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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report 3 The Next Logical Steps A mission to the edge of the heliosphere will take at least two decades (if we count the time for a probe to reach that distance). However, progress can be made in exploring the outer heliosphere using current state-of-the-art technology and relatively small space missions. Such missions will both complement the Voyager and Ulysses discoveries and serve as valuable and necessary precursors to Interstellar Probe. This chapter summarizes possible experimental directions that can be undertaken relatively inexpensively. First, it notes the crucial importance of the pervasive interstellar wind of neutral atoms that blows through the solar system and carries gas to within 3 AU of Earth (see, e.g., Möbius et al., 2001). Securing rides on missions that are focused on and optimized for other objectives has enabled great advances in determining the physical parameters and composition of interstellar gas through ultraviolet glow (see, e.g., Bertaux and Blamont, 1971; Lallement, 1996), pickup ions (Möbius et al., 1985; Gloeckler and Geiss, 1998), and neutral He atoms (Witte et al., 1996). Dedicated efforts with instruments optimized for interstellar gas studies in the inner heliosphere will substantially advance knowledge of the local interstellar medium (LISM)-solar wind interaction. As discussed above, suprathermal and energetic ion populations generated at the heliospheric termination shock produce energetic neutral atoms (ENAs) through charge exchange with the interstellar neutral gas and can be used to provide a full-sky image of this region (see, e.g., Gruntman, 1997; Gruntman et al., 2001). Likewise, remote sensing in the extreme ultraviolet (EUV) of singly charged ions should provide extremely valuable information through a full-sky image of the heliopause (Gruntman and Fahr, 2000; Gruntman, 2001a,b), providing that the instrumentation can achieve the necessary sensitivities and spectral resolution and that the inherent background from heliospheric ENAs is small. Such studies are extremely timely, as the Voyagers’ approach to the termination shock and heliopause will provide essential constraints to the imaging techniques, such as limits on intensities and spectral shapes of ENAs and the distance to the boundary regions. In situ studies within 1 to 4 AU will remain critical to furthering our understanding of the fundamental coupling of LISM material and solar wind plasma. Interstellar pickup ions and energetic particle acceleration processes at shocks and other interaction regions can be studied in detail within the inner heliosphere. Focusing especially on radial variation, composition, charge states, injection, and sources will yield new and valuable information, which can be extrapolated to the heliospheric boundary. This will require
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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report spacecraft with instrumentation that can study the inflowing neutral gas and dust, pickup ions, and cosmic rays (anomalous and galactic), energetic particles, and magnetic field directly, since these are essential if we are eventually to probe both the LISM and the heliospheric boundaries with new spacecraft (and even remotely). Our understanding of the critical microphysics will be revealed best by in situ measurements. PROBING THE LOCAL INTERSTELLAR MEDIUM IN THE INNER HELIOSPHERE With a renewed emphasis on dedicated observations within the inner heliosphere substantial steps can be taken to address the first, second, and fourth science objectives listed in Chapter 1. Studying nucleosynthesis or the evolution of matter in our galaxy requires extensive modeling of the production of elements and isotopes in stars and their dissemination into the interstellar medium. Constraints on the models require precise composition observations of samples from different ages of the galaxy. While the solar system is a sample from 4.5 billion years ago, the local interstellar cloud (LIC) provides material of today’s galaxy and is thus much more evolved. The necessary composition measurements can be achieved through pickup ion observations (Gloeckler and Geiss, 1998). To make substantial progress requires a pickup ion mass spectrometer with good resolution and a large geometric factor. Such an instrument can be built today, based on time-of-flight instruments used on board the Ulysses and ACE spacecraft (Gloeckler et al., 1998; Mason et al., 1998). Mass resolution, energy range and geometric factor (up to 500×) must be tailored specifically to the pickup ion investigation, because