Search

The long-anticipated encounter by Voyager 2 (V2) of the region between the heliosphere and the very local interstellar medium (VLISM) occurred toward the end of 2018. Here, we report measurements of energetic (>28 keV) charged particles on V2 from the interface region between the heliosheath, dominated by heated solar wind plasma, and the VLISM, expected to contain cold non-solar plasma and the Galactic magnetic field. The number of particles of solar origin began a gradual decrease on 7 August 2018 (118.2 au), while those of Galactic origin (Galactic cosmic rays) increased ~20% in number over a period of a few weeks. An abrupt change occurred on 5 November when V2 was located at 119 au, with a decrease in the number of particles at energies of >28 keV and a corresponding increase in the number of Galactic cosmic rays of energy E > 213 MeV. This signature of the transition to the VLISM resembles, but is very different from, that observed on Voyager 1 at ~121.6 au, associated with the putative crossing of the heliopause some six years earlier. Measurements of energetic ions and electrons with the Low-Energy Charged Particle instrument on Voyager 2 are presented from the boundary of the heliosphere and from the interstellar medium. Voyager 2’s heliopause crossing bears some similarity to that of Voyager 1, despite differing solar wind conditions.

The variation of the charge state of iron [Fe] ions is used to trace volume elements of plasma in the solar wind into the magnetosphere and to determine the time scales associated with the entry into and the action of the magnetospheric energization process working on these plasmas. On 2–3 May 1998 the Advanced Composition Explorer (ACE) spacecraft located at the L1 libration point observed a series of changes to the average charge state of the element Fe in the solar wind plasma reflecting variation in the coronal temperature of their original source. Over the period of these two days the average Fe charge state was observed to vary from + 15 to + 6 both at the Polar satellite in the high latitude dayside magnetosphere and at ACE. During a period of southward IMF the observations at Polar inside the magnetosphere of the same Fe charge state were simultaneous with those at ACE delayed by the measured convection speed of the solar wind to the subsolar magnetopause. Comparing the phase space density as a function of energy at both ACE and Polar has indicated that significant energization of the plasma occurred on very rapid time scales. Energization at constant phase space density by a factor of 5 to 10 was observed over a range of energy from a few keV to about 1 MeV. For a detector with a fixed energy threshold in the range from 10 keV to a few hundred keV this observed energization will appear as a factor of ~103 increase in its counting rate. Polar observations of very energetic O+ ions at the same time indicate that this energization process must be occurring in the high latitude cusp region inside the magnetosphere and that it is capable of energizing ionospheric ions at the same time.Key words. Magnetospheric physics (magnetopause, cusp, and boundary layers; magnetospheric configuration and dynamics; solar wind-magnetosphere interactions)

Department of Atmospheric, Oceanic and Space Sciences, University of Michigan2455 Hayward St., Ann Arbor, MI 48109, USAemail: gglo@umich.eduAbstract.For our understanding of the origin and evolution of baryonic matter in the Universe,the Protosolar Cloud (PSC) is of unique importance in two ways: 1) Up to now, many of thenaturally occurring nuclides have only been detected in the solar system. 2) Since the timeof solar system formation, the Sun and planets have been virtually isolated from the galacticnuclear evolution, and thus the PSC is a galactic sample with a degree of evolution intermediatebetween the Big Bang and the present.The abundances of the isotopes of hydrogen andhelium in the Protosolar Cloud are primarilyderivedfromcompositionmeasurementsinthesolarwind,theJovianatmosphereand“planetarynoble gases” in meteorites, and also from observations of density profiles inside the Sun. Afterapplying the changes in isotopic and elemental composition resulting from processes in the solarwind, the Sun and Jupiter, PSC abundances of the four lightest stable nuclides are given.Keywords.Sun: solar wind, corona, abundances

In the last decade a significant discovery has been made in the heliosphere: the spectrum of particles accelerated in both the inner heliosphere and in the heliosheath is the same: a power law in particle speed with a spectral index of −5, when the spectrum is expressed as a distribution function; or equivalently, a differential intensity spectrum that is a power law in energy with a spectral index of −1.5. In the inner heliosphere this common spectrum occurs at quite low energies and is most evident in instruments designed to measure suprathermal particles. In the heliosheath, the common spectrum is observed over the full energy range of the Voyager energetic particle instruments, up to energies of ~100 MeV. The remarkable discovery of a common spectrum is compounded by the realization that no traditional acceleration mechanism, i.e., diffusive shock acceleration or stochastic acceleration, can account for the common spectrum. There is thus an opportunity to once again demonstrate the relevance of heliospheric physics by developing a new acceleration mechanism that yields the common spectrum, with the expectation that such a new acceleration mechanism will find broader applications in astrophysics. In this paper, the observations of the common spectrum in the heliosphere are summarized, with emphasis on those that best reveal the conditions in which the acceleration must operate. Then, building on earlier work, a complete derivation is presented of an acceleration mechanism, a pump acceleration mechanism, that yields the common spectrum, and the various subtleties associated with this derivation are discussed.

Interstellar neutrals penetrating into the inner heliosphere are ionized by photoionization, charge exchange with solar wind ions, and electron impact ionization. These processes comprise the first step in the evolution of interstellar pickup ion (PUI) distributions. Typically, PUI distributions have been described in terms of velocity distribution functions that cool adiabatically under solar wind expansion, with a cooling index of 3/2. Recently, the cooling index has been determined experimentally in observations of He PUI distributions with Advanced Composition Explorer (ACE)/Solar Wind Ion Composition Spectrometer and found to vary substantially over the solar cycle. The experimental determination of the cooling index depends on the knowledge of the ionization rates and their spatial variation. Usually, ionization rates increase with 1/r2 as neutral particles approach the Sun, which is not exactly true for electron impact ionization, because the electron temperature increases with decreasing distance from the Sun due to the complexity of its distributions and different radial gradients in temperature. This different dependence on distance may become important in the study of the evolution of PUI distributions and is suspected as one of the potential reasons for the observed variation of the cooling index. Therefore, we investigate in this paper the impact of electron ionization on the variability of the cooling index. We find that the deviation of the electron ionization rate from the canonical 1/r2 behavior of other ionization processes plays only a minor role.

The Voyager 1 spacecraft is currently in the vicinity of the heliopause, which separates the heliosphere from the local interstellar medium. There has been a precipitous decrease in particles accelerated in the heliosphere and a substantial increase in galactic cosmic rays (GCRs). The evidence is unclear, however, as to whether Voyager 1 has crossed the heliopause into the local interstellar medium or remains within the heliosheath. In this Letter we propose a test that will determine whether Voyager 1 has crossed the heliopause: If Voyager 1 remains in the heliosheath, the high densities observed must be due to compressed solar wind, with the consequence that Voyager 1 will encounter another current sheet, where the polarity of the magnetic field reverses. Voyager 1 observations can be used to predict that the next current sheet crossing is likely to occur during 2015.

