Composition of Anomalous Cosmic Rays

The “classic” anomalous cosmic ray (ACR) component originates as interstellar neutral atoms that drift into the heliosphere, become ionized and picked up by the solar wind, and carried to the outer heliosphere where the pickup ions are accelerated to hundreds of MeV, presumably at the solar wind termination shock. These interstellar ACRs are predominantly singly charged, although higher charge states are present and become dominant above ～350 MeV. Their isotopic composition is like that of the solar system and unlike that of the source of galactic cosmic rays. A comparison of their energy spectra with the estimated flux of pickup ions flowing into the termination shock reveals a mass-dependent acceleration efficiency that favors heavier ions. There is also a heliospheric ACR component as evidenced by “minor” ACR ions, such as Na, Mg, S, and Si that appear to be singly-ionized ions from a source likely in the outer heliosphere.     

The Solar Wind in the Outer Heliosphere and Heliosheath

The solar wind environment has a large influence on the transport of cosmic rays. This chapter discusses the observations of the solar wind plasma and magnetic field in the outer heliosphere and the heliosheath. In the supersonic solar wind, interaction regions with large magnetic fields form barriers to cosmic ray transport. This effect, the “CR-B” relationship, has been quantified and is shown to be valid everywhere inside the termination shock (TS). In the heliosheath, this relationship breaks down, perhaps because of a change in the nature of the turbulence. Turbulence is compressive in the heliosheath, whereas it was non-compressive in the solar wind. The plasma pressure in the outer heliosphere is dominated by the pickup ions which gain most of the flow energy at the TS. The heliosheath plasma and magnetic field are highly variable on scales as small as ten minutes. The plasma flow turns away from the nose roughly as predicted, but the radial speeds at Voyager 1 are much less than those at Voyager 2, which is not understood. Despite predictions to the contrary, magnetic reconnection is not an important process in the inner heliosheath with only one observed occurrence to date.     

The multifuid solar wind termination shock and its influence on the threedimensional plasma structure upstream and downstream

The solar wind termination shock is described as a multi-fluid phenomenon taking into account the magnetohydrodynamic self-interaction of a multispecies plasma consisting of solar wind ions, pick-up ions and shock-generated anomalous cosmic ray particles. The spatial diffusion of these high energy particles relative to the resulting, pressure-modified solar wind flow structure is described by a coupled system of differential equations describing mass-, momentum-, and energy-flow continuities for all plasma components. The energy loss due to escape of energetic particles (MeV) from the precursor into the inner heliosphere is taken into account. We determine the integrated properties of the anomalous cosmic ray gas and the low-energy solar wind. Also the variation of the compression ratio of the shock structure is quantitatively determined and is related to the pick-up ion energization efficiency and to the mean energy of the downstream anomalous cosmic ray particles. The variation of the resulting shock structure and of the solar wind sheath plasma extent beyond the shock is discussed with respect to its consequences for the LISM neutral gas filtration and the threedimensional shape of the heliosphere.     

Shock interactions in the outer heliosphere

Observations of plasma and magnetic fields by Pioneer 10 and 11 and Voyager 1 and 2 reveal that MHD shocks are an important component of the large-scale solar wind structures in the outer heliosphere. This review discusses recent progress in simulation studies of the nonlinear evolution of the solar wind structures, and in particular concentrates on the theoretical development and applications of the shock interactions model. Various stream propagation models, which do not use the Rankine-Hugoniot relations to calculate the jump conditions at shock crossings, have been used to simulate the essential evolution process of isolated streams and the formation and propagation of corotating and transient shocks. They produce fairly good results in the region up to a few AU. In 1984, the shock interactions model was introduced to study the evolution of large-scale solar wind structures in the region outside 1 AU up to several tens of AU. The model uses the exact Rankine-Hugoniot relations to calculate the shock speed and shock strength at all shock crossings. So that the model can more accurately calculate the shock speeds and the accumulated irreversible shock heating of plasma at several tens of AU. The applications of the shock interactions model are presented in three groups. (a) The first group covers the basic interaction of a shock with the ambient solar wind, the formation and propagation of shock pairs, and the collision and merging of shocks. (b) The second group covers the use of the shock interactions model to simulate the nonlinear evolution of large-scale solar wind structures in the outer heliosphere. These simulation results can provide the detailed evolution process for large-scale solar wind structures in the vast region not directly observed. Two selected studies are reported. (c) Finally, the shock interactions model is applied to studying the heating of the solar wind in the outer heliosphere. The model calculations support shocks being chiefly responsible for the heating of the solar wind plasma in the outer heliosphere at least up to 30 AU.     