past and current instruments were designed for other purposes. The composition of elements and isotopes with high ionization potential, such as H, He, N, O, Ne, and Ar, can be studied from the neutral gas entering the inner heliosphere. Key isotopic ratios, such as 3He/4He, 14N/15N, 18O/16O, and 22Ne/20Ne, can be observed without any significant alteration through heliospheric interface processes. Elemental ratios may require corrections for selective filtration at the interface, another important topic that can be understood more thoroughly through observations in the inner heliosphere. Progress in constraining the physical parameters of the LIC and the interaction with the heliosphere has been made recently through a coordinated analysis of interstellar He using three complementary techniques: direct neutral gas measurements, pickup ion measurements, and ultraviolet backscattering observations. A benchmark set of physical He parameters has been derived that yields consistent results for all three techniques (Gloeckler and Geiss, 2004; Lallement et al., 2004; Witte et al., 2004) if the ionization environment and the solar illumination are taken into account correctly. A lesson learned from this analysis is the recognition that direct observations of the neutral gas provide the most accurate and complete information on the arriving neutral distribution. The key to these observations is the use of the Sun as a gigantic gravitational lens for the inflow of interstellar gas, as depicted in Figure 3.1. The controlled deflection of interstellar trajectories on Keplerian orbits leads to a distinctive pattern for the gas distribution in the inner heliosphere and to a unique dependence of the flow direction on the location of the observer (Fahr, 1968; see, e.g., Zank, 1999a). It is the latter characteristic that is utilized in the form of an image of the neutral gas flow in the sky. Unlike He, which is not affected by heliospheric interface filtration, the distributions of H and O experience substantial depletion, deceleration, and heating entering the heliosphere when compared with their pristine state in the LIC (see, e.g., Izmodenov et al., 1999a; Müller et al., 2000). As depletion, deceleration, and heating are intimately coupled to interface processes, deviations in the flow pattern and temperature of the H and O distributions compared with that of He will allow us to evaluate the filtration and original LIC distribution from interplanetary neutral gas observations. Such studies will rely on well-developed models. The analysis of neutral He observations (Witte et al., 1996, 2004) has shown that highly accurate LISM parameters can be derived, provided the angular resolution and precision of the observa-
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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 3.1 Typical orbit, main objectives, and viewing of projected Interstellar Pathfinder with trajectories of the interstellar gas flow and a qualitative distribution of interstellar gas. tions are commensurate with each other. The logical next step is to use a neutral imaging instrument, similar to the one flown on Ulysses but now with the ability to distinguish H and O (Möbius et al., 2001). Concepts to increase angular resolution and geometric factor have been developed (Livi et al., 2003). A sensor with front-end, surface conversion of neutrals into negative ions, and subsequent time-of-flight analysis similar to the LENA (Low-Energy Neutral Atom) instrument on the IMAGE (Imager for Magnetosphere to Aurora Global Exploration) spacecraft (Burch, 2000), can extend the neutral gas observations to O and possibly H (see, e.g., Wurz et al., 1995). The gravitational deflection of the interstellar flow by the Sun used by a neutral gas instrument to infer the flow velocity from the direction of the incoming neutrals is most pronounced very close to the Sun. Conversely, interstellar pickup ions other than He can only be observed effectively at distances >2 AU. For example, at 1 AU, inner source pickup O ions—that is, ions created from dust close to the Sun—dominate the interstellar O population. Only beyond 1.4 AU can we distinguish unambiguously between inner source and interstellar pickup ions (Geiss et al., 1995). The farther away from the Sun, the more the interstellar pickup ion distribution becomes visible without contamination by the inner source. To measure both inner source and interstellar pickup ions, an elliptical orbit in the ecliptic plane into the side-wind direction relative to the interstellar gas flow would be an ideal compromise (see Figure 3.1) and would also cut through the gravitational focusing cone of He.