[1] The velocity distribution of interstellar pickup ions (PUIs) has typically been described as evolving through fast pitch angle scattering followed by adiabatic cooling while being transported radially outward with the solar wind. In combination, the ionization rate, which controls the radial profile of the interstellar neutrals, and the cooling process determine the slope of the observed PUI distributions. Thus far, a cooling index of 3/2 for the PUI velocity distributions has been used in almost all studies. This value is based on the implicit assumptions of immediate PUI isotropization due to pitch angle scattering and solar wind expansion with the square of the distance from the Sun. Here we determine the observed cooling index in a comparison of He+ PUI distributions taken for 1 month in the upwind direction with ACE SWICS from 1999 through 2010 over the past solar cycle with such an isotropic PUI model, treating the cooling index as a free parameter. The ionization rate is obtained simultaneously from independent observations. To separate effects of slow pitch angle scattering of PUIs, the comparison is repeated for times restricted to perpendicular interplanetary magnetic field (IMF). When averaged over the entire data set, the cooling index is very close to 3/2. However, it varies substantially from 1.1 to 1.9 between samples, shows a distinct variation with solar activity, and has a significant correlation with sunspot number when data are restricted to nearly perpendicular IMF (θBvSW>60°) excluding the slow pitch angle scattering in the radial IMF direction. The potential influence of slow pitch angle scattering, solar wind structures, and electron ionization on the cooling index and its variations is discussed.

[1] We have analyzed measurements of planetary ions near Mercury made by the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) Fast Imaging Plasma Spectrometer (FIPS) over the first three Mercury years of orbital observations (25 March 2011 through 31 December 2011). We determined the composition and spatial distributions of the most abundant species in the regions sampled by the MESSENGER spacecraft during that period. In particular, we here focus on altitude dependence and relative abundances of species in a variety of spatial domains. We used observed density as a proxy for ambient plasma density, because of limitations to the FIPS field of view. We find that the average observed density is 3.9 × 10–2 cm–3 for He2+, 3.4 × 10–4 cm–3 for He+, 8.0 × 10–4 cm–3 for O+-group ions, and 5.1 × 10–3 cm–3 for Na+-group ions. Na+-group ions are particularly enhanced over other planetary ions (He+ and O+ group) in the northern magnetospheric cusp (by a factor of ~2.0) and in the premidnight sector on the nightside (by a factor of ~1.6). Within 30° of the equator, the average densities of all planetary ions are depressed at the subsolar point relative to the dawn and dusk terminators. The effect is largest for Na+-group ions, which are 49% lower in density at the subsolar point than at the terminators. This depression could be an effect of the FIPS energy threshold. The three planetary ion species considered show distinct dependences on altitude and local time. The Na+ group has the smallest e-folding height at all dayside local times, whereas He+ has the largest. At the subsolar point, the e-folding height for Na+-group ions is 590 km, and that for the O+ group and He+ is 1100 km. On the nightside and within 750 km of the geographic equator, Na+-group ions are enhanced in the premidnight sector. This enhancement is consistent with nonadiabatic motion and may be observational evidence that nonadiabatic effects are important in Mercury's magnetosphere.

During the MESSENGER spacecraft's interplanetary trajectory to Mercury, the Fast Imaging Plasma Spectrometer (FIPS) measured the first helium pickup ion distributions at a heliocentric distance (R) ranging between 0.3 and 0.7 AU. From several transits of MESSENGER through the interstellar helium gravitational focusing cone, we map the cone structure in the ecliptic at R=0.3 AU and compare it with observations of He+ at R=1 AU made with the Solar Wind Ion Composition Spectrometer (SWICS) on the Advanced Composition Explorer (ACE). Average downwind enhancements of ≈8 and average cone widths of ≈30° measured by each sensor match known models of neutral helium during this most recent, unusually quiet, solar minimum. The average cone center direction is calculated to be at J2000 ecliptic longitude λ_∞=76.0°(±6.0°) and 77.0°(±1.5°) from FIPS and SWICS observations, respectively. These parameters are also in agreement with previous determinations of the downwind direction of interstellar flow and demonstrate the effectiveness of using pickup ion observations inside 1 AU to map the structure of the neutral helium distribution in the heliosphere, as well as to diagnose the conditions of the interstellar medium.

There has been a remarkable discovery concerning particles that are accelerated in the solar wind. At low energies, in the region where the particles are being accelerated, the spectrum of the accelerated particles is always the same: when expressed as a distribution function, the spectrum is a power law in particle speed with a spectral index of −5, and a rollover at higher particle speeds that can often be described as exponential. This common spectral shape cannot be accounted for by any conventional acceleration mechanism, such as diffusive shock acceleration or traditional stochastic acceleration. It has thus been necessary to invent a new acceleration mechanism to account for these observations, a pump mechanism in which particles are pumped up in energy through a series of adiabatic compressions and expansions. The conditions under which the pump acceleration is the dominant acceleration mechanism are quite general and are likely to occur in other astrophysical plasmas. In this paper, the most compelling observations of the −5 spectra are reviewed; the governing equation of the pump acceleration mechanism is derived in detail; the pump acceleration mechanism is applied to acceleration at shocks; and, as an illustration of the potential applicability of the pump acceleration mechanism to other astrophysical plasmas, the pump mechanism is applied to the acceleration of galactic cosmic rays in the interstellar medium.

The MESSENGER Fast Imaging Plasma Spectrometer (FIPS) measured the bulk plasma characteristics of Mercury's magnetosphere and solar wind environment during the spacecraft's first two flybys of the planet on 14 January 2008 (M1) and 6 October 2008 (M2), producing the first measurements of thermal ions in Mercury's magnetosphere. In this work, we identify major features of the Mercury magnetosphere in the FIPS proton data and describe the data analysis process used for recovery of proton density (np) and temperature (Tp) with a forward modeling technique, required because of limitations in measurement geometry. We focus on three regions where the magnetospheric flow speed is likely to be low and meets our criteria for the recovery process: the M1 plasma sheet and the M1 and M2 dayside and nightside boundary-layer regions. Interplanetary magnetic field (IMF) conditions were substantially different between the two flybys, with intense reconnection signatures observed by the Magnetometer during M2 versus a relatively quiet magnetosphere during M1. The recovered ion density and temperature values for the M1 quiet-time plasma sheet yielded np∼1–10 cm−3, Tp∼2×106 K, and plasma β∼2. The nightside boundary-layer proton densities during M1 and M2 were similar, at np∼4–5 cm−3, but the temperature during M1 (Tp∼4–8×106 K) was 50% less than during M2 (Tp∼8×106 K), presumably due to reconnection in the tail. The dayside boundary layer observed during M1 had a density of ∼16 cm−3 and temperature of 2×106 K, whereas during M2 this region was less dense and hotter (np∼8 cm−3 and Tp∼10×106 K), again, most likely due to magnetopause reconnection. Overall, the southward interplanetary magnetic field during M2 clearly produced higher Tp in the dayside and nightside magnetosphere, as well as higher plasma β in the nightside boundary, ∼20 during M2 compared with ∼2 during M1. The proton plasma pressure accounts for only a fraction (24% for M1 and 64% for M2) of the drop in magnetic pressure upon entry into the dayside boundary layer. This result suggests that heavy ions of planetary origin, not considered in this analysis, may provide the “missing” pressure. If these planetary ions were hot due to “pickup” in the magnetosheath, the required density for pressure balance would be an ion density of ∼1 cm−3 for an ion temperature of ∼108 K.

The second and third flybys of Mercury by the MESSENGER spacecraft occurred, respectively, on 6 October 2008 and on 29 September 2009. In order to provide contextual information about the solar wind properties and the interplanetary magnetic field (IMF) near the planet at those times, we have used an empirical modeling technique combined with a numerical physics-based solar wind model. The Wang–Sheeley–Arge (WSA) method uses solar photospheric magnetic field observations (from Earth-based instruments) in order to estimate the inner heliospheric radial flow speed and radial magnetic field out to 21.5 solar radii from the Sun. This information is then used as input to the global numerical magnetohydrodynamic model, ENLIL, which calculates solar wind velocity, density, temperature, and magnetic field strength and polarity throughout the inner heliosphere. WSA-ENLIL calculations are presented for the several-week period encompassing the second and third flybys. This information, in conjunction with available MESSENGER data, aid in understanding the Mercury flyby observations and provide a basis for global magnetospheric modeling. We find that during both flybys, the solar wind conditions were very quiescent and would have provided only modest dynamic driving forces for Mercury's magnetospheric system.

Magnetic field and plasma data from the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft on the outbound portions of the first (M1) and second (M2) flybys of Mercury reveal a region of depressed magnetic field magnitude and enhanced proton fluxes adjacent to but within the magnetopause, which we denote as a dayside boundary layer. The layer was present during both encounters despite the contrasting dayside magnetic reconnection, which was minimal during M1 and strong during M2. The overall width of the layer is estimated to be between 1000 and 1400 km, spanning most of the distance from the dayside planetary surface to the magnetopause in the mid-morning. During both flybys the magnetic pressure decrease was ∼1.6 nPa, and the width of the inner edge was comparable to proton gyro-kinetic scales. The maximum variance in the magnetic field across the inner edge was aligned with the magnetic field vector, and the magnetic field direction did not change markedly, indicating that the change in field intensity was consistent with an outward plasma-pressure gradient perpendicular to the magnetic field. Proton pressures in the layer inferred from reduced distribution observations were 0.4 nPa during M1 and 1.0 nPa during M2, indicating either that the proton pressure estimates are low or that heavy ions contribute substantially to the boundary-layer plasma pressure. If the layer is formed by protons drifting westward from the cusp, there should be a strong morning–afternoon asymmetry that is independent of the interplanetary magnetic field (IMF) direction. Conversely, if heavy ions play a major role, the layer should be strong in the morning (afternoon) for northward (southward) IMF. Future MESSENGER observations from orbit about Mercury should distinguish between these two possibilities.

Global measurements by MESSENGER of the fluxes of heavy ions at Mercury, particularly sodium (Na(+)) and oxygen (O(+)), exhibit distinct maxima in the northern magnetic-cusp region, indicating that polar regions are important sources of Mercury's ionized exosphere, presumably through solar-wind sputtering near the poles. The observed fluxes of helium (He(+)) are more evenly distributed, indicating a more uniform source such as that expected from evaporation from a helium-saturated surface. In some regions near Mercury, especially the nightside equatorial region, the Na(+) pressure can be a substantial fraction of the proton pressure.

At energies above the bulk solar wind and pick-up ion cutoff, observations reveal an interplanetary suprathermal ion population extending to ∼1 MeV/nucleon and even higher energies. These suprathermal ions are found under a wide variety of conditions including periods when there are no obvious nearby accelerating shocks. We review the observational properties of these ions in quiet solar wind periods near 1 AU, including transient Corotating Interaction Region (CIR) events, and other, quieter periods in between transient enhancements. The particle energy spectra are power laws close to E −1.5 in the range above the solar wind, rolling over at energies of a few hundred keV/nucleon to a few MeV/nucleon. Although the C/O and Fe/O ratios of the tails is close to that of the solar wind, pickup ions and 3He found in the tails indicate sources distinct from the solar wind. We briefly review several mechanisms that have been proposed to explain these ions.

Particle spectra directly measured by a plasma or low-energy particle experiment on spacecraft often contain instrumental effects due to a limited field of view or angular resolution. It is because the particle distribution function at low energies is highly anisotropic in the spacecraft frame of reference, in which the measurements are made. In this paper, we present a new mathematical method of transforming the particle spectrum to the solar wind frame of reference, where the particle distribution can be assumed to be nearly isotropic. The transformed particle spectrum allows us to investigate the properties of the solar wind, pickup ions, and suprathermal particles without concern for instrumental effects. We apply the method to the measurements made by the Solar Wind Ion Composition Spectrometer on Ulysses. Our results demonstrate that the transformed spectrum can improve the determination of the solar wind density and temperature from the previously published methods. A brief survey of Ulysses data shows that suprathermal ions in the slow solar wind frequently display a velocity distribution very close to the v −5 power law.

The MESSENGER spacecraft flyby of Mercury on 14 January 2008 provided a new opportunity to study the intrinsic magnetic field of the innermost planet and its interaction with the solar wind, The model presented in this paper is based on the solution of the three-dimensional, bi-f1uid equations for solar wind protons and electrons in the absence of mass loading, In this study we provide new estimates of Mercury's intrinsic magnetic field and the solar wind conditions that prevailed at the time of the flyby. We show that the location of the boundary layers and the strength of the magnetic field along the spacecraft trajectory can be reproduced with a solar wind ram pressure P(sub sw) = 6.8 nPa and a planetary magnetic dipole having a magnitude of 210 R(sub M)(exp 3)- nT and an offset of 0.18 R(sub M) to the north of the equator, where R(sub M) is Mercury's radius. Analysis of the plasma flow reveals the existence of a stable drift belt around the planet; such a belt can account for the locations of diamagnetic decreases observed by the MESSENGER Magnetometer. Moreover, we determine that the ion impact rate at the n011hern cusp was four times higher than at the southern cusp, a result that provides a possible explanation for the observed north-south asymmetry in exospheric sodium in the neutral tail.

The suprathermal tails on the distribution functions of ions in the solar wind are observed to have a common spectral shape in many different circumstances: a power law in particle speed with spectral index of −5. Three possible approaches for explaining these observations are considered: (1) the acceleration mechanism of Fisk & Gloeckler in which energy is redistributed from a core particle population into the suprathermal tail; (2) traditional stochastic acceleration in which particles are accelerated by damping turbulence; and (3) the statistical approach introduced by Schwadron et al. in which the −5 spectrum is formed by averaging over individual spectra. The acceleration mechanism of Fisk & Gloeckler has advantages: (1) it appears to occur in conditions that are readily satisfied: compressive turbulence that is thermally isolated (no largescale spatial gradients), with a core distribution of particles with a sharp initial cutoff in particle speed, above which particles can spatially diffuse; and (2) it yields spectra that are consistent with observations. Traditional stochastic acceleration has the disadvantage that it is unlikely to yield spectral shapes consistent with observations, and the spectra are dependent upon the plasma conditions and thus unlikely to be the same in different circumstances. The statistical approach of Schwadron et al. can yield the −5 spectrum and is consistent with the results of Fisk & Gloeckler when the assumed distribution functions for individual events and the averaging technique are taken to be compatible with the assumptions and averaging in Fisk & Gloeckler.

What's Happening in the Heliosphere The influence of the Sun is felt well beyond the orbits of the planets. The solar wind is a stream of charged particles emanating from the Sun that carves a bubble in interstellar space known as the heliosphere and shrouds the entire solar system. The edge of the heliosphere, the region where the solar wind interacts with interstellar space, is largely unexplored. Voyager 1 and 2 crossed this boundary in 2004 and 2007, respectively, providing detailed but only localized information. In this issue (see the cover), McComas et al. (p. 959 , published online 15 October), Fuselier et al. (p. 962 , published online 15 October), Funsten et al. (p. 964 , published online 15 October), and Möbius et al. (p. 969 , published online 15 October) present data taken by NASA's Interstellar Boundary Explorer (IBEX). Since early 2009, IBEX has been building all-sky maps of the emissions of energetic neutral atoms produced at the boundary between the heliosphere and the interstellar medium. These maps have unexpectedly revealed a narrow band of emission that bisects the two Voyager locations at energies ranging from 0.2 to 6 kiloelectron volts. Emissions from the band are two- to threefold brighter than outside the band, in contrast to current models that predict much smaller variations across the sky. By comparing the IBEX observations with models of the heliosphere, Schwadron et al. (p. 966 , published online 15 October) show that to date no model fully explains the observations. The model they have developed suggests that the interstellar magnetic field plays a stronger role than previously thought. In addition to the all-sky maps, IBEX measured the signatures of H, He, and O flowing into the heliosphere from the interstellar medium. In a related report, Krimigis et al. (p. 971 , published online 15 October) present an all-sky image of energetic neutral atoms with energies ranging between 6 and 13 kiloelectron volts obtained with the Ion and Neutral Camera onboard the Cassini spacecraft orbiting Saturn. It shows that parts of the structure observed by IBEX extend to high energies. These data indicate that the shape of the heliosphere is not consistent with that of a comet aligned in the direction of the Sun's travel through the galaxy as was previously thought.

The magnetic field strength of Mercury at the planet’s surface is approximately 1% that of Earth’s surface field. This comparatively low field strength presents a number of challenges, both theoretically to understand how it is generated and observationally to distinguish the internal field from that due to the solar wind interaction. Conversely, the small field also means that Mercury offers an important opportunity to advance our understanding both of planetary magnetic field generation and magnetosphere-solar wind interactions. The observations from the Mariner 10 magnetometer in 1974 and 1975, and the MESSENGER Magnetometer and plasma instruments during the probe’s first two flybys of Mercury on 14 January and 6 October 2008, provide the basis for our current knowledge of the internal field. The external field arising from the interaction of the magnetosphere with the solar wind is more prominent near Mercury than for any other magnetized planet in the Solar System, and particular attention is therefore paid to indications in the observations of deficiencies in our understanding of the external field. The second MESSENGER flyby occurred over the opposite hemisphere from the other flybys, and these newest data constrain the tilt of the planetary moment from the planet’s spin axis to be less than 5°. Considered as a dipole field, the moment is in the range 240 to 270 nT-R M 3 , where R M is Mercury’s radius. Multipole solutions for the planetary field yield a smaller dipole term, 180 to 220 nT-R M 3 , and higher-order terms that together yield an equatorial surface field from 250 to 290 nT. From the spatial distribution of the fit residuals, the equatorial data are seen to reflect a weaker northward field and a strongly radial field, neither of which can be explained by a centered-dipole matched to the field measured near the pole by Mariner 10. This disparity is a major factor controlling the higher-order terms in the multipole solutions. The residuals are not largest close to the planet, and when considered in magnetospheric coordinates the residuals indicate the presence of a cross-tail current extending to within 0.5R M altitude on the nightside. A near-tail current with a density of 0.1 μA/m2 could account for the low field intensities recorded near the equator. In addition, the MESSENGER flybys include the first plasma observations from Mercury and demonstrate that solar wind plasma is present at low altitudes, below 500 km. Although we can be confident in the dipole-only moment estimates, the data in hand remain subject to ambiguities for distinguishing internal from external contributions. The anticipated observations from orbit at Mercury, first from MESSENGER beginning in March 2011 and later from the dual-spacecraft BepiColombo mission, will be essential to elucidate the higher-order structure in the magnetic field of Mercury that will reveal the telltale signatures of the physics responsible for its generation.

Stochastic acceleration in the heliosheath appears to be a likely mechanism by which Anomalous Cosmic Rays (ACRs) are accelerated. However, most stochastic acceleration mechanisms are not appropriate. The energy density in the ACRs and in the interstellar pickup ions out of which the ACRs are accelerated greatly exceeds the energy density in the turbulence in the heliosheath. Thus, a traditional stochastic acceleration mechanism in which particles are accelerated by damping the turbulence will not work. A stochastic acceleration mechanism has been developed in which the total energy of the pickup ions and the ACRs is conserved. Energy is redistributed from the core pickup ions into a suprathermal tail to create the ACRs. A model for the acceleration of the ACRs in the heliosheath, based on this stochastic acceleration mechanism, is presented. The model provides reasonable fits to the spectra of suprathermal particles and ACRs observed by Voyager.

MESSENGER from Mercury The spacecraft MESSENGER passed by Mercury in October 2008, in what was the second of three fly-bys before it settles into the planet's orbit in 2011. Another spacecraft visited Mercury in the mid-1970s, which mapped 45% of the planet's surface. Now, after MESSENGER, only 10% of Mercury's surface remains to be imaged up close. Denevi et al. (p. 613 ) use this near-global data to look at the mechanisms that shaped Mercury's crust, which likely formed by eruption of magmas of different compositions over a long period of time. Like the Moon, Mercury's surface is dotted with impact craters. Watters et al. (p. 618 ) describe a well-preserved impact basin, Rembrandt, which is second in size to the largest known basin, Caloris. Unlike Caloris, Rembrandt is not completely filled by material of volcanic origin, preserving clues to its formation and evolution. It displays unique patterns of tectonic deformation, some of which result from Mercury's contraction as its interior cooled over time. Mercury's exosphere and magnetosphere were also observed (see the Perspective by Glassmeier ). Magnetic reconnection is a process whereby the interplanetary magnetic field lines join the magnetospheric field lines and transfer energy from the solar wind into the magnetosphere. Slavin et al. (p. 606 ) report observations of intense magnetic reconnection 10 times as intense as that of Earth. McClintock et al. (p. 610 ) describe simultaneous, high-resolution measurements of Mg, Ca, and Na in Mercury's exosphere, which may shed light on the processes that create and maintain the exosphere.

We discuss a consolidation of determinations of the density of neutral interstellar H at the nose of the termination shock carried out with the use of various data sets, techniques, and modeling approaches. In particular, we focus on the determination of this density based on observations of H pickup ions on Ulysses during its aphelion passage through the ecliptic plane. We discuss in greater detail a novel method of determination of the density from these measurements and review the results from its application to actual data. The H density at TS derived from this analysis is equal to 0.087 \pm 0.022 cm-3, and when all relevant determinations are taken into account, the consolidated density is obtained at 0.09 \pm 0.022 cm-3. The density of H in CHISM based on literature values of filtration factor is then calculated at 0.16 \pm 0.04 cm-3., Submitted to Space Science Reviews

Suprathermal particles are ubiquitously present in the solar wind, with energies typically from ~1 keV nucleon−1 to ~a few MeV nucleon−1. Remarkably, the suprathermal particles exhibit a common spectral shape in many different circumstances; the distribution function is a power law in particle speed, with spectral index of –5. The observations cannot be explained by traditional stochastic acceleration, which yields spectra that depend on, e.g., the momentum diffusion coefficient, and thus the spectra are expected to be different in different circumstances. A theory is presented in which the particles are accelerated in thermally isolated compressional turbulence. Thermal isolation is valid in spatially homogeneous conditions in the solar wind, and when properly applied, yields suprathermal tails that always have the required spectral shape. The theory describes the time evolution of the spectrum to its equilibrium form and predicts the high-speed cutoff on the acceleration. The high-speed cutoff is shown to be in good agreement with observations of quiet-time spectra at Earth and is consistent with observations throughout the outer heliosphere.

Observations by MESSENGER show that Mercury's magnetosphere is immersed in a comet-like cloud of planetary ions. The most abundant, Na + , is broadly distributed but exhibits flux maxima in the magnetosheath, where the local plasma flow speed is high, and near the spacecraft's closest approach, where atmospheric density should peak. The magnetic field showed reconnection signatures in the form of flux transfer events, azimuthal rotations consistent with Kelvin-Helmholtz waves along the magnetopause, and extensive ultralow-frequency wave activity. Two outbound current sheet boundaries were observed, across which the magnetic field decreased in a manner suggestive of a double magnetopause. The separation of these current layers, comparable to the gyro-radius of a Na + pickup ion entering the magnetosphere after being accelerated in the magnetosheath, may indicate a planetary ion boundary layer.

Broad regions on both sides of the solar wind termination shock are populated by high intensities of non-thermal ions and electrons. The pre-shock particles in the solar wind have been measured by the spacecraft Voyager 1 (refs 1-5) and Voyager 2 (refs 3, 6). The post-shock particles in the heliosheath have also been measured by Voyager 1 (refs 3-5). It was not clear, however, what effect these particles might have on the physics of the shock transition until Voyager 2 crossed the shock on 31 August-1 September 2007 (refs 7-9). Unlike Voyager 1, Voyager 2 is making plasma measurements. Data from the plasma and magnetic field instruments on Voyager 2 indicate that non-thermal ion distributions probably have key roles in mediating dynamical processes at the termination shock and in the heliosheath. Here we report that intensities of low-energy ions measured by Voyager 2 produce non-thermal partial ion pressures in the heliosheath that are comparable to (or exceed) both the thermal plasma pressures and the scalar magnetic field pressures. We conclude that these ions are the >0.028 MeV portion of the non-thermal ion distribution that determines the termination shock structure and the acceleration of which extracts a large fraction of bulk-flow kinetic energy from the incident solar wind.

The precise interstellar neutral (ISN) flow direction is important because of its strong leverage on the plane subtended by the ISN and magnetic field vectors, which controls the heliospheric shape and interaction with the interstellar medium. IBEX measurements provide a very precise relation between ISN flow longitude and speed via the hyperbolic trajectory equation, forming a 4-dimensional tube in the ISN parameter space, with substantially larger uncertainty along this tube and thus for the longitude alone. As demonstrated before, the interstellar pickup ion (PUI) cut-off speed is a function of the ratio of the radial ISN flow component and the solar wind speed at the observer location. The former is largest precisely upwind and decreases symmetrically with the angle from the upwind direction. Using this functional dependence and the observed solar wind speed, the PUI cut-off can be constructed solely as a function of the ISN flow longitude. From ACE SWICS and STEREO PLASTIC, data sets that span 18+ years are available. We will show, in particular, that by selecting observations for local interplanetary magnetic fields perpendicular to the solar wind and transforming the observed distributions into the solar wind frame, a comparison with data can be devised that is much less sensitive to PUI production and transport effects than methods that rely on pickup ion fluxes.

Comet McNaught was the brightest comet observed from Earth in the last 40 years. For a period of five days in early 2007 February, four instruments on the Ulysses spacecraft directly measured cometary ions and key properties of the interaction of the comet's ion tail with the high-speed solar wind from the polar regions of the Sun. Because of the record-breaking duration of the encounter, the data are unusually comprehensive. O3(+) ions were detected for the first time in a comet tail, coexisting with singly charged molecular ions with masses in the range 28-35 amu. The presence of magnetic turbulence and of ions with energies up to approximately 200 keV indicate that at a distance of approximately 1.6 AU from the comet nucleus, the ion tail McNaught had not yet reached equilibrium with the surrounding solar wind.

The Energetic Particle and Plasma Spectrometer (EPPS) package on the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission to Mercury is composed of two sensors, the Energetic Particle Spectrometer (EPS) and the Fast Imaging Plasma Spectrometer (FIPS). EPS measures the energy, angular, and compositional distributions of the high-energy components of the in situ electrons (>20 keV) and ions (>5 keV/nucleon), while FIPS measures the energy, angular, and compositional distributions of the low-energy components of the ion distributions (

Johannes Geiss is a world leader and foremost expert on measurements and interpretation of the composition of matter that reveals the history, present state, and future of astronomical objects. With his Swiss team he was first to measure the composition of the noble gases in the solar wind when in the late 1960s he flew the brilliant solar wind collecting foil experiments on the five Apollo missions to the moon. Always at the forefront of the art of composition measurements, he with his colleagues determined the isotopic and elemental composition of the solar wind using instruments characterized by innovative design that have provided the most comprehensive record of the solar wind composition under all solar wind conditions at all helio-latitudes. He discovered heavy interstellar pickup ions, from which the composition of the neutral gas of the Local Interstellar Cloud is determined, and the “Inner Source” of pickup ions. Johannes Geiss played a key role both in the in-situ measurements and modeling of molecular ions in comets, and the interpretation of these data. He and co-workers measured the composition of plasmas in the magnetospheres of Earth and Jupiter. Here we highlight Johannes Geiss’ many discoveries and seminal contributions to our knowledge of the composition of matter of the Sun, solar wind, interstellar gas, early universe, comets and magnetospheres.

Evolution and composition of baryonic matter is influenced by the evolution of other forms of matter and energy in the universe. At the time of primordial nucleosynthesis the universal expansion and thus the decrease of the density and temperature of baryonic matter were controlled by leptons and photons. Non-baryonic dark matter initiated the formation of clusters and galaxies, and to this day, dark matter largely determines the dynamics and geometries of these baryonic structures and indirectly influences their chemical evolution. Chemical analyses and isotopic abundance measurements in the solar system established the composition in the protosolar cloud (PSC). The abundances of nuclear species in the PSC led to the discovery of the magic numbers and the nuclear shell model, and they allowed the identification of nucleosynthetic sites and processes. To this day, we know the abundances of the ∼300 stable and long-lived nuclides infinitely better in the PSC than in any other sample of matter in the universe. Thus, we know the exact composition of a Galactic sample of intermediate age, allowing us to check on theories of Galactic evolution before and after the formation of the solar system. This paper specifically discusses the nucleosynthesis in the early universe and the Galactic evolution during the last 5 Gyr.

The solar wind charge state and elemental compositions have been measured with the Solar Wind Ion Composition Spectrometers (SWICS) on Ulysses and ACE for a combined period of about 25 years. This most extensive data set includes all varieties of solar wind flows and extends over more than one solar cycle. With SWICS the abundances of all charge states of He, C, N, O, Ne, Mg, Si, S, Ar and Fe can be reliably determined (when averaged over sufficiently long time periods) under any solar wind flow conditions. Here we report on results of our detailed analysis of the elemental composition and ionization states of the most unbiased solar wind from the polar coronal holes during solar minimum in 1994–1996, which includes new values for the abundance S, Ca and Ar and a more accurate determination of the 20Ne abundance. We find that in the solar minimum polar coronal hole solar wind the average freezing-in temperature is ∼1.1×106 K, increasing slightly with the mass of the ion. Using an extrapolation method we derive photospheric abundances from solar wind composition measurements. We suggest that our solar-wind-derived values should be used for the photospheric ratios of Ne/Fe=1.26±0.28 and Ar/Fe=0.030±0.007.

Observations in the solar wind have revealed important insights into how energetic particles are accelerated in astrophysical plasmas. In circumstances where stochastic acceleration is expected, a suprathermal tail on the distribution function is formed with a common spectral shape: the spectrum is a power law in particle speed with a spectral index of −5. Recent theories for this phenomenon, in which thermodynamic constraints are applied to explain the common spectral shape, are reviewed. As an example of potential extensions of this theoretical work, consideration is given to the acceleration of Anomalous Cosmic Rays in the heliosheath.

The combination of recent observational and theoretical work has completed the catalog of the sources of heliospheric Pickup Ions (PUIs). These PUIs are the seed population for Anomalous Cosmic Rays (ACRs), which are accelerated to high energies at or beyond the Termination Shock (TS). For elements with high First Ionization Potentials (high-FIP atoms: e.g., H, He, Ne, etc.), the dominant source of PUIs and ACRs is from neutral atoms that drift into the heliosphere from the Local Interstellar Medium (LISM) and, prior to ionization, are influenced primarily by solar gravitation and radiation pressure (for H). After ionization, these interstellar ions are pickup up by the solar wind, swept out, and are either accelerated near the TS or beyond it. Elements with low first ionization potentials (low-FIP atoms: e.g., C, Si, Mg, Fe, etc.) are also observed as PUIs by Ulysses and as ACRs by Wind and Voyager. But the low-FIP composition of this additional component reveals a very different origin. Low-FIP interstellar atoms are predominantly ionized in the LISM and therefore excluded from the heliosphere by the solar wind. Remarkably, a low-FIP component of PUIs was hypothesized by Banks (J. Geophys. Res. 76, 4341, 1971) over twenty years prior to its direct detection by Ulysses/SWICS (Geiss et al., J. Geophys. Res. 100(23), 373, 1995) The leading concept for the generation of Inner Source PUIs involves an effective recycling of solar wind on grains near the Sun, as originally suggested by Banks. Voyager and Wind also observe low-FIP ACRs, and a grain-related source appears likely and necessary. Two concepts have been proposed to explain these low-FIP ACRs: the first concept involves the acceleration of the Inner Source of PUIs, and the second involves a so-called Outer Source of PUIs generated from solar wind interaction with the large population of grains in the Kuiper Belt. We review here the observational and theoretical work over the last decade that shows how solar wind and heliospheric grains interact to produce pickup ions, and, in turn, anomalous cosmic rays. The inner and outer sources of pickup ions and anomalous cosmic rays exemplify dusty plasma interactions that are fundamental throughout the cosmos for the production of energetic particles and the formation of stellar systems.

For ~2.5 yr before the crossing of the termination shock of the solar wind, Voyager 1 observed a series of highly anisotropic beams, which are believed to result from downstream particles leaking upstream across a broad range of heliographic longitudes. We show that the downstream spectrum of the accelerated particles is a power law with spectral index of -5 when expressed as a distribution function over this same broad longitude range. We also show that the upstream beams contain too little heat flux to influence the shock structure or the processes occurring downstream. We argue that underlying the beams is an isotropic distribution of suprathermal particles, which also has a power-law spectrum with spectral index of -5, and we find that these conditions are consistent with a simple shock-heating model for the acceleration of energetic particles at the termination shock.

The Voyager 1 investigators have reported that the spacecraft crossed the termination shock of the solar wind at 94 AU from the Sun. The intensity of low-energy ions (3 MeV nucleon-1) increases abruptly at the shock. It is argued in this paper that the spectral shapes of the low-energy ions observed at the termination shock, both upstream and downstream, require that the pressure of the accelerated particles is behaving like that of a simple ideal gas, without heat flux, and that the intensity increase across the termination shock can be determined by assuming that the pressure of the accelerated particles behaves according to the Rankine-Hugoniot relationship. The approach taken in this paper is contrasted with diffusive shock acceleration. The implications of the conclusions from this work for the acceleration of suprathermal tails on the distribution of particles in the solar wind and for the origin of anomalous cosmic rays are discussed.

Suprathermal tails, formed in part from accelerated interstellar pickup ions, are observed ubiquitously in the solar wind. In quiet-time conditions, the tails are found to have a common spectral shape, a distribution function in velocity space that is a power law with a spectral index of -5. It is shown that such a spectral index is to be expected if the tails are formed by stochastic acceleration due to compressional turbulence in the solar wind, in which the particles are accelerated by, and do an equal amount of work on, the turbulence. The spectrum is formed by a cascade in energy, analogous to turbulent cascades.

Voyager 1 (V1) began measuring precursor energetic ions and electrons from the heliospheric termination shock (TS) in July 2002. During the ensuing 2.5 years, average particle intensities rose as V1 penetrated deeper into the energetic particle foreshock of the TS. Throughout 2004, V1 observed even larger, fluctuating intensities of ions from 40 kiloelectron volts (keV) to ≥50 megaelectron volts per nucleon and of electrons from >26 keV to ≥350 keV. On day 350 of 2004 (2004/350), V1 observed an intensity spike of ions and electrons that was followed by a sustained factor of 10 increase at the lowest energies and lesser increases at higher energies, larger than any intensities since V1 was at 15 astronomical units in 1982. The estimated solar wind radial flow speed was positive (outward) at ∼+100 kilometers per second (km s –1 ) from 2004/352 until 2005/018, when the radial flows became predominantly negative (sunward) and fluctuated between ∼–50 and 0 km s –1 until about 2005/110; they then became more positive, with recent values (2005/179) of ∼+50 km s –1 . The energetic proton spectrum averaged over the postshock period is apparently dominated by strongly heated interstellar pickup ions. We interpret these observations as evidence that V1 was crossed by the TS on 2004/351 (during a tracking gap) at 94.0 astronomical units, evidently as the shock was moving radially inward in response to decreasing solar wind ram pressure, and that V1 has remained in the heliosheath until at least mid-2005.

Over the past several decades, a number of groups have successfully developed and flown space instruments based on ultrathin (0.5–3.5 μg cm−2 or ∼2–17 nm nominal thickness) carbon foils suspended on fine metallic grids. These foils have proven to be remarkably robust in surviving the rigors of the launch and space environments and have functioned as required over long mission lifetimes. This study examines the scientific basis of possible failure mechanisms and the actual flight histories of ultrathin carbon foils. In all, we were able to document 138 separate foils used in 17 independent space instruments on 16 different spacecraft with a cumulative functioning flight history of 93 foil-years in space. All of these foils clearly survived their launches, and there is no evidence for any significant degradation over their various flight histories. This successful flight heritage, combined with laboratory data on foil survivability, shows that properly implemented ultrathin carbon foils are a mature and rel...

The helium gravitational focusing cone has been observed using pickup He + , first during the solar minimum in 1984-1985 with the AMPTE/IRM spacecraft, and again in more detail from 1998 to 2002 with ACE and in 2000 with Nozomi. Five traversals of the cone allow us to obtain an accurate determination of the ecliptic longitude of the interstellar wind flow direction, A = 74.43° ± 0.33°, while observations of pickup He ++ with Ulysses give us an estimate, relatively free of instrumental systematic uncertainties, of the neutral He density, n He = 0.0151 ± 0.0015 cm -3 , in the Local Interstellar Cloud. From best fits to the measured velocity distributions of pickup He + using time-stationary models we deduce the radial dependence and magnitude of electron-impact ionization rates that cannot presently be measured, and find this to be an important ionization process in the inner (≤0.5 AU) heliosphere. We obtain excellent model fits to the 1998 cone profile using measured or deduced rates and known interstellar He parameters, and from this conclude that cross-field diffusion of pickup He + is small. Furthermore, we find no evidence for extra sources of He in or near the cone region. Best fits to the velocity distributions of He + are obtained assuming isotropic solar-wind-frame distributions, and we conclude from this that the scattering mean free path for pickup He + in the turbulent slow solar wind is small, probably less than 0.1 AU. We argue that application of 3D, time-dependent models for computation of the spatial distribution of interstellar neutral helium in the inner heliosphere may lead to excellent fits of short-term averaged pickup He + data without assuming loss rates that are significantly different from production rates.

The magnetospheric imaging instrument (MIMI) is a neutral and charged particle detection system on the Cassini orbiter spacecraft designed to perform both global imaging and in-situ measurements to study the overall configuration and dynamics of Saturn’s magnetosphere and its interactions with the solar wind, Saturn’s atmosphere, Titan, and the icy satellites. The processes responsible for Saturn’s aurora will be investigated; a search will be performed for substorms at Saturn; and the origins of magnetospheric hot plasmas will be determined. Further, the Jovian magnetosphere and Io torus will be imaged during Jupiter flyby. The investigative approach is twofold. (1) Perform remote sensing of the magnetospheric energetic (E > 7 keV) ion plasmas by detecting and imaging charge-exchange neutrals, created when magnetospheric ions capture electrons from ambient neutral gas. Such escaping neutrals were detected by the Voyager 1 spacecraft outside Saturn’s magnetosphere and can be used like photons to form images of the emitting regions, as has been demonstrated at Earth. (2) Determine through in-situ measurements the 3-D particle distribution functions including ion composition and charge states (E > 3 keV/e). The combination of in-situ measurements with global images, together with analysis and interpretation techniques that include direct “forward modeling” and deconvolution by tomography, is expected to yield a global assessment of magnetospheric structure and dynamics, including (a) magnetospheric ring currents and hot plasma populations, (b) magnetic field distortions, (c) electric field configuration, (d) particle injection boundaries associated with magnetic storms and substorms, and (e) the connection of the magnetosphere to ionospheric altitudes. Titan and its torus will stand out in energetic neutral images throughout the Cassini orbit, and thus serve as a continuous remote probe of ion flux variations near 20R S (e.g., magnetopause crossings and substorm plasma injections). The Titan exosphere and its cometary interaction with magnetospheric plasmas will be imaged in detail on each flyby. The three principal sensors of MIMI consists of an ion and neutral camera (INCA), a charge-energy-mass-spectrometer (CHEMS) essentially identical to our instrument flown on the ISTP/Geotail spacecraft, and the low energy magnetospheric measurements system (LEMMS), an advanced design of one of our sensors flown on the Galileo spacecraft. The INCA head is a large geometry factor (G ~ 2.4 cm2 sr) foil time-of-flight (TOF) camera that separately registers the incident direction of either energetic neutral atoms (ENA) or ion species (≥5° full width half maximum) over the range 7 keV/nuc < E < 3 MeV/nuc. CHEMS uses electrostatic deflection, TOF, and energy measurement to determine ion energy, charge state, mass, and 3-D anisotropy in the range 3 ≤ E ≤ 220 keV/e with good (~0.05 cm2 sr) sensitivity. LEMMS is a two-ended telescope that measures ions in the range 0.03 ≤ E ≤ 18 MeV and electrons 0.015 ≤ E < 0.884 MeV in the forward direction (G ~ 0.02 cm2 sr), while high energy electrons (0.1–5 MeV) and ions (1.6–160 MeV) are measured from the back direction (G ~ 0.4 cm2 sr). The latter are relevant to inner magnetosphere studies of diffusion processes and satellite microsignatures as well as cosmic ray albedo neutron decay (CRAND). Our analyses of Voyager energetic neutral particle and Lyman-a measurements show that INCA will provide statistically significant global magnetospheric images from a distance of ~60 RS every 2–3 h (every ~10 min from ~20 RS). Moreover, during Titan flybys, INCA will provide images of the interaction of the Titan exosphere with the Saturn magnetosphere every 1.5 min. Time resolution for charged particle measurements can be

Ion tails of comets are known to extend radially away from the Sun over very large distances. Crossing these tails by spacecraft not specifically targeted to intercept them was believed to be extremely improbable, since that requires precise angular alignment of the spacecraft with a comet. We report here the fortuitous detection of cometary ions at large angular separation far from the comet. To explain this unexpected discovery, we conclude that these ions were ducted laterally along magnetic fields that were randomly distorted by a coronal mass ejection and that such transport increases the probability of an unplanned detection of comets.

In this Letter we report on our study of the filtration of interstellar atoms of hydrogen, oxygen and nitrogen in the interaction region between the solar wind and the local interstellar medium. The filtration has great importance for the deter- mination of local interstellar abundances of these elements, which becomes now possible due to measurements of interstellar pickup ions by Ulysses and ACE, and anomalous cosmic rays by Voyagers, Ulysses, ACE, SAMPEX and Wind. The filtra- tion of the different elements depends on the level of their coupling with the plasma in the interaction region. We study the dependence of the filtration on the local interstellar proton and H atom number densities and evaluate the effects of charge exchange and electron impact ionization on the filtration. We explore the influence of electron temperature in the inner he- liosheath on the filtration process. Using our filtration coefficients and SWICS/Ulysses pickup ion measurements we conclude nOI,LIC = (7.8 ± 1.3) × 10 −5 cm −3 and nNI,LIC = (1.1 ± 0.2) × 10 −5 cm −3 .

Since the rapid flybys of Mariner 10 in the late seventies, Mercury is known to have an intrinsic magnetic field that dominates the interactions of Mercury with its space environment. Even though important aspects of this interaction have been probed by the Mariner 10 instruments, crucial aspects remain unknown and unexplored. In particular there are fundamental uncertainties concerning the ion content of Mercury's magnetosphere as well as issues related to the expected time-dependence of its magnetosphere due to the forcing by the interplanetary environment or due to substorm-like behavior. This paper will focus on a few specific topics that are related to the anticipated in situ investigations of MESSENGER and Bepi-Colombo later this decade.

SOHO's detection of heliospheric energetic H and He atoms in the energy range ∼55–280 keV has led to simulations of the production of these particles in the heliosheath and inner heliosphere that could be compared with observation. Improved and extended observation of heliospheric energetic neutral atoms (ENA) in this energy range should be useful in at least three related areas of study. These are: (1) the acceleration of pickup ions in the inner heliosphere, then again at the termination shock to become anomalous cosmic rays, and their subsequent propagation in the heliosheath; (2) the anomalous cosmic ray spectrum at the termination shock; and (3) the latitudinal distribution of heliospheric shock structures. Future advances in this field will require a new generation of ENA spectrometers. Essential to all ENA spectrometers is an effective ion-rejecting collimator. Taking advantage of the lessons learned from the first generation of ENA instruments and new computer simulations, we report on a preliminary design for an ion-rejecting collimator suitable for second-generation instrument to monitor ENA of both inner and outer heliospheric origins at the readily accessible L1 location at 1 AU.

Pickup ions provide us with a powerful tool to probe remote regions in and beyond the heliosphere and determine the composition of the neutral gas in these regions. Interstellar pickup ions, observed inside the heliosphere at 3–5 AU from the Sun, are created by charge exchange and photoionization of the local interstellar gas that penetrates deep into the heliosphere. Comprehensive measurements of H, He, C, N, O, and Ne, especially with the Solar Wind Ion Composition Spectrometer (SWICS) on both the Ulysses and ACE spacecraft, are giving us a wealth of data that we use to infer the chemical and physical properties of the Local Interstellar Cloud (LIC). Using updated and improved measurements of pickup ions and state-of-the-art modeling, we find that in the LIC the proton and neutral hydrogen densities are respectively (0.061 ± 0.018) and (0.176 ± 0.019) cm −3 . The neutral helium density is (0.0154 ± 0.0015) cm −3 and the ionized helium density (0.0083 ± 0.0021) cm −3 , giving a LIC total gas density of hydrogen plus helium of (0.261 ± 0.026) cm −3 . Using these updated densities, the location of the heliospheric termination shock is calculated to be at about 96 AU in 2002, and the crossing of Voyager 1 through this shock is predicted to occur between 2003 and 2009. The metallicity of the gas in the LIC is found to be comparable to that of the Protosolar Cloud. This unexpected result may be explained by a recent model of quasi-continuous excess infall of extra-galactic material that is only moderately processed by stellar nucleosynthesis [Astrophys. J. 578 (2002) 862].

The outer limit of the Solar System is often considered to be at the distance from the Sun where the solar wind changes from supersonic to subsonic flow1. Theory predicts that a termination shock marks this boundary, with locations ranging2 from a few to over 100 au (1 au ≈ 1.5 × 108 km, the distance from Earth to the Sun). ‘Pick-up ions’ that originate3,4 as interstellar neutral atoms should be accelerated to tens of MeV at the termination shock, generating anomalous cosmic rays5,6,7. Here we report a large increase in the intensity of energetic particles in the outer heliosphere, as measured by an instrument on the Voyager 1 spacecraft. We argue that the spacecraft exited the supersonic solar wind and passed into the subsonic region (possibly beyond the termination shock) on about 1 August 2002 at a distance of ∼85 au (heliolatitude ∼34° N), then re-entered the supersonic solar wind about 200 days later at ∼87 au from the Sun. We show that the composition of the ions accelerated at the putative termination shock is that of anomalous cosmic rays and of interstellar pick-up ions.

Observations and measurements in the solar wind, the Jovian atmosphere and the gases trapped in lunar surface material provide the main evidence from which the isotopic composition of H, He and Ne in the Protosolar Cloud (PSC) is derived. These measurements and observations are reviewed and the corrections are discussed that are needed for obtaining from them the PSC isotopic ratios. The D/H,3He/4He (D+3He)/H,20Ne/22Ne and 21Ne/22Ne ratios adopted for the PSC are presented. Protosolar abundances provide the basis for the interpretation of isotopic ratios measured in the various solar system objects. In this article we discuss constraints derived from the PSC abundances on solar mixing, the origin of atmospheric neon, and the nature of the “SEP” component of neon trapped at the lunar surface. We also discuss constraints on the galactic evolution provided by the isotopic abundances of H and He in the PSC.

A significant fraction of the gas in the Galactic disk comes from quasi-continuous infall of external material that is moderately processed by stellar nucleosynthesis. Based on a comparison of the abundances in the protosolar cloud (PSC) and the Local Interstellar Cloud (LIC), we propose that the infalling matter carries the nucleosynthetic signature of dwarf galaxies. The dwarfs have low metallicity, and secondary products of stellar nucleosynthesis are scarce as compared to primary products. Using a simple model, we show that the differences found in the LIC and PSC compositions can be explained by adding material processed in dwarf galaxies to the material processed in the Galactic disk. Moreover, some previously unexplained composition observations in the Galaxy, such as the apparently anomalously high metallicity of the Sun, or the "18O puzzle" could be resolved in this way.