Pickup protons in the heliosphere

We calculate the conditions of pickup protons inside the termination shock. Outside 50 AU the partial pressure of pickup protons is greater than the magnetic pressure by a factor of > 10, and greater than the partial pressure of solar wind protons by a factor of > 100. Thus, pickup protons have a significant dynamical influence on the structures of the solar wind in the outer heliosphere.     

Voyager observations of the magnetic field, interstellar pickup ions and solar wind in the distant heliosphere

Voyagers 1 and 2 are now observing the latitudinal structure of the heliospheric magnetic field in the distant heliosphere (the legion between - 30 AU and the termination shock). Voyager 2 is observing the influence of the interstellar medium on the solar wind. The pressure of the interstellar pickup protons, measured by their contribution to pressure balanced structures, is greater than or equal to the magnetic pressure and much greater than the thermal pressures of the solar wind protons and electrons in the distant heliosphere. The solar wind speed is observed to decrease and the proton temperature increase with increasing distance from the sun. This may result from the production of pickup ions by the charge exchange process with the interstellar neutrals. The introduction of the pickup ions into the dynamics of the magnetized solar wind plasma appears to be an important new process which must be considered in future theoretical studies of the termination shock and boundary with the local interstellar medium.     

The termination shock of the solar wind

An overview of the solar wind termination shock is presented including: its place in the heliosphere and its origin; its structure including the role of interstellar pickup ions and galactic and anomalous cosmic rays; its inferred location based on Lyman- backscatter, Voyager radio signals, and anomalous cosmic rays; its shape and movement.     

IBEX—Interstellar Boundary Explorer

The Interstellar Boundary Explorer (IBEX) is a small explorer mission that launched on 19 October 2008 with the sole, focused science objective to discover the global interaction between the solar wind and the interstellar medium. IBEX is designed to achieve this objective by answering four fundamental science questions: (1) What is the global strength and structure of the termination shock, (2) How are energetic protons accelerated at the termination shock, (3) What are the global properties of the solar wind flow beyond the termination shock and in the heliotail, and (4) How does the interstellar flow interact with the heliosphere beyond the heliopause? The answers to these questions rely on energy-resolved images of energetic neutral atoms (ENAs), which originate beyond the termination shock, in the inner heliosheath. To make these exploratory ENA observations IBEX carries two ultra-high sensitivity ENA cameras on a simple spinning spacecraft. IBEX’s very high apogee Earth orbit was achieved using a new and significantly enhanced method for launching small satellites; this orbit allows viewing of the outer heliosphere from beyond the Earth’s relatively bright magnetospheric ENA emissions. The combination of full-sky imaging and energy spectral measurements of ENAs over the range from ～10 eV to 6 keV provides the critical information to allow us to achieve our science objective and understand this global interaction for the first time. The IBEX mission was developed to provide the first global views of the Sun’s interstellar boundaries, unveiling the physics of the heliosphere’s interstellar interaction, providing a deeper understanding of the heliosphere and thereby astrospheres throughout the galaxy, and creating the opportunity to make even greater unanticipated discoveries.     

The Theory of Anomalous Cosmic Rays

Anomalous cosmic rays are a heliospheric phenomenon in which interstellar neutral atoms stream into the heliosphere, are ionized by either solar radiation or the solar wind, and are subsequently accelerated to very high energies, greater than 1 GeV. Current thinking has the bulk of the acceleration to very-high energies taking place, by the mechanism of diffusive shock acceleration, at the termination shock of the solar wind. Detailed two-dimensional numerical simulations and models based on this picture show broad agreement with a number of the observed properties of anomalous cosmic rays. Recent improvements to this picture include the observation of multiply charged cosmic rays and the suggestion that some "preacceleration" of the initially ionized particles occurs in the inner heliosphere.     

On the off-ecliptic distributions of anomalous cosmic rays and their relation to the large-scale geometry of the heliospheric shock

The scenario explaining the origin of the anomalous component of cosmic rays (ACR) implies a close relation between these high energy particles and the solar wind termination shock representing their main acceleration region. Consequently, one should expect the ACR distributions in the heliosphere to reflect some information about the structure as well as the large-scale geometry of the shock. We study the influence of a non-spherically symmetric heliospheric shock on the off-ecliptic — i.e. high latitude — ACR distributions using a two-dimensional model including their anisotropic diffusion and drift in the heliospheric magnetic field as well as a solar wind flow dependent on the heliographic latitude. The model calculations are used to investigate the probability of a possible polar elongation of the heliospheric shock from observations of the distributions of the ACR at high latitudes during solar minimum conditions.     

Galactic Cosmic Rays in the Outer Heliosphere: Theory and Models

We review recent advances in the field of galactic cosmic ray transport in the distant heliosphere. The advent of global MHD models brought about a better understanding of the three-dimensional structure of the interface between the solar system and the surrounding interstellar space, and of the magnetic field topology in the outer heliosphere. These results stimulated a development of galactic cosmic ray transport models taking the advantage of the available detailed plasma backgrounds and of the new Voyager results from the heliosheath. It emerges that the heliosheath plays a prominent role in the process of modulation and filtration of low-energy galactic ions and electrons. The heliosheath stores particles for a duration of several years thus acting as a large reservoir of galactic cosmic rays. Cosmic-ray trajectories, transit times, and entry locations across the heliopause are discussed. When compared to observations model calculations of low energy electrons show almost no radial gradient up to the termination shock, irrespective of solar activity, but a large gradient in the inner heliosheath. Intensities are however sensitive to heliospheric conditions such as the location of the heliopause and shock. In contrast, high energy proton observations by both the Voyager spacecraft show a clear solar cycle dependence with intensities also increasing with increasing distance. By comparing these observations to model calculations we can establish whether our current understanding of long-term modulation result in computed intensities compatible to observations.     

The Heliospheric Termination Shock

The heliospheric termination shock is a vast, spheroidal shock wave marking the transition from the supersonic solar wind to the slower flow in the heliosheath, in response to the pressure of the interstellar medium. It is one of the most-important boundaries in the outer heliosphere. It affects energetic particles strongly and for this reason is a significant factor in the effects of the Sun on Galactic cosmic rays. This paper summarizes the general properties and overall large-scale structure and motions of the termination shock. Observations over the past several years, both in situ and remote, have dramatically revised our understanding of the shock. The consensus now is that the shock is quite blunt, is with the front, blunt side canted at an angle to the flow direction of the local interstellar plasma relative to the Sun, and is dynamical and turbulent. Much of this new understanding has come from remote observations of energetic charged particles interacting with the shock, radio waves and radiation backscattered from interstellar neutral atoms. The observations and the implications are discussed.     

Local Interstellar Parameters as They Are Inferred from Analysis of Observations Inside the Heliosphere

This paper provides a brief summary on the current knowledge of the properties of the Circum-Heliospheric Interstellar Medium (CHISM). It discusses what can be learnt on the parameters of CHISM’s components from analysis of measurements performed inside the heliosphere. The analysis is based on the kinetic-gasdynamic models of the solar wind/interstellar medium interaction. We focus the analysis on three types of diagnostics: 1) interstellar H atom number density at the heliospheric termination shock inferred from pickup ion measurements, 2) the location and time of the Voyager 1 and 2 termination shock crossings, 3) the deflection of the interstellar H atom flow inside the heliosphere as been measured by SOHO/SWAN. From these results estimations of the unknown local interstellar parameters are deduced. The parameters are the number densities of interstellar H⁺ and H and the magnitude and direction of the interstellar magnetic field in the vicinity of the solar system.     

Particle acceleration at the Sun and in the heliosphere

Energetic particles are accelerated in rich profusion at sites throughout the heliosphere. They come from solar flares in the low corona, from shock waves driven outward by coronal mass ejections (CMEs), from planetary magnetospheres and bow shocks. They come from corotating interaction regions (CIRs) produced by high-speed streams in the solar wind, and from the heliospheric termination shock at the outer edge of the heliospheric cavity. We sample many populations near Earth, but can distinguish them readily by their element and isotope abundances, ionization states, energy spectra, angular distributions and time behavior. Remote spacecraft have probed the spatial distributions of the particles and examined new sources in situ. Most acceleration sources can be ‘seen’ only by direct observation of the particles; few photons are produced at these sites. Wave-particle interactions are an essential feature in acceleration sources and, for shock acceleration, new evidence of energetic-proton-generated waves has come from abundance variations and from local cross-field scattering. Element abundances often tell us the physics of the source plasma itself, prior to acceleration. By comparing different populations, we learn more about the sources, and about the physics of acceleration and transport, than we can possibly learn from one source alone. This revised version was published online in August 2006 with corrections to the Cover Date.     