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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report REMOTE SENSING OF THE HELIOSPHERIC BOUNDARY REGIONS Interaction processes in the nose direction1 can be deduced from the interstellar gas flow, but only remote-sensing techniques with full-sky imaging allow us to investigate the characteristics and the three-dimensional topology of the heliospheric termination shock. The heliosphere is an essentially asymmetric three-dimensional object on a scale of several hundred AU, which calls for remote techniques to study its boundary. Only remote observations (complemented by the Voyager and the proposed Interstellar Probe “ground truth” in situ measurements) can provide a global view of the time-varying heliosphere and its boundary on a continuous basis. Two remote-sensing experimental techniques, using fluxes of ENAs (Gruntman, 1997; Gruntman et al., 2001) and EUV photons (Gruntman and Fahr, 2000; Gruntman, 2001a,b) with different degrees of maturity, promise significant advances in imaging the heliospheric boundaries. Energetic Neutral Atom Imaging The inner heliosheath, between the termination shock and the heliopause, contains shocked solar wind plasma and pickup ions. Energetic protons charge exchange with interstellar hydrogen to produce ENAs, some of which reach 1 AU, where they can be reliably detected by instrumentation based on well-tested technology. Provided that the background from heliospheric ENAs is sufficiently small compared with ENA fluxes from the termination shock and beyond, ENA imaging of the heliosphere could establish the nature of the termination shock and its directional dependence and could also determine the asymmetry of the interstellar magnetic field. The experimental concept and the instrumentation to carry out these observations are mature and ready for implementation. For example, an ENA imager that is optimized for the energy distribution of H neutrals from the termination shock with energies that range from a few hundred eV to several keV could be derived from instruments on Cassini, IMAGE, and TWINS2 (Two Wide-Angle Imaging Neutral-Atom Spectrometers) (Funsten et al., 2001). As mentioned above, three spacecraft missions to image the heliospheric boundaries through ENA observations have already been proposed—two Interstellar Pathfinder (e.g., McComas et al., 2003) missions and the IBEX (McComas et al., 2004), which is currently in Phase A study. Clearly, ENA imaging has reached a level of maturity that the EUV and x-ray imaging techniques discussed below have yet to attain. Extreme Ultraviolet Imaging As a complementary technique, heliospheric imaging in the EUV with high sensitivity and spectral resolution will significantly advance our knowledge of the physical processes at and beyond the heliopause—that is, in the region where the expanding solar wind meets the galactic medium. The 1 The direction of the nose is 180 degrees opposite to the direction of the flow and is therefore approximately 254 degrees ecliptic longitude and +5.6 degrees ecliptic latitude (Geiss and Witte, 1996). The most recent data puts the nose at 254.7 ± 0.6 degrees ecliptic longitude and +5.3 ± 0.3 degrees ecliptic latitude (Möbius et al., 2004; Witte et al., 2004). The direction to the nose is, of course, a key consideration in selection of a flight path for Interstellar Probe. 2 Scheduled for launch in 2005, the TWINS mission will provide a new capability for stereoscopically imaging the magnetosphere. By imaging the charge exchange neutral atoms over a broad energy range (~1 to 100 keV) using two identical instruments on two widely spaced, high-altitude, high-inclination spacecraft, TWINS will enable the three-dimensional visualization and the resolution of large-scale structures and dynamics within the magnetosphere for the first time. See the TWINS home page at http://niswww.lanl.gov/nis-projects/twins/.
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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report interstellar plasma beyond the heliopause reflects line emissions of the Sun in the EUV range, thus providing information on the distance to the heliopause and on interstellar plasma properties. Observations in the He+ 30.4-nm line are most promising. Mapping of the heliopause at 30.4 nm could allow us to establish the shape of the heliopause, determine the ionization degree of interstellar helium, and reveal the asymmetry of the interstellar magnetic field. It may also tell us whether there is a heliospheric bow shock. This measurement requires the development of a new generation of extremely sensitive diffuse EUV spectrometers with high spectral resolution. The experimental concepts have been formulated, and a feasibility study of the proposed instrumentation is being conducted. In addition, full-sky images of the heliosphere at 30.4 nm with high spectral resolution will allow us to establish remotely, from 1 AU, the time-varying flow properties (velocity and number density) of the solar wind plasma flow in all directions, including over the Sun’s poles and on its far side (Gruntman, 2001b). X-ray Imaging Recently also the diffuse x-ray background has received attention as a potentially powerful diagnostic technique for the interstellar gas/solar wind interaction (see, e.g., Cravens, 2000; Robertson et al., 2001). This radiation is produced through the excitation of heavy solar wind ions when they collide with interstellar gas atoms. The serendipitous observation of x-rays from comets (Dennerl et al., 1997; Cravens, 1997) paved the way for this promising new tool. When fully developed, it could provide a full-sky image of the solar wind/interstellar gas interaction and provide an independent account of the spatial distribution of interstellar gas.
Representative terms from entire chapter: