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Science Summary: The Interaction of the Solar Wind and the Local Interstellar Medium

This chapter summarizes the community’s basic understanding of the physics of the outer heliosphere, focusing on the influence exerted on the solar wind by the interstellar medium (Box 2.1). In particular, it describes several approaches that allow us to infer the properties and structure of the heliospheric boundaries. The term “boundaries” is used interchangeably in this report to refer both to specific boundaries, such as the termination shock or heliopause, and to regions enclosed by the boundaries, such as the inner and outer heliosheath, which would more properly be referred to as the heliospheric boundary regions. The context should make clear the word’s meaning.

In presenting the many models and concepts that have been developed to explain different aspects of interactions between the solar wind and the local interstellar medium (LISM),1 it should be emphasized that the underlying physics is not always well constrained. This is a consequence of only a single spacecraft mission into the outer heliosphere (Voyager) and of spacecraft whose instrumentation is not typically designed to answer questions about the outer heliosphere, with the obvious and very important exception of Ulysses (and possibly ACE). Remote sensing has placed some constraints on theoretical models, such as Lyman-alpha absorption measurements, but these are not always unique (see, for example, Florinski et al., 2003). Furthermore, data interpretation can be strongly model dependent. The only way to properly resolve the fundamental physics of the solar wind-LISM interaction will be through in situ measurements and remote sensing.

It is a trivial observation that the greatest part of the heliosphere—that beyond some 10 AU—is mostly unexplored. Less obvious, as noted above, is that the outer heliosphere beyond 10 AU consists primarily of material that is of interstellar rather than solar origin. Although the large-scale dynamics is still dominated

1  

The LISM is that region of space in the local galactic arm where the Sun is located (Thomas, 1978), the local interstellar cloud is the cloud within it in which the Sun resides, and the heliosphere is the region in space filled with solar wind material (both supersonic and subsonic flow).



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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report 2 Science Summary: The Interaction of the Solar Wind and the Local Interstellar Medium This chapter summarizes the community’s basic understanding of the physics of the outer heliosphere, focusing on the influence exerted on the solar wind by the interstellar medium (Box 2.1). In particular, it describes several approaches that allow us to infer the properties and structure of the heliospheric boundaries. The term “boundaries” is used interchangeably in this report to refer both to specific boundaries, such as the termination shock or heliopause, and to regions enclosed by the boundaries, such as the inner and outer heliosheath, which would more properly be referred to as the heliospheric boundary regions. The context should make clear the word’s meaning. In presenting the many models and concepts that have been developed to explain different aspects of interactions between the solar wind and the local interstellar medium (LISM),1 it should be emphasized that the underlying physics is not always well constrained. This is a consequence of only a single spacecraft mission into the outer heliosphere (Voyager) and of spacecraft whose instrumentation is not typically designed to answer questions about the outer heliosphere, with the obvious and very important exception of Ulysses (and possibly ACE). Remote sensing has placed some constraints on theoretical models, such as Lyman-alpha absorption measurements, but these are not always unique (see, for example, Florinski et al., 2003). Furthermore, data interpretation can be strongly model dependent. The only way to properly resolve the fundamental physics of the solar wind-LISM interaction will be through in situ measurements and remote sensing. It is a trivial observation that the greatest part of the heliosphere—that beyond some 10 AU—is mostly unexplored. Less obvious, as noted above, is that the outer heliosphere beyond 10 AU consists primarily of material that is of interstellar rather than solar origin. Although the large-scale dynamics is still dominated 1   The LISM is that region of space in the local galactic arm where the Sun is located (Thomas, 1978), the local interstellar cloud is the cloud within it in which the Sun resides, and the heliosphere is the region in space filled with solar wind material (both supersonic and subsonic flow).

