting that external magnetic as well as external gas dynamic forces both play essential roles in the heliosphere/ LISM interaction. IBEX measurements show that the heliospheric interaction is squarely in the middle between the two limiting cases (external dynamic and magnetic dominance) considered by Parker [1961] [McComas et al., 2009b]. Funsten et al. [2009b] calculated a line of sight integrated plasma pressure in the ribbon of 2 pdyne cm 2 for an inner heliosheath source 50 AU thick. Schwadron et al. [2009b] estimated that this is 2.5 times the LISM ram pressure, Figure 3. (a) ENA sky map at 1.1 kev, from IBEX s first six months of science operations [from McComas et al., 2009b]. The emissions are mapped in solar ecliptic coordinates using a Mollweide projection; the red line indicates the galactic equator. The ribbon passes between the locations of the termination shock crossings by the two Voyagers (marked V1 and V2), which did not sample the parent ion population of the ribbon ENAs. (b) Fine structure in the ribbon, possibly due to extrusions of dense, hot plasma from the inner heliosheath into the outer heliosheath, small scale compressions of the draped ISMF, magnetic reconnection, or heliopause instabilities [McComas et al., 2010]. 3 of 9

4 Figure 4. Contour lines showing where the IBEX radial line of sight forms an angle between 82 and 99 (i.e., is perpendicular) to the calculated draped ISMF superposed on the 1.1 kev ribbon. The ISMF is assumed to be in the hydrogen deflection plane (titled 60 to the ecliptic), have a magnitude of 3 mg, and be inclined 30 into the southern hemisphere from the LISM flow vector [from Schwadron et al., 2009b]. structure of the solar wind near solar minimum [McComas et al., 2009b; Dayeh et al., 2011]. Spectral analysis of the ENA flux maps also reveals that the spectrum in the tail is significantly steeper (softer) than that in the nose region at energies >0.7 kev (Figure 5b) [McComas et al., 2009b; Schwadron et al., 2011; Dayeh et al., 2011]. The steeper spectrum could be from longer line of sight (LOS) integrations of low energy ions toward the tail [McComas et al., 2009b] or evidence that the TS is weaker toward the tail than at the nose [Schwadron et al., 2011]. [15] The ribbon was initially thought to have the same spectral properties as the non ribbon flux, but more recent analysis by Schwadron et al. [2011] reveals a knee (a downturn in the slope) whose location varies between 1 and 4 kev with longitude and latitude. Generally, the knee occurs at higher energies at higher latitudes, which is consistent with the latitudinally ordered behavior of the non ribbon spectra and indicative of the influence of the solar wind structure on the ribbon as well as on the globally distributed flux. The knee distinguishes the ribbon spectrally from the globally distributed flux, which at mid to low latitudes can generally be described by a simple power law [McComas et al., 2009b; Dayeh et al., 2011; Livadiotis et al., 2011]. This difference suggests that [the ribbon s] source plasma population is generated via a distinct physical process [Schwadron et al., 2011, p. 1]. [16] Applying Tsallis statistical mechanics to the analysis of ENA spectra, Livadiotis et al. [2011] calculated temperatures for parent ion populations with energies >0.7 kev. The derived temperatures vary over the sky but are typically 106 at low to mid latitudes (Figure 6), which is consistent with an inner heliosheath source population. Interestingly, requiring a 2.5 mg interstellar magnetic field (ISMF) to provide the additional (JxB) pressure needed to achieve pressure balance ENA Spectra [13] The ENA energy spectra provide essential information about the identities of parent ions and the nature of the heliosphere/lism interaction. Using a sophisticated masking technique, Schwadron et al. [2011] separated the globally distributed flux (Figure 5a) from the ribbon flux at energies > 0.5 kev, making it possible to analyze the spectral as well as other properties of the two populations independently. [14] A key result from the analysis of IBEX data is a latitudinal ordering of the ENAs spectral shape, flattening (hardening) at higher energies (> 1 kev) toward the poles and steepening (softening) toward the equator [McComas et al., 2009b, 2010; Funsten et al., 2009b; Schwadron et al., 2011; Dayeh et al., 2011]. The hardening of globally distributed flux spectra at high latitudes (Figure 5b) is most likely due to higher energy PUIs entrained in the fast (high latitude) solar wind; this latitudinal ordering thus reflects the velocity Figure 5. Sky map in solar ecliptic coordinates showing the distribution of (a) the globally distributed ENA flux at 4.29 kev and (b) the spectral index of this ENA flux. The sharply reduced emissions seen in panel A between 0 and 60 longitude and extending south toward 30 are from the heliotail, which is deflected from the LISM downstream flow direction in both longitude and latitude by the interstellar magnetic field. (Adapted from Schwadron et al. [2001, Figures 8 and 17]. Reproduced by permission of the AAS.) 4 of 9

5 Figure 6. Map of derived temperatures for an ENA source population composed of solar wind plasma and entrained pickup ions and describable by a kappa distribution. (Ion temperatures near the poles cannot be derived because the PUI dominated source population is not in a kappa distribution.) The dominant temperature is 10 6 K, consistent with that predicted for the inner heliosheath; lower temperature regions are also present, including the locations of the Voyager crossings (V1 and V2). White numerals identify the northern knot along with four other possible knots, which generally have higher temperatures than the rest of the ribbon. N marks the location of the heliospheric nose. (From Livadiotis et al. [2011, Figure 3]. Reproduced by permission of the AAS.) the derived temperatures at the two Voyager locations are similar to the IBEX derived temperatures (Figure 6). The analysis also shows the ribbon to be thermodynamically distinct from the global distributed flux and possibly composed of multiple separate knots with higher average temperatures of K Temporal Variability [17] The structure and dynamics of the heliospheric boundaries are influenced by the solar wind properties, which vary with the solar activity cycle as well as on shorter time scales [Zank and Müller, 2003; Izmodenov et al., 2005; Washimi et al., 2011]; the effects of such variability should be reflected in the ENA flux. The correlation of the solar wind structure near solar minimum with the ordering of the ENA spectra noted above may be indicative of a solar cycle effect on the outer heliosphere. Longer term variations from changes in the interstellar environment are theoretically possible as well [e.g., Frisch et al., 2010], but unlikely to occur over IBEX s lifetime. Short term (during IBEX s first year of observations), both the ribbon and the global distributed flux have proven to be largely stable [McComas et al., 2010]. Nonetheless, comparison of the first two sky maps reveals some statistically significant changes [McComas et al., 2010]. [18] Two changes seen between the first two 6 month sets of maps are reductions in the emissions from 1) the north and south polar regions (10 15%) and 2) the knot in the northern part of ribbon (25 35%). The reduced polar emissions may reflect the effects of the long term decrease in the mass and momentum flux of the high latitude solar wind observed by Ulysses over the last solar minimum [McComas et al., 2008]. In addition to a reduction in flux, the knot also appears to have spread out along the ribbon as well as to have undergone a softening of its spectrum (Figure 7). The knot occurs near the boundary of the fast and slow solar wind so these changes might be explained by a shift in that boundary to higher latitudes, with the slower wind PUIs providing more parent ions for the knot ENAs [McComas et al., 2010] The Origin of the Ribbon [19] Even though the ribbon appears to be aligned with the direction of the ISMF (Figure 4), the origin of the enigmatic ribbon ENAs where and how they are produced remains an open and crucial question. Several possible sources have been proposed, both inside and outside the HP as well as involving interactions at the HP and TS. McComas et al. [2009a] suggested six possible explanations for the ribbon; these were subsequently discussed in more detail (Figure 8) [McComas et al., 2010; Schwadron et al., 2011]. In addition, several other ideas have been independently posed ENAs From Inside the Heliopause [20] The occurrence of the ribbon where the LOS is perpendicular to the draped ISMF suggests that ribbon ENAs might originate in a region of enhanced plasma pressure (Figure 8a) that forms inside the HP to balance the combined ram pressure of the LISM and the magnetic pressure of the draped ISMF. Increased plasma density in this region, resulting from stagnation of the bulk plasma flow in the inner heliosheath [McComas et al., 2009b], could produce ribbon shaped enhanced ENA fluxes. Additionally, a highdensity, high pressure plasma region might create localized, small scale extrusions of the HP into the outer heliosheath that are reflected in the fine scale structure observed in the portions of the ribbon (Figure 3b). [21] Plasma pressures calculated for the ribbon [Funsten et al., 2009b; Schwadron et al., 2009b; Livadiotis et al., Figure 7. Close up of the knot ENA emissions from the (top) first and (bottom) second sky maps showing a reduction in emission intensity and a change in the distribution of the emissions during the six month interval between the two maps (adapted from McComas et al. [2010]). 5of9

6 Figure 8. Graphic [from McComas et al., 2010] illustrating the six possible sources of the IBEX ribbon from outside as well as inside the heliopause initially suggested by McComas et al. [2009b]. The various possible mechanisms are not mutually exclusive, McComas et al. [2010, paragraph 15] point out; in fact, some combination or combinations may well ultimately explain the ribbon. 2011] are consistent with the stagnation region hypothesis, as are temperatures calculated by Livadiotis et al. [2011], which point to an inner heliosheath origin for both the ribbon and the global distributed flux. However, it should be noted that 1) pre IBEX models [e.g., Heerikhuisen et al., 2007] included external magnetic forces as well as dynamic forces and did not predict the ribbon; and 2) global pressure balance requirements can also be satisfied by assuming that the ribbon s contribution to the total plasma pressure in the inner heliosheath is small and that ribbon fluxes elevated over globally distributed fluxes result from integration over a longer line of sight [McComas et al., 2009b; Schwadron et al., 2011]. Further, although ribbon and non ribbon ENAs may share a common source population in the inner heliosheath, their different energy spectra suggest somewhat different processes. [22] PUIs accelerated at the TS have been proposed as another possible source population for the ribbon ENAs (Figure 8e) [McComas et al., 2009b; Fahr et al., 2011] and are consistent with the ribbon s spectral properties [Schwadron et al., 2011]. However, a source near the TS does not easily account for the apparent ordering of the ribbon by the ISMF. Investigation is ongoing, and improved modeling of the TS and HP geometry may produce a better match with the observed ribbon morphology ENAs From Outside the Heliopause [23] In addition to traveling sunward, ENAs produced in both the unshocked solar wind and inner heliosheath propagate outward into the outer heliosheath as well. There, these primary ENAs can be re ionized through charge exchange with the shocked LISM plasma and captured as PUIs in the outer heliosheath. Charge exchange between the captured PUIs and LISM neutral hydrogen creates secondary ENAs, some of which can, like those from the inner heliosheath, propagate to 1 AU [Izmodenov et al., 2009]. An isotropic population of secondary ENAs (assumed by Izmodenov et al.) cannot explain the ribbon. However, newly created PUIs initially form a ring or partial shell distribution. ENAs produced from PUIs that remain in a ring distribution will propagate sunward preferentially from locations on the HP defined by B r = 0 (Figure 8c). [24] Production of the ribbon by secondary ENAs created from a PUI ring distribution was first proposed by McComas et al. [2009b] and subsequently modeled by Heerikhuisen et al. [2010] (Figure 9). Secondary ENAs offer an attractive explanation for the ribbon: it is consistent with the spectral knee [Schwadron et al., 2011], and is not excluded by the analysis of Livadiotis et al. [2011], who note that their methodology would not distinguish between primary and secondary ENAs if the two have similar spectral character- Figure 9. Sky map for a simulated secondary ENA source in the outer heliosheath. The red line indicates the galactic equator; the black line shows the best fit to the observed ribbon. (From Heerikhuisen et al. [2010, Figure 3]. Reproduced by permission of the AAS.) 6of9

7 istics. The viability of the secondary ENA source, however, depends on stability of the PUI ring population, that is, on whether the new born PUIs preserve the preferred velocity distribution long enough to undergo charge exchange before scattering through wave particle and other interactions. As pointed out by McComas et al. [2009b], these distributions would need to be stable for a couple of years. Stability was assumed by Heerikhuisen et al. [2010]; however, Florinski et al. [2010] calculated that resonant scattering by PUIexcited waves would isotropize PUIs within only a few days. Gamayunov et al. [2010] analyzed the effect of largescale interstellar turbulence and locally generated smallscale turbulence on the evolution of PUI pitch angles prior to re neutralization; their simulations generally support the secondary ENA model. [25] Another possible mechanism for the production of the ribbon outside the HP involves the compression of the outer heliosheath plasma as the ISMF drapes around the HP (Figure 8b) [McComas et al., 2009b, 2010; Schwadron et al., 2009b, 2011]. Compression increases the field strength, density, and perpendicular velocity of the ions (conservation of the first adiabatic invariant, m) such that more of the ENAs created when these ions charge exchange are directed radially inward on lines of sight transverse to the draped field. These ENAs come from suprathermal LISM protons as well as secondary ENAs. Small scale compressions might explain ribbon fine structure [McComas et al., 2009b]. However, for large scale compression, the predicted ribbon is 3 wider than observed at energies <2.73 kev [Schwadron et al., 2011]. These authors conclude that compression of the outer heliosheath plasma cannot be the primary mechanism responsible for the ribbon. In a model of secondary ENAs with compression, Chalov et al. [2010] considered the motion of the parent PUIs along the compressed interstellar magnetic field lines, which can produce ribbon like features, again assuming very slow pitch angle scattering ENAs From Heliopause Interactions [26] Rayleigh Taylor (R T) and Kelvin Helmholz (K H) instabilities, shown theoretically to form on the HP [e.g., Florinski et al., 2005; Borovikov et al., 2008], represent potential sites of elevated ENA emissions from hot inner heliosheath plasma confined in the narrow structure of the instability with possibly lengthened LOS s (Figure 8f) [McComas et al., 2009b, 2010; Schwadron et al., 2009b, 2011]. If such large scale structures are the source of the ribbon, their motion should be reflected in changes in the ribbon s location. As discussed above, comparison of the first two sky maps shows the ribbon to be quite stable over six months; if anything, there is a possible shift of up to 6 (1 pixel) in the southern segment of the ribbon, but toward rather than away from the nose as would be expected for tailward convecting instabilities [McComas et al., 2010]. [27] Magnetic reconnection of the heliospheric magnetic field (HMF) and the ISMF at the HP has also been suggested as another possible ribbon formation mechanism (Figure 8d) [McComas et al., 2009b]. Given the alternating polarity structure of the IMF at the HP [Suess, 2004], however, it is not obvious how reconnection could produce the observed morphology of the ribbon [McComas et al., 2010] nor are estimates of