The Heliospheric Magnetic Field and Its Extension to the Inner Heliosheath

The general structure of the heliospheric magnetic field is well known and has been extensively studied, mostly in the inner heliosphere, out to the orbit of Saturn. Beyond 10 AU, the Pioneer and now the Voyager spacecraft have provided a view of the outer heliosphere. Its structure is strongly affected by large-scale phenomena originating in the Sun’s activity, such as the pattern of fast and slow solar wind streams around solar minimum that lead to Corotating Interaction Regions, and the increased frequency and strength of Coronal Mass Ejections around solar maximum. The large current sheet that separates the dominant magnetic polarities in the heliospheric medium, the Heliospheric Current Sheet, provides a variable structure that evolves from a relatively simple geometry close to the solar equatorial plane to what is likely to be a highly complex and dynamic surface reaching to high heliolatitudes at high levels of solar activity. The magnetic field observed in a fluctuating, dynamical heliosheath differs considerably from that in a static heliosheath. In particular, the time between current-sheet crossings (sectors) is quite sensitive to the radial speed of the solar-wind termination shock. If an inwardly moving termination shock moves past an observer on a slowly moving spacecraft, the time between current-sheet crossings in the heliosheath becomes larger, and can become very large, for reasonably expected inward shock speeds. This effect may help to explain recent observations of the magnetic field from the Voyager 1 spacecraft, where, in the heliosheath, the magnetic field remained directed outward from the Sun for several months without a current-sheet crossing. The crossings finally resumed and now occur somewhat regularly. In addition, the magnetic fluctuations in the heliosheath are observed to be quite different from those in the supersonic upstream solar wind.     

Heliospheric Lessons for Galactic Cosmic-ray Acceleration

The heliosphere is bathed in the supersonic solar wind, which generally creates shocks at any obstacles it encounters: magnetic structures such as coronal mass ejections and planetary magnetospheres, or fast-slow stream interactions such as corotating interaction regions (CIRs) or the termination shock. Each of these shock structures has an associated energetic particle population whose spectra and composition contain clues to the acceleration process and the sources of the particles. Over the past several years, the solar wind composition has been systematically studied, and the long-standing gap between high energy (>1 MeV amu^–1) and the plasma ion populations has been closed by instruments capable of measuring the suprathermal ion composition. In CIRs, where it has been possible to observe all the relevant populations, it turns out that the suprathermal ion population near 1.8–2.5 times the solar wind speed is the seed population that gets accelerated, not the bulk particles near the solar wind peak. These new results are of interest to the problem of Galactic Cosmic-Ray (GCR) Acceleration, since the injection and acceleration of GCRs to modest energies is likely to share many features with processes we can observe in detail in the heliosphere.     

Shock Acceleration of Ions in the Heliosphere

Energetic particles constitute an important component of the heliospheric plasma environment. They range from solar energetic particles in the inner heliosphere to the anomalous cosmic rays accelerated at the interface of the heliosphere with the local interstellar medium. Although stochastic acceleration by fluctuating electric fields and processes associated with magnetic reconnection may account for some of the particle populations, the majority are accelerated by the variety of shock waves present in the solar wind. This review focuses on “gradual” solar energetic particle (SEP) events including their energetic storm particle (ESP) phase, which is observed if and when an associated shock wave passes Earth. Gradual SEP events are the intense long-duration events responsible for most space weather disturbances of Earth’s magnetosphere and upper atmosphere. The major characteristics of gradual SEP events are first described including their association with shocks and coronal mass ejections (CMEs), their ion composition, and their energy spectra. In the context of acceleration mechanisms in general, the acceleration mechanism responsible for SEP events, diffusive shock acceleration, is then described in some detail including its predictions for a planar stationary shock, shock modification by the energetic particles, and wave excitation by the accelerating ions. Finally, some complexities of shock acceleration are addressed, which affect the predictive ability of the theory. These include the role of temporal and spatial variations, the distinction between the plasma and wave compression ratios at the shock, the injection of thermal plasma at the shock into the process of shock acceleration, and the nonlinear evolution of ion-excited waves in the vicinity of the shock.     

The Acceleration Mechanism of Anomalous Cosmic Rays

This paper reviews our current understanding of the acceleration mechanism of anomalous cosmic rays (ACRs). ACRs were first discovered in the early 1970s and soon afterwards it was recognized that they were accelerated interstellar pickup ions that obtained most of their energization in the outer heliosphere. Their observed composition and charge state suggest they are accelerated to over 200 MeV total energy in about a year. Diffusive shock acceleration at the solar-wind termination shock, which provided a natural explanation for spacecraft observations prior to the Voyager crossings of the termination shock in 2004 and 2007, was the long-held paradigm for the acceleration mechanism. But when both Voyagers crossed the shock, the ACR energy spectrum remained modulated, suggesting a source more distant than the shock. While shock acceleration remains a popular mechanism, other ideas have emerged recently to explain the observations. This review focuses on three main acceleration mechanisms that have been proposed: (a) acceleration at the termination shock including new effects such as the global blunt-shape of the shock and large-scale turbulence, (b) acceleration by magnetic reconnection in the heliosheath, and (c) acceleration by diffusive compression acceleration in the heliosheath.     

The anomalous component of cosmic rays in the 3-D heliosphere

More than 20 years ago, in 1972, anomalous flux increases of helium and heavy ions were discovered during solar quiet times. These flux increases in the energy range<50 MeV/nucleon showed peculiar elemental abundances and energy spectra, e.g. a C/O ratio0.1 around 10 MeV/nucleon, different from the abundances of solar energetic particles and galactic cosmic rays. Since then, this anomalous cosmic ray component (ACR) has been studied extensively and at least six elements have been found (He,N,O,Ne,Ar,C) whose energy spectra show anomalous increases above the quiet time solar and galactic energetic particle spectrum. There have been a number of models proposed to explain the ACR component. The presently most plausible theory for the origin of ACR ions identifies neutral interstellar gas as the source material. After penetration into the inner heliosphere, the neutral particles are ionized by solar UV radiation and by charge exchange reactions with the solar wind protons. After ionization, the now singly charged ions are picked up by the interplanetary magnetic field and are then convected with the solar wind to the outer solar system. There, the ions are accelerated to high energies, possibly at the solar wind termination shock, and then propagate back into the inner heliosphere. A unique prediction of this model is that ACR ions should be singly ionized. Meanwhile, several predictions of this model have been verified, e.g. low energy pick-up ions have been detected and the single charge of ACR ions in the energy range at MeV/nucleon has been observed. However, some important aspects such as, for example, the importance of drift effects for the acceleration and propagation process and the location of the acceleration site are still under debate. In this paper the present status of experimental and theoretical results on the ACR component are reviewed and constraints on the acceleration process derived from the newly available ACR ionic charge measurements will be presented. Possible new constraints provided by correlative measurements at high and low latitudes during the upcoming solar pole passes of the ULYSSES spacecraft in 1994 and 1995 will be discussed.     

Composition of anomalous cosmic rays and implications for the heliosphere

We use energy spectra of anomalous cosmic rays (ACRs) measured with the Cosmic Ray instrument on the Voyager 1 and 2 spacecraft during the period 1994/157-313 to determine several parameters of interest to heliospheric studies. We estimate that the strength of the solar wind termination shock is 2.42 (–0.08, +0.04). We determine the composition of ACRs by estimating their differential energy spectra at the shock and find the following abundance ratios: H/He = 5.6 (–0.5, ⁺0.6), C/He = 0.00048 ± 0.00011, N/He = 0.011 ± 0.001, O/He = 0.075 ± 0.006, and Ne/He = 0.0050 ± 0.0004. We correlate our observations with those of pickup ions to deduce that the long-term ionization rate of neutral nitrogen at 1 AU is 8.3 × 10^–7 s^–1 and that the charge-exchange cross section for neutral N and solar wind protons is 1.0 × 10^–15 cm² at 1.1 keV. We estimate that the neutral C/He ratio in the outer heliosphere is 1.8(–0.7, +0.9) × 10^–5. We also find that heavy ions are preferentially injected into the acceleration process at the termination shock.