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report BOX 2.1 THE INTERSTELLAR MEDIUM, THE CRADLE OF THE STARS The interstellar medium is the cradle of the stars and provides the raw material for all bodies in stellar systems, including those of our own. This material has undergone continuous evolution, from the big bang until today. The big bang produced only light nuclei, such as H, He, their isotopes 3He and D, and some 7Li (Schramm, 1998). Stars synthesize the heavier elements (Prantzos, 1998) and high-energy galactic cosmic rays contribute very rare elements, such as Be and B. Consequently, the abundance of elements and isotopes changes over time, and knowledge of it for several points in time will allow us to understand nucleosynthetic evolution. Our current knowledge of the origin of the elements and their isotopes is derived mainly from composition measurements in the solar system. The relative abundance of nearly 300 nuclear species has been derived for the proto-solar nebula, which represents a sample of galactic matter from 4.5 billion years ago. Meteorites also provide some isotopic ratios in stellar grains; these ratios represent very specific information about certain stellar sources, such as supernovae. Finally, spectroscopic data on elemental abundances (rarely on isotopes) are available for a variety of astrophysical objects. Missing is a sample of the present-day galaxy with reliable observations of a number of important elemental and isotopic abundance ratios. In situ measurements of interstellar material inside and just outside the heliosphere, combined with remote absorption spectroscopy, will fill this gap. by solar wind ram pressure, the LISM begins to introduce distinctly new physical processes that have little counterpart in the inner solar wind. The LISM plays a fundamental role in determining the physics of the outer heliosphere. NASA recognized the importance of improving scientific understanding of this region in its 2000 Sun-Earth Connection (SEC) roadmap, which lists “understanding how the sun and galaxy interact” as one of its four top-level quests. Similarly, the 2003 SEC roadmap has the following as one of its three principal objectives: “Understand the changing flow of energy and matter throughout the Sun, heliosphere, and planetary environments.”2 At a practical level, interest in the outer heliosphere has been strongly fostered by the Voyager Interstellar Mission (Voyagers 1 and 2). The Voyager spacecraft are the last remaining operational spacecraft in the very distant heliosphere and the only currently available platforms from which to explore the boundary regions of the heliosphere in situ. Interstellar Probe, a mission shown in the 2003 SEC roadmap as being ready for launch in 2015-2018, would require 15 years of travel to reach 200 AU (see also JPL, 1999). The SEC roadmap objectives recognize, too, that the mutual interaction of large-scale and microscale processes is a fundamental ingredient describing the space physics of the outer heliosphere. In the context of the outer heliosphere, spacecraft such as Ulysses and ACE have provided new and important insights into the coupling between the solar wind and the LISM, revealing the importance of both macro- and microscale processes. Large-scale, sometimes termed “meso-“ or “macroscale,” phenomena contribute to the gross morphology of the heliosphere, in either a time-dependent or a steady-state sense. The solar 2   See Table 1.1 in “The Sun-Earth Connection 2003 Roadmap: Understand the Sun, Heliosphere and Planetary Environments as a Single Connected System,” available at http://sec.gsfc.nasa.gov/sec_roadmap.htm.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report wind, the interplanetary magnetic field (IMF), interplanetary shocks, streams and stream-interaction regions, and heliospheric boundary structures are all examples of macro-/mesoscale structures, whereas turbulence, waves, particle scattering, magnetic field line wandering, dissipation, and so forth are microscale processes. Nonetheless, the mutual feedback between these disparate scales serves to determine almost all aspects of heliospheric physics. In many respects, the complex coupling between the solar wind and the interstellar medium is the quintessential example of multiscale, regional, and disparate populations (solar wind plasma, pickup ions, anomalous and galactic cosmic rays, neutral atoms) coupling across complex boundaries. Thus, the study of the solar wind interaction with the LISM is of fundamental interest and importance to space physics. There can be little doubt that sending a spacecraft beyond the heliopause to begin the exploration of our local galactic neighborhood will yield enormous scientific advances for a wide array of astrophysical questions, such as the state and evolution of matter in our galaxy, the interaction of radiation and interstellar medium, the interaction of star systems with their neighborhood, as well as the interaction of shock waves with their environment and their role in particle acceleration. The following sections begin with a basic overview of the structure of the heliosphere and a discussion of the fundamental role of neutral interstellar gas and then focus on several processes that are particularly promising for determining large-scale structure by probing the physical character of the boundary regions remotely. GLOBAL STRUCTURE Perhaps the most important lesson learned from the Voyager and Ulysses missions, complemented by theory and modeling efforts, is that the physics of the outer heliosphere is influenced profoundly by the LISM since we are embedded in the local interstellar cloud (LIC). This is because neutral hydrogen atoms flow into the heliosphere and are the dominant component, by mass, from 10 AU outward (i.e., beyond Saturn). As described below, the ionization of interstellar neutrals in the supersonic solar wind can even affect the size and structure of the global heliosphere. The detection of anomalous cosmic ray fluxes by Garcia-Muñoz et al. (1973), Hovestadt et al. (1973), and McDonald et al. (1974) led to the construction of a remarkable chain linking interstellar neutral atoms, interstellar pickup ions (their prediction and eventual detection ~10 (He) and ~20 (H) years later; Fisk et al., 1974; Mobius et al., 1985; and Gloeckler et al., 1993), anomalous cosmic rays (ACRs) experiencing diffusive shock acceleration at a postulated termination shock (Pesses et al., 1981; Jokipii, 1986), and energetic neutral atoms created by charge exchange between ACRs and interstellar neutrals (Hsieh et al., 1992a,b). Figure 2.1 is a schematic diagram of the chain. That the various elements are linked is now well established; however, the precise details underlying the coupling are still not properly understood, and models remain incomplete. The coupling of the different elements within the chain reveals the interplay of large-scale heliospheric structure and detailed microphysics. Major advances in understanding the physics of pickup ions resulted from Ulysses (and, subsequently, ACE) observations, demonstrating the importance of a well-instrumented mission. Ulysses, and to a lesser extent ACE, are the most vital missions for revealing the direct coupling of the interstellar medium to the solar wind plasma. Unfortunately, the observations returned by Ulysses have far outstripped theoretical understanding, leaving numerous fundamental puzzles to be resolved. The importance of similarly instrumented (but improved) missions within some 4 AU cannot be overstated, since Ulysses measurements address the absolutely fundamental question of LISM-solar wind coupling. As summarized in Figure 2.2 (also see the review by Zank, 1999a), if the LISM flows supersonically with respect to the motion of the Sun, a bow shock diverts the LISM flow about the heliosphere while a termination shock decelerates the

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 2.1 Overview of the heliosphere, with termination shock, heliopause, bow shock, and outer and inner heliosheath (HS). Some sample plasma (H+), pickup ion (PU ion), and solar wind plasma (VHS) trajectories are shown, as well as trajectories of neutral hydrogen (H) coming from the interstellar medium (HISM) and experiencing charge exchange (_), and galactic cosmic rays (GCRs). The solar and interstellar magnetic fields (BHS and BISM) are sketched. supersonic solar wind. The heliopause is a contact—or, in a magnetohydrodynamic (MHD) description, a tangential discontinuity (see, e.g., Washimi and Tanaka, 1996; Linde et al., 1998; and Pogorelov et al., 2004)—that separates the LISM and solar wind plasmas. Whereas solar wind and interstellar plasmas respond to electromagnetic fields (being decelerated at the heliospheric boundary shocks, for example), interstellar neutrals flow relatively unimpeded. Plasma

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 2.2 Schematic of the heliosphere. The global heliosphere is created by the supersonic solar wind diverting the interstellar plasma flow around the Sun. Interstellar ions and neutral atoms flow at 26 km/s relative to the Sun. The solar wind, flowing outward at 400 to 800 km/s, makes a transition to subsonic flow at the termination shock. Beyond this, the solar wind is turned toward the heliotail, carrying with it the spiraling interplanetary magnetic field. The heliopause separates solar material and magnetic fields from interstellar material and magnetic fields. Interstellar neutral atoms and higher-energy galactic cosmic rays can penetrate the heliosphere, but interstellar ions are diverted around it. Beyond the heliopause there may also be a bow shock formed in the interstellar medium. SOURCE: Jet Propulsion Laboratory (1999a), courtesy of Steven T. Suess. and neutral atoms are coupled weakly but directly through various kinetic processes, these being charge exchange, photoionization, recombination, and electron-impact ionization, besides direct collisions between the neutral atoms and charged particles. The weak coupling of neutral interstellar hydrogen and plasma affects both interstellar hydrogen and plasma distributions in important ways. In the partially

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report BOX 2.2 THE NATURE OF THE HELIOSPHERIC BOUNDARY REGIONS The Knudsen number, Kn, is the ratio λ/L, where λ is the mean free path of neutral atoms and L is a characteristic macroscopic length scale, such as the size of the solar heliosphere, ~100 AU. It is a measure of the neutral distribution relaxation distance and is >>1 inside the heliosphere and ~1 in the very local interstellar medium. Thus, within the heliosphere neutral and plasma distributions cannot equilibrate and may possess quite distinct bulk flow speeds and temperatures. Charge exchange between the coupled, nonequilibrated neutral and charged particle populations can therefore introduce distinct new populations of neutral atoms and plasma whose characteristics reflect their parent population. The subsequent interaction and assimilation of the newly created plasma and neutral populations into the existing plasma and neutral distributions may then lead to the substantial modification of the overall partially ionized plasma system. Thus, the total neutral distribution cannot relax to a single Maxwellian distribution, and either a multicomponent transport (Zank et al., 1996a) or a kinetic description for the neutral populations (Baranov and Malama, 1993; Izmodenov et al., 1999; Müller et al., 2000) is used. Furthermore, since the time for a neutral atom to enter the heliosphere and reach 1 AU is ~15 to 20 years, the local neutral atom distribution has experienced a variable charge exchange and photoionization rate, as well as a supersonic solar wind whose extent (in both latitude and longitude), velocity, and density are variable (McComas et al., 2000; Pauls and Zank, 1996, 1997). Neutral atom characteristics can therefore depend on the solar cycle, with the overall distribution being a mixture of atoms created in temporally different solar wind environments, since they cannot be lost to the system on time scales shorter than the solar cycle. ionized LISM, the charge exchange mean free path for neutral H atoms is ~50 to 100 AU (Box 2.2; Knudsen number, Kn << 1), implying complete equilibration between neutral H atoms and LISM plasma some distance upstream of the bow shock (i.e., the charge exchange merely relabels H atoms and protons). The bow shock diverts, decelerates, and heats the LISM plasma, but neutral H is unaffected by the boundary. Between the bow shock and the heliopause (see Figure 2.1, outer heliosheath), the charge exchange mean free path is ~50 AU (Kn ~ 1), and so a large proportion of the LISM neutral H atoms experience charge exchange with slightly heated, diverted and slowed LISM protons. This implies a related slowing, heating, and diverting of the neutral H atom distribution, and thus the formation of the hydrogen wall (Figure 2.3). This process filters neutral H as it enters the heliosphere. Although predicted theoretically using two distinct approaches (Baranov and Malama, 1993; Pauls et al., 1995; Zank et al., 1996a), the hydrogen wall was discovered serendipitously by Linsky and Wood (1996) and Gayley et al. (1997), making it the first of the heliospheric boundary structures to be detected directly. Indirect evidence for the filtration of interstellar neutral hydrogen was provided initially by the Voyager and Pioneer observations of a radial gradient in the neutral gas (Hall et al., 1993) and an apparent deceleration of the interstellar gas flow in the heliosphere (Lallement et al., 1993). Within the heliosphere (i.e., inside the heliopause), the charge exchange mean free path increases dramatically (~1,000 AU, Kn >>1) and a simple single-fluid hydrodynamic treatment of the neutral H atoms is inadequate. Use of either a multifluid (Zank et al., 1996a) or a kinetic description (Baranov and Malama, 1993, 1996; Müller et al., 2000) of the neutral H reveals that neutrals created via charge exchange in the hot inner heliosheath (see Figure 2.1) stream into the LISM (Gruntman, 1982; Baranov and Malama, 1993; Zank et al., 1996a). These neutral atoms (“component 2”) experience secondary charge

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 2.3 The two-dimensional steady-state, two-shock heliosphere showing (top plot) the temperature distribution of the solar wind and interstellar plasma and (bottom plot) the density distribution of neutral hydrogen. The plasma boundaries, termination shock, heliopause, and bow shock are labeled, and the wall of neutral hydrogen is also identified. The solid lines of the top plot show the streamlines of the plasma. The plasma temperature is plotted logarithmically and the neutral density linearly. The distances along the x and y axes are measured in astronomical units (AU). exchange with the LISM, thus introducing very hot (~106 K compared with 6,500 K) new protons into the LISM. The component 2 neutrals, although tenuous, are energetically important and transport heat anomalously from the subsonic heated solar wind plasma to the cooler interstellar medium plasma. The heating of the LISM in turn modifies the incoming neutral H distribution via charge exchange. This complex, nonlinear feedback of plasma and neutrals yields complicated neutral distributions that are different from region to region; some of these distributions are illustrated in the center row of Figure 2.4. When neutral H atoms drifting at ~20 km/s are ionized in the supersonic solar wind (400-800 km/s), they are picked up almost instantaneously by the motional electric field, forming a ring-beam distribution. The turbulence excited by the unstable ring-beam was expected to “isotropize” the pickup ions rapidly, producing a distinct, stable suprathermal population of ions in the solar wind plasma. That neither of the latter expectations has been met fully poses challenges to existing theoretical plasma models (Zank and Cairns, 2000), and the relative scarcity of observations has so far not allowed for further refinement of

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 2.4 An example of a kinetic simulation described by Müller et al. (2000) for a two-shock model. One-dimensional profiles of plasma density nP and plasma temperature TP are shown as dashed lines over radial distance R from the Sun, and neutral density nH and (averaged) temperature TH are shown as solid lines. The profiles are obtained in the upstream direction, antiparallel to the flow of the local interstellar medium (LISM). The middle row depicts normalized two-dimensional neutral velocity distribution functions (logarithmic density scale) at various locations on that axis, with prominent HLISM and H1 26 km/s neutrals, and evidence of 100 to 300 km/s component 2 and 400 km/s component 3 neutrals. BS, bowshock; HP, heliopause; TS, termination shock. theory. The pickup process decelerates the solar wind flow (Holzer, 1972; Wang et al., 2000) and adds considerably to the level of turbulence in the outer heliosphere (Fisk and Goldstein, 1978; Lee and Ip, 1987; Zank et al., 1996c). Pickup ions can, by virtue of their dominant pressure contribution, mediate all MHD processes in the solar wind, including small-scale structures such as shock waves and pressure-balanced structures (Burlaga et al., 1994; Whang and Burlaga, 1993; Liewer et al., 1993; Zank et al., 1996b; Zank and Pauls, 1997; Whang et al., 1999; Rice and Zank, 1999; Lu et al., 1999). Pickup ions act to weaken interplanetary shocks, even when magnetic fields (Rice and Zank, 1999) and cosmic rays (Rice and Zank, 2000) are included. This, in turn, must affect the nature of the radio emissions observed by the Voyager spacecraft (Kurth et al., 1984, 1987; Gurnett et al., 1993) and interpreted (Gurnett et al., 1993; Gurnett and Kurth, 1995; McNutt et al., 1995; Cairns and Zank, 1999, 2002) as the generation of radiation by global merged interaction-region-associated shocks interacting with the heliopause and outer heliosheath—especially in terms of the timing of the turn-on and the shock strength and speed in the heliospheric boundary regions, discussed further below.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report NEUTRAL INTERSTELLAR ATOMS AND PICKUP IONS Perhaps the most crucial factor for determining global heliospheric structure and the properties of the boundary regions is also the most unconstrained. The structure and extent of the heliosphere depend critically on the state of the LISM (electron and neutral hydrogen density, temperature, velocity, magnetic field strength and orientation, ionization state, and so on). As is discussed below, only two of these, temperature and velocity, are now known within reasonable error bounds and the rest are very uncertain. Evidence for the presence of interstellar hydrogen and helium in the heliosphere was provided initially by the measurement of resonantly scattered solar ultraviolet light (Bertaux and Blamont, 1971; Thomas and Krassa, 1971). From subsequent measurements solar physicists have tried to infer the density, temperature, and (relative) velocities of hydrogen and helium (Adams and Frisch, 1977; Bertaux et al., 1985; Ajello et al., 1987). Analyses have concentrated on the strongest resonance lines, H 1216 Å (Lyman-alpha) and He 584 Å. As solar photons travel outward they scatter resonantly from heliospheric H and He atoms. These scattered photons are detected by the Voyager, Pioneer 10, and the SOHO Solar Wind Anisotropies (SWAN) ultraviolet instruments as well as by the Hubble Space Telescope, and intensities vary with heliocentric distance, pointing direction, and the illuminating solar flux (Hall, 1992). H 1216-Å photons are expected to travel on the order of 10 AU before scattering, and He 584-Å photons scatter after traversing on the order of 100 AU. Most H 1216-Å measurements have been obtained by spacecraft within 5 AU of the Sun, where the lines are dominated by scattering from within 10 to 20 AU. The Voyager and Pioneer 10 spacecraft observed resonance lines beyond 5 AU. Finally, it was suggested by M. Gruntman, at this workshop, that the heliopause might possibly be mapped by measuring backscattered solar light at wavelengths 30.4 nm and 83.4 nm, since doing so directly probes interstellar plasma at the heliopause and beyond. This technique is in its infancy, however. With the launch of Ulysses and ACE, several novel instruments now provide direct measurements for the physical parameters of the interstellar gas in the heliosphere: Close to the Sun (within ~5 AU), the local distribution of interstellar neutral helium has been measured using an impact-ionization method by the Ulysses Interstellar Neutral Gas Experiment (GAS) (Witte et al., 1992). These measurements provide the velocity and direction of the flow of interstellar He as well as its temperature and density. The densities and velocity distribution functions of the pickup ions H+, 4He+, 3He+, N+, O+, and Ne+ have been determined by the Ulysses Solar Wind Ionic Composition Spectrometer (SWICS) experiment (Gloeckler et al., 1993; Geiss et al., 1994; Gloeckler and Geiss, 1996). If a model is assumed for the transport of interstellar neutrals within the heliosphere—for example, the hot model (Thomas, 1978; Rucinski and Bzowski, 1995)—the pickup ion data can then be used to infer the neutral parameters at the heliospheric termination shock or possibly even beyond, depending on the importance of filtration. Similarly, the ACE SWICS and Solar Wind Ion Mass Spectrometer (SWIMS) instruments also measure interstellar Ne, He, and O at 1 AU (Gloeckler, 1996; Gloeckler and Geiss, 2004). Table 2.1 shows the hydrogen and helium parameters and their method of determination.3 As can be seen, there is disagreement about the inferred number density of neutral hydrogen in the LISM. This is a result of H filtration at the heliospheric boundaries, which makes it very difficult to relate heliospheric 3   The latest determination of the direction of the He flow, based on a combination of neutral gas, pickup ion, and ultraviolet measurements, is 74.7 ± −0.6 degrees ecliptic longitude; −5.3 ± 0.3 degrees ecliptic latitude (Möbius et al., 2004).

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report TABLE 2.1 Hydrogen and Helium Parameters Derived from Various Experiments Method Velocity (km/s) Density (10−2 cm−3) Temperature (K) Observation Interstellar Helium Pickup He+ a 23-30 0.9-1.2 4,800-7,200 AMPTE (Active Magnetospheric Particle Tracer Explorer) Pickup He+ b   1.5 ± 0.15   Ulysses/SWICS (Solar Wind Ionic Composition Spectrometer) Pickup He++ b   1.5   Ulysses/SWICS Directc 26.3 ± 0.4 1.5 ± 0.3 6,300 ± 340 Ulysses/GAS (Energetic Particle and Interstellar Neutral Gas Experiment) UVd 19-24 0.5-1.4 8,000 Prognoz, V1, V2 Heliospheric Hydrogen Pickup H+ b   11.5-5   Ulysses/SWICS UVe 18-20   8,000 HST (Hubble Space Telescope) UVf 19-21   8,000 Prognoz UVg     <20,000 Copernicus UVh 18-0   30,000 HST (downstream) NOTE: Primary references only. See, for example, Lallement et al. (1996) for a review. aMöbius (1996). bGloeckler (1996), Gloeckler and Geiss (2004). cWitte et al. (1996), Witte et al. (2004). dChassefière et al. (1988). eLallement (1996). fBertaux et al. (1985). gAdams and Frisch (1977). hClarke et al. (1995). pickup ion data directly to the LISM H. Secondly, there is some disagreement about the temperature of neutral hydrogen in the interplanetary medium. However, the interstellar helium temperature now appears to be well constrained and, by implication, the temperature of the interstellar H as well. The results presented in Table 2.1 provide a baseline against which to begin to constrain global heliospheric models, but a crucial goal of future missions will be the determination of the state of the LISM. HELIOSPHERIC LYMAN-ALPHA ABSORPTION TOWARD NEARBY STARS Rather serendipitously, the first of the heliospheric boundaries to be discovered, the hydrogen wall, was found by measuring the absorption of Lyman-alpha light toward our nearest stellar neighbor α-Centauri (Linsky and Wood, 1996; Gayley et al., 1997). This technique has since evolved into a very promising approach both for investigating the structure of our heliosphere and for discovering stellar winds and hydrogen walls associated with neighboring solarlike stars (Box 2.3). The effectiveness of the Lyman-alpha absorption technique for investigating the outer heliosheath resides in its ability to probe local density and temperature enhancements. The Lyman-alpha absorption

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report BOX 2.3 ASTROSPHERES The heliosphere is not unique, but it is the one astrosphere that can be studied in detail with in situ observations. Astrospheres around other stars should certainly exist. Because many stars with a magnetic field and a stellar wind are surrounded by a partially ionized interstellar medium like that surrounding our heliosphere, much of the physics that we are learning locally about neutral atoms, pickup ions, anomolous cosmic rays, and so forth will carry over to the astrospheres of other stars. The deceleration and accumulation of neutral hydrogen on the upwind side of the heliosphere and other astrospheres have already led to the identification of hydrogen walls in both our own solar system and at several nearby star systems, using typical absorption features in the Lyman-alpha profile (Linsky, 1996; Wood et al., 2000a,b). In situ studies of the outer reaches of our home system will sharpen these tools, allowing us to understand the surroundings of many star systems and thus providing a direct link to astrophysics. seen in high-resolution stellar ultraviolet spectra obtained by the Hubble Space Telescope can be explained only in part by the ubiquitous interstellar absorption observed toward nearby stars (see, for example, Linsky and Wood, 1996; Gayley et al., 1997; Wood et al., 2000b). Since the assumed stellar Lyman-alpha profile as well as the intervening interstellar absorption is rather well constrained through analysis of the corresponding absorption line of deuterium, one requires the existence of a further hydrogen absorption component. There is strong evidence for excess absorption in various spatial directions, including toward 36 Oph, α Cen, and Sirius, for example (Wood et al., 2000a). This excess absorption has been linked convincingly to heliospheric neutrals, since heliospheric models predict that neutral hydrogen in the heliosphere should be hot, with temperatures on the order of 20,000 to 40,000 K. This high-temperature gas produces neutral H Lyman-alpha absorption broad enough to be separable from the interstellar absorption. In upwind directions, such as that toward α Cen (Gayley et al., 1997) and 36 Oph (Wood et al., 2000a), the heliospheric H I column density is dominated by compressed, heated, and decelerated material in the hydrogen wall. For downwind lines of sight (e.g., toward ε Eri), the H I density is much lower than in the hydrogen wall, but the sightline through the heated heliospheric H is longer, potentially allowing heliospheric Lyman-alpha absorption to be observed also in downwind directions. There have already been successful attempts to match observations with heliospheric models through parameter studies that vary key LISM parameters (Gayley et al., 1997; Wood et al., 2000b) (see Figure 2.5), which underscores the value of this approach for constraining LISM parameters and the global heliospheric structure. While successful in predicting observed Lyman-alpha profiles, the models are not necessarily constrained uniquely by the observations (Florinski et al., 2003). PARTICLE ACCELERATION IN THE HELIOSPHERE AND AT THE TERMINATION SHOCK The termination shock is likely to provide our first opportunity to study in situ the acceleration of anomalous cosmic rays, providing us with a tangible glimpse into how galactic cosmic rays are thought to be accelerated by shock waves associated with supernova remnants. The development of diffusive shock acceleration theory as an explanation for the universal form of the cosmic ray spectrum over many decades in energy space has made it one of the most important and widely used results in astrophysics today. Investigating the origin of anomalous cosmic rays within and at the heliospheric boundaries will therefore

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 2.5 Lyman-alpha profiles on the red side for six sightlines toward neighboring stars. The solid lines show observations made using the Space Telescope Imaging Spectrograph instrument (see http://www.stsci.edu/instruments/stis/) and the dashed lines are theoretical predictions from a self-consistent model of the global heliosphere. SOURCE: Wood et al. (2000b). embed much of the theory firmly in observations, allowing the termination shock to be viewed as an example of an astrophysical shock (see Box 2.2). However, as is discussed below, the termination shock is also distinguished by the presence of pickup ions, by its comparatively weak strength (due to pickup ion momentum loading), and by its largely quasi-perpendicular character. The origin of anomalous cosmic rays is intimately related to the interaction of the LISM with the solar wind. It is generally accepted that ACRs are formed when a fraction of the interstellar pickup ion population4 is injected into the diffusive shock acceleration process at the solar wind termination shock (Fisk et 4   There is a possibility that, besides the interstellar source, there is another source of ACRs in the outer heliosphere. This “‘outer source’ is thought to consist of atoms that are sputtered from small grains of material from the Kuiper Belt. These atoms are subsequently ionized, picked up by the solar wind, and transported to the termination shock, where they are accelerated by the same processes that act on the interstellar pickup ions.” See Schwadron et al. (2002).

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report al., 1974; Pesses et al., 1981). However, without direct observations of the termination shock structure and the injection and acceleration process, sufficient constraints are not available to form a detailed understanding of this process, and competing theories have been developed. Preliminary observations should be returned by Voyager 2, but these will be limited by the absence of a working plasma instrument and pickup ion instrumentation. Observations of energized pickup ions in the inner heliosphere have presented challenges to theory. Observations of co-rotating interaction regions (CIRs), for example, suggest that pickup ions created close to the Sun (the inner source pickup ions) are not effectively accelerated at 1 AU. In CIRs, energetic heavy ions exhibit a charge state compatible with that of the solar wind with only a small contribution of Ne+, while He+—clearly of interstellar origin—is effectively accelerated (Möbius et al., 2002). However, Gloeckler (1999) has found strong suprathermal tail distributions in the solar wind and pickup ion distributions that appear to strengthen with distance from the Sun. This observation suggests that a preacceleration mechanism that becomes increasingly effective with heliocentric distance may be at work, thereby enabling particles to be more easily accelerated at the termination shock. To study such behavior quantitatively and to draw inferences about acceleration processes in the outer heliosphere, pickup ion, suprathermal, and energetic particle populations need to be observed at 1 to 5 AU with elemental and ionic charge resolution and a collection power comparable to those of the energetic particle instruments on ACE. By following the acceleration of particles in CIRs and coronal mass ejections at increasing distances from the Sun, it will be possible to delineate the injection and preacceleration processes necessary to understand particle acceleration at the termination shock. In combination with quantitative modeling and simulations, this will allow reasonable extrapolation to the outer heliosphere. These objectives can be achieved with state-of-the-art instrumentation similar to that flown on ACE,5 but on a 1 to 5 AU orbit, and will allow us to achieve the first quantitative understanding for the origin of cosmic rays throughout the universe. GALACTIC COSMIC RAYS: ENTRY INTO THE HELIOSPHERE Like interstellar neutrals, galactic cosmic rays allow us to probe the interstellar medium, with the distinction being that their origin is not necessarily local. However, unlike interstellar neutrals, galactic cosmic rays, being charged, respond to the (electro)magnetic structure of the heliosphere-LISM interaction region. Observations (McDonald et al., 2000) and models are beginning to use galactic cosmic rays flowing into the heliosphere to infer the structure of the boundary regions. Such studies can be accomplished only by spacecraft placed far out in the heliosphere since diffusion effectively washes out any signature or imprint of the heliospheric boundaries on the cosmic ray flux observed within 40 to 50 AU (Florinski et al., 2003). The global heliospheric transport of galactic cosmic rays (GCRs) has tended to focus on the region inside the termination shock. However, it has become increasingly apparent that we need to address GCR interaction with the complex three-dimensional structure of the heliosphere (Jokipii, 1989; Jokipii et al., 1993; McDonald et al., 2001; Florinski et al., 2003). Most importantly, the heliospheric interface is very inhomogeneous, containing regions with vastly different flow patterns and magnetic fields, resulting in very different patterns of cosmic ray propagation. As GCRs approach the heliopause they encounter the modulation or magnetic wall (Figure 2.6), a region with a strongly amplified magnetic field. At least half of 5   A description of instruments on ACE is available at http://www.srl.caltech.edu/ACE/.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 2.6 Schematic showing the overall structure of the heliosphere and its expected effect on the modulation of galactic cosmic rays. the particles with energies below 1 GeV are filtered by the magnetic wall. The second obstacle is the inner heliosheath itself, a highly asymmetric region around the termination shock. As the heliosphere extends in the tailward direction, the solar wind flow is decelerated by the process of charge exchange with neutral interstellar atoms; that is, charge exchange in the heliotail decelerates and cools the shocked plasma flow, creating a convergent flow structure. GCRs propagating with the convergent flow experience re-acceleration through large-scale compression (Figure 2.7). The final region that GCRs must traverse before reaching Earth is the supersonic solar wind upstream of the termination shock. During solar minima, transport in this region is strongly dominated by polar diffusion and drift, effectively erasing the asymmetry present in the GCR distribution in the heliosheath. This necessitates in situ detection methods, preferably outside the termination shock region. Although the picture described so far is already quite complex, it is still not complete. In addition to the above regions, the outer heliosheath may play an important role in GCR modulation if the heliopause is unstable to a charge exchange instability (Zank et al., 1996a; Liewer et al., 1996; Florinski et al., 2004). The evolution of the turbulence in the solar wind and LISM has yet to be modeled. By enabling measuring of the level of modulation that occurs in the inner heliosheath compared with that in the supersonic wind, GCRs may provide a mechanism for probing the large-scale magnetic structure of the heliospheric boundary region. This will require that the observations be made in the very distant heliosphere. It is conceivable, too, that the LISM magnetic field can be probed indirectly via cosmic rays, again provided that the spacecraft is sufficiently deep within the boundary region.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 2.7 Galactic cosmic ray spectra based on a self-consistent heliospheric model that includes H atoms explicitly. The lines identify the theoretical cosmic ray differential flux at various heliospheric locations. LISM (solid); 1.1 rs 0° (dashed), nose region; 1.1 rs 90° (dash-double-dotted), polar region; 800 AU 180° (dotted), heliotail; 10 AU 0° (green). An example of the spectrum at 10 AU when no H atoms (red) are included in the self-consistent model. The effect of the modulation cavity reduction due to neutrals is not significant (~5 percent) at small heliocentric distances. SOURCE: Florinski et al. (2003). RADIO EMISSIONS FROM THE OUTER HELIOSPHERE Another avenue for investigating the heliospheric boundaries remotely is through radio emissions of the kind observed by Voyager. Approximately every 11 years (solar cycle), the Voyager spacecraft detect bursts of radio waves at frequencies of 2 to 3 kHz in the outer regions of our solar system (Kurth et al., 1984, 1987; McNutt, 1988, 1989; Gurnett et al., 1993). The radiation is from the strongest radio source in our solar system but does not come from the Sun or planets (Gurnett and Kurth, 1994; Cairns, 1995). Instead it appears to be generated when shock waves caused by solar activity reach the vicinity of the heliopause (Gurnett et al., 1993; Kurth and Gurnett, 1995; Gurnett et al., 2003; McNutt et al., 1995; Cairns and Zank, 1999). This radiation provides an opportunity to probe the outer boundaries and plasma characteristics of the solar system remotely, including the heliopause region where the solar wind plasma and LISM plasma interact (as distinct from the interstellar neutral atoms and the solar wind plasma). Gurnett et al. (1993) and Gurnett and Kurth (1995) argue persuasively that the radiation is generated when certain global merged interaction regions (GMIRs) and associated shocks reach the vicinity of the heliopause and move into the outer heliosheath. The main arguments are based on the observation of GMIRs and shock waves for each major outburst of radiation, the existence of almost identical time lags 415 ± 5 days between when the GMIRs originate at the Sun and when the radiation starts, very similar GMIR propagation speeds, and estimates based on these speeds and time lags that the source is at distances R ~ 115 to 180 AU, comparable to those predicted for the heliopause. Another argument is that the emission frequency is approximately equal to the electron plasma frequency, fp = 8.98(ne/m−3)1/2 Hz, predicted for the outer heliosheath and the LISM, based on the number densities of pickup ions observed in the inner heliosphere and associated modeling (Gloeckler et al., 1993) and the average plasma number density inferred from the column densities to nearby stars (e.g., Zank, 1999a). The emission frequencies are larger by factors of ≥ 8 and ≥ 4 than the values of fp predicted for the solar wind near the termination shock

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report and in the inner heliosheath, respectively. It is worth emphasizing that the radiation is only a few decibels above the noise levels of the Voyager radio receivers, despite being the most powerful known radio source in our solar system (Gurnett et al., 1993). This means that other weaker radio events or components of the observed emissions may exist and be observable closer to the source, as suggested by Cairns and Zank (1999, 2002). ENERGETIC NEUTRAL ATOMS IN THE HELIOSPHERE Like energetic neutral atoms used to image Earth’s magnetosphere (Burch et al., 2001), energetic neutral atoms (ENAs) born in the inner heliosheath can be used to image the heliospheric boundaries and to infer plasma properties. ENA imaging can be done within a few astronomical units of the Sun as well as by outer heliospheric missions. Neutral hydrogen detected close to the Sun (within ~5 AU) with an energy above about 10 eV is unlikely to be of interstellar origin. Instead, the spectrum of energetic neutral H in the range from 10 eV to 1 keV is dominated by ENAs of heliospheric origin (Hsieh, 1992a; Gruntman, 1992), born through charge exchange with the hot plasma (106 K) of the inner heliosheath (between the termination shock and the heliopause)—that is, component 2 neutrals. At those temperatures, the ions have large thermal velocities in mainly random directions. During charge exchange, the initial ion velocity is preserved, but the now energetic neutral decouples from the plasma and is no longer affected by magnetic fields. The sunward component of such a heliospheric ENA distribution can reach spacecraft detectors in the vicinity of the Sun traveling in almost direct trajectories from the site of charge exchange. The distribution is depleted only by ionization events on this path and can therefore serve to probe the ion distribution in the heliosheath. The energy range above 1 keV is dominated by ENAs that are born through charge exchange of anomalous cosmic rays with LISM neutrals (Hsieh et al., 1992b). Efforts to use <1 keV ENAs as a tool for mapping the global structure of the heliosphere and the associated ion distributions have begun (Gruntman, 1992, 1997; Gruntman et al., 2001). Analysis of the dependence of ENA fluxes on observation direction (e.g., by SOHO; Hilchenbach et al., 1998) will provide a measure of heliospheric asymmetry. Asymmetry is suggested by numerical studies that incorporate a non-parallel interstellar magnetic field (Ratkiewicz et al., 1998; Linde et al., 1998; Pogorelov et al., 2004), or heliolatitude-dependent solar wind parameters (Pauls and Zank, 1996, 1997), or both (McNutt et al., 1998, 1999a,b). The asymmetry that can be expected from the inclusion of both the interplanetary and interstellar magnetic fields is illustrated in Figure 2.8, where a very complex topology for the LISM streamlines and the magnetic field lines is shown. Considerable interest has been expressed in using ENA imaging, which has been successfully employed on the IMAGE mission, to image the termination shock and regions beyond. High-sensitivity ENA observations will help constrain global models of the heliosphere and will advance our theoretical understanding of the nature of the termination shock and the pickup ion population in the heliosheath. At least three spacecraft missions employing ENA imaging have been proposed: two Interstellar Pathfinder missions (e.g., McComas et al., 2003) and the Interstellar Boundary Explorer (IBEX) (McComas et al., 2004), which is currently in Phase A study. Both the imaging and the designs demonstrating the required instrumentation for such interstellar missions are well documented in the refereed literature. The application of this technique to imaging the termination shock will greatly benefit from determining the degree of interference from background ENAs inside the heliosphere. This ENA background is produced from strong, time-variable suprathermal ion tails that are observed between 1 and 5 AU by ACE and Ulysses. Both current theoretical and experimental studies can be used to characterize this background population.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 2.8 Projection of a three-dimensional magnetohydrodynamic-only simulation (i.e., excluding interstellar H atoms) in the presence of an interplanetary and an interstellar magnetic field. The interstellar magnetic field is oriented 20° from the interstellar flow direction and the interstellar magnetic field strength is assumed to be 1.6 μG. Density isolines are plotted in the meridional plane, together with three-dimensional steamlines starting in this plane. SOURCE: Pogorelov et al. (2004).