the energy of ions accelerated at the reconnection sites consistent with the energy spectrum of the ribbon [Schwadron et al., 2011]. Nonetheless, reconnection, like R T and K H instabilities and magnetic compressions, may play a role in temporally varying fine scale structure in the ribbon [McComas et al., 2010] An Interstellar Source [28] The ribbon formation mechanisms discussed above assume that the ribbon results from the heliosphere/lism interaction and, in most cases, that it is organized by the draped ISMF. In contrast, Grzedzielski et al. [2010] propose that the ribbon (and possibly the globally distributed flux) are produced by charge exchange between the hot, tenuous plasma of the Local Bubble (LB) and neutral hydrogen evaporating from the Local Interstellar Cloud (LIC), with the LIC/LB interface AU away. Their model accounts for ENA extinction in the LIC and assumes <2% of suprathermal protons over the IBEX energy range in the parent ion population. This population could account for the knee in the ribbon energy spectrum for PUIs created in the turbulent LB/LIC interface [Schwadron et al. 2011]. For angles between the IBEX LOS and the shortest distance to the LIC/LB interface comparable to the 72 opening halfangle determined by Funsten et al. [2009b] for the ribbon, the calculated ENA intensity maximizes along a circular arc for both plane parallel and curved interfaces, although the latter provides the better fit to the IBEX data. [29] Owing to its large scale, the LB/LIC interaction cannot explain fine structure or short term changes in the heliospheric ENA emissions. (In fact, all extraheliospheric source models have difficulty accounting for short term temporal variations because of the long LOS integration on the order of >100 AU that they require.) The observed temporal variability does not rule out an interstellar source, however, but requires an additional, time variable heliospheric source [McComas et al., 2010]. More problematic is the fact that the LB/LIC model, by attributing the morphology of the ribbon to the interface geometry, does not address the apparent organization of the ribbon emissions by the ISMF Properties and Shape of the Outer Heliosphere and the ISMF [30] One of the primary goals of the IBEX mission is to investigate the global strength and structure of the TS. A sky map of the parent ion temperatures derived from the ENA energy spectra reveals inner heliosheath temperatures of 10 6 K over most of the sky, which Livadiotis et al. [2011] interpret as indicative of a shock stronger than seen by the in situ Voyager measurements. Temperatures were not derived for the polar regions, however, where the plasma cannot be described by a kappa distribution. Here, the TS may be weaker. Further evidence for the strength and structure of the shock is provided by the slope of the ENA spectra, which is steeper near the nose than toward the tail (Figure 5b), suggesting that the shock is weaker toward the tail [Schwadron et al., 2011]. [31] IBEX data also allow the structure and orientation of the heliosphere to be estimated, indicating that 1) the density of the inner heliosheath decreases and the width increases from the nose toward the tail [Livadiotis et al., 2011]; 2) the distance to the TS in the tail is 145 AU [Schwadron et al., 2011]; and 3) the tail appears to be offset by 44 from the direction of the downwind LISM flow [Schwadron et al., 2011] (Figure 5a). There is no comparable offset in the 7of9

8 magnetic field direction, or possibly small scale magnetic turbulence. Figure 10. IMAP, a follow on mission to IBEX, promises an advance in our capability to image the heliosphere/lism interaction comparable to the advance achieved by WMAP in the high resolution imaging of cosmic background radiation. In addition to measuring a broader range of ENAs, IMAP would be instrumented to make contextual measurements of the solar wind and IMF as well as direct measurements of interstellar material within the heliosphere (e.g., cosmic dust, interstellar neutrals). direction of the tail observed by Doppler shifted stellar Ly a lines, which may suggest that the heliosphere has both neutral and plasma heliotails [Schwadron et al., 2011]. The longitudinal offset of the plasma heliotail is likely caused by the influence of the ISMF; a significant offset toward this direction again supports a heliospheric interaction intermediate between Parker s two limiting cases [Parker, 1961; McComas et al., 2009b]. [32] Prior to the IBEX mission, information about the ISMF was derived from interstellar polarization measurements of a 4 deflection of the interstellar neutral hydrogen flow relative to the interstellar helium flow [Lallement et al., 2005], and the locations of the Voyager shock crossings [Ratkiewicz and Grygorczuk, 2008]. IBEX provides valuable new constraints on the magnitude and orientation of the ISMF as well as clues to the nature of the heliosphere s magnetic environment. Grygorczuk et al. [2011] model the draped ISMF using different field directions and magnitudes to determine the best fit to the observed ribbon. They assume that the ribbon is produced by secondary ENAs from the outer heliosheath and find that the best fit is obtained with a field strength of 3.0 ± 1.0 mg and a direction toward ecliptic (l,b) = (225 ± 5, 35 ± 5 ). Very similar results B LISM 2 3 mgand(l,b) = ( , ) are reported by Heerikhuisen and Pogorelov [2011]. In both cases, the calculated field direction is close to the center of the ribbon arc determined by Funsten et al. [2009b], and the field magnitude is comparable to the values ( 2.5 mg to 3.3 mg) obtained by Schwadron et al. [2009b, 2011] using heliosheath pressures derived from the ENA data. Frisch et al. [2010, p. 1479] compare the ISMF direction determined by IBEX with that from interstellar polarization data and find a marginally significant difference of 33 between the two, which could reflect either large scale distortion of the 4. What s Next? [33] IBEX has proven to be a remarkable mission of firsts and discovery. It has made the first ENA observations of the heliospheric boundary and the first direct measurements of interstellar neutral H and O [Möbius et al., 2009]. IBEX s discovery of the heliospheric ribbon is leading to the development of a new paradigm for the heliosphere/lism interaction as the IBEX science team and others work to solve the mystery of the ribbon s origin. Moreover, IBEX s firsts are not limited to its primary, heliospheric objectives, but also include the 1) first observations of neutralized/reflected solar wind from the lunar regolith [McComas et al., 2009c]; 2) first ENA images of Earth s subsolar magnetosheath [Fuselier et al., 2010]; 3) first ENA observations of the magnetosphere s cusps and their seasonal asymmetry [Petrinec et al., 2011]; and 4) first images of Earth s plasma sheet, showing both its quiescent structure and a possible plasmoid disconnection event in the mid magnetotail [McComas et al., 2011]. [34] IBEX completed its primary mission in the February 2011 and is now in its extended mission, promising much more exciting discovery science to come. As ground breaking as the current IBEX all sky maps are and future additional IBEX observations will be, they are still but a first glimpse of our solar system s outer boundaries. Fundamental questions will remain about the heliosphere/lism interaction, the properties of the LISM, and the charged particles, neutrals, and cosmic dust that populate the dynamic heliospheric frontier. To answer such questions, development of the concept for an Interstellar MApping Probe (IMAP) as a follow on mission to IBEX has already begun. With a factor of enhanced sensitivity, higher angular and energy resolution and an extended energy range, IMAP promises to be to IBEX what WMAP was to COBE (Figure 10). [35] Acknowledgments. This work was funded by NASA s IBEX project. [36] The Editor thanks two anonymous reviewers for their assistance in evaluating this paper. References Baranov, V. B., M. G. Lebedev, and I. G. Malama (1991), The influence of the interface between the heliosphere and the local interstellar medium on the penetration of H atoms to the solar system, Astrophys. J., 375, 347, doi: / Borovikov, S. N., N. V. Pogorelov, G. P. Zank, and I. A. Kryukov (2008), Consequences of the heliopause instability caused by charge exchange, Astrophys. J., 682, 1404, doi: / Chalov, S. V., et al. (2010), Scatter free pickup ions beyond the heliopause as a model for the Interstellar Boundary Explorer ribbon, Astrophys. J., 716, L99, doi: / /716/2/l99. Dayeh, M. A., et al. (2011), Spectral properties of regions and structures in the Interstellar Boundary Explorer (IBEX) sky maps, Astrophys. J., 734, doi: / x/734/1/29. Fahr, H. J., T. Kausch, and H. Scherer (2000), A 5 fluid hydrodynamic approach to model the solar system interstellar medium interaction, Astron. Astrophys., 357, 268. Fahr, H. J., et al. (2011), The inner heliospheric source for kev energetic IBEX ENAs: The anomalous cosmic ray induced component, Astron. Astrophys., 531, A77, doi: / / Florinski, V., G. P. Zank, and N. V. Pogorelov (2005), Heliopause stability in the presence of neutral atoms: Rayleigh Taylor dispersion analysis 8of9

9 and axisymmetric MHD simulations, J. Geophys. Res., 110, A07104, doi: /2004ja Florinski, V., et al. (2010), Stability of a pickup ion ring beam population in the outer heliosheath: Implications for the IBEX ribbon, Astrophys. J., 719, 1097, doi: / x/719/2/1097. Frisch, P. C., et al. (2009), The galactic environment of the Sun: Interstellar material inside and outside of the heliosphere, Space Sci. Rev., 146, 235, doi: /s Frisch, P. C., et al. (2010), Comparisons of the interstellar magnetic field directions obtained from the IBEX ribbon and interstellar polarizations, Astrophys. J., 724, 1473, doi: / x/724/2/1473. Funsten, H. O., et al. (2009a), Interstellar Boundary Explorer High Energy (IBEX Hi) Neutral Atom Imager, Space Sci. Rev., 146, 75, doi: / s y. Funsten, H. O., et al. (2009b), Structures and spectral variations of the outer heliosphere in IBEX energetic neutral atom maps, Science, 326, 964, doi: /science Fuselier, S. A., et al. (2009a), The IBEX Lo sensor, Space Sci. Rev., 146, 117, doi: /s Fuselier, S. A., et al. (2009b), Width and variation of the ENA flux ribbon observed by the Interstellar Boundary Explorer, Science, 326, 962, doi: /science Fuselier, S. A., et al. (2010), Energetic neutral atoms from the Earth s subsolar magnetosphere, Geophys. Res. Lett., 37, L13101, doi: / 2010GL Gamayunov, K., M. Zhang, and H. Rassoul (2010), Pitch angle scattering in the outer heliosheath and formation of the Interstellar Boundary Explorer ribbon, Astrophys. J., 725, 2251, doi: / x/ 725/2/2251. Gruntman, M. A. (1992), Anisotropy of the energetic neutral atom flux in the heliosphere, Planet. Space Sci., 40, 439, doi: / (92) H. Gruntman, M. (1997), Energetic neutral atom imaging of space plasmas, Rev. Sci. Instrum., 68, 3617, doi: / Gruntman, M., E. C. Roelof, D. G. Mitchell, H. J. Fahr, H. O. Funsten, and D. J. McComas (2001), Energetic neutral atom imaging of the heliospheric boundary region, J. Geophys. Res., 106, 15,767, doi: / 2000JA Grygorczuk, J., et al. (2011), IBEX ribbon: What would it tell about the local interstellar magnetic field?, Astrophys. J., 727, L48, doi: / /727/2/L48. Grzedzielski, S., et al. (2010), A possible generation mechanism for the IBEX ribbon from outside the heliosphere, Astrophys. J., 715, L84, doi: / /715/2/l84. Heerikhuisen, J., and N. V. Pogorelov (2011), An estimate of the nearby interstellar magnetic field using neutral atoms, Astrophys. J., 738, 29, doi: / x/738/1/29. Heerikhuisen, J., et al. (2007), The effects of global heliospheric asymmetries on energetic neutral atom sky maps, Astrophys. J., 655, L53, doi: / Heerikhuisen, J., et al. (2010), Pick up ions in the outer heliosphere: A possible mechanism for the Interstellar Boundary Explorer ribbon, Astrophys. J., 708, L126, doi: / /708/2/l126. Izmodenov, V., Y. Malama, and M. S. Ruderman (2005), Solar cycle influence on the interaction of the solar wind with the local interstellar cloud, Astron. Astrophys., 429, 1069, doi: / : Izmodenov, V. V., et al. (2009), Kinetic gasdynamic modeling of the heliospheric interface, Space Sci. Rev., 146, 329, doi: /s Lallement, R., et al. (2005), Deflection of the interstellar neutral hydrogen flow across the heliospheric interface, Science, 307, 1447, doi: / science Livadiotis, G., et al. (2011), First sky map of the inner heliosheath temperature using IBEX spectra, Astrophys. J., 734, doi: / x/ 734/1/1. McComas, D. J., et al. (2004), The Interstellar Boundary Explorer (IBEX), in Physics of the Outer Heliosphere, Third Annual IGPP Conference, AIP CP719, edited by V. Florinski, N. V. Pogorelov, and G. P. Zank, p. 162, Am. Inst. of Phys., Melville, N. Y., doi: / McComas, D. J., R. W. Ebert, H. A. Elliott, B. E. Goldstein, J. T. Gosling, N. A. Schwadron, and R. M. Skoug (2008), Weaker solar wind from the polar coronal holes and the whole Sun, Geophys. Res. Lett., 35, L18103, doi: /2008gl McComas, D. J., et al. (2009a), IBEX Interstellar Boundary Explorer, Space Sci. Rev., 146, 11, doi: /s McComas, D. J., et al. (2009b), Global observations of the interstellar interaction from the Interstellar Boundary Explorer (IBEX), Science, 326, 959, doi: /science McComas, D. J., et al. (2009c), Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon, Geophys. Res. Lett., 36, L12104, doi: /2009gl McComas, D. J., et al. (2010), Evolving outer heliosphere: Large scale stability and time variations observed by the Interstellar Boundary Explorer, J. Geophys. Res., 115, A09113, doi: /2010ja McComas, D. J., et al. (2011), First IBEX observations of the terrestrial plasma sheet and a possible disconnection event, J. Geophys. Res., 116, A02211, doi: /2010ja Möbius, E., et al. (2009), Direct observations of interstellar H, He, and O by the Interstellar Boundary Explorer, Science, 326, 969, doi: / science Parker, E. N. (1961), The stellar wind regions, Astrophys. J., 134, 20, doi: / Petrinec, S. M., et al. (2011), Neutral atom imaging of the magnetospheric cusps, J. Geophys. Res., 116, A07203, doi: /2010ja Ratkiewicz, R., and J. Grygorczuk (2008), Orientation of the local interstellar magnetic field inferred from Voyagers positions, Geophys. Res. Lett., 35, L23105, doi: /2008gl Schwadron, N. A., et al. (2009a), The Interstellar Boundary Explorer science operations center, Space Sci. Rev., 146, 207, doi: /s x. Schwadron, N. A., et al. (2009b), Comparison of Interstellar Boundary Explorer Observations with 3D global heliospheric models, Science, 326, 966, doi: /science Schwadron, N. A., et al. (2011), Separation of the Interstellar Boundary Explorer ribbon from globally distributed energetic neutral atom flux, Astrophys. J., 731, 56, doi: / x/731/1/56. Slavin, J. D. (2009), The origins and physical properties of the complex of local interstellar clouds, Space Sci. Rev., 143, 311, doi: /s Stone, E. C., et al. (2005), Voyager 1 explores the termination shock region and the heliosheath beyond, Science, 309, 2017, doi: / science Stone, E. C., et al. (2008), An asymmetric solar wind termination shock, Nature, 454, 71, doi: /nature Suess, S. T. (2004), The magnetic field in the outer heliosphere, AIP Conf. Proc., 719, 10, doi: / Washimi, H., et al. (2011), Realistic and time varying outer heliospheric modelling, Mon. Not. R. Astron. Soc., doi: /j x, in press. Witte, M. (2004), Kinetic parameters of interstellar neutral helium. Review of results obtained during one solar cycle with the Ulysses/GAS instrument, Astron. Astrophys., 426, , doi: / : Zank, G. P. (1999), Interaction of the solar wind with the local interstellar medium: A theoretical perspective, Space Sci. Rev., 89, 413, doi: /a: Zank, G. P., and H. R. Müller (2003), The dynamical heliosphere, J. Geophys. Res., 108(A6), 1240, doi: /2002ja Zank, G. P., et al. (2009), Physics of the solar wind local interstellar medium interaction: The role of magnetic fields, Space Sci. Rev., 146, 295, doi: /s H. O. Funsten, Los Alamos National Laboratory, Los Alamos, NM 87545, USA. S. A. Fuselier, Lockheed Martin Advanced Technology Center, Palo Alto, CA 94304, USA. W. S. Lewis and D. J. McComas, Southwest Research Institute, San Antonio, TX 78228, USA. (dmccomas@swri.org) E. Möbius, Physics Department, University of New Hampshire, Durham, NH 03824, USA. N. A. Schwadron, Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, NH 03824, USA. 9of9