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.     

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.     

Physical Processes in the Outer Heliosphere

This chapter covers the theory of physical processes in the outer heliosphere that are particularly important for the IBEX Mission, excluding global magnetohydrodynamic/Boltzmann modeling of the entire heliosphere. Topics addressed include the structure and parameters of the solar wind termination shock, the transmission of ions through the termination shock including possible reflections at the shock electrostatic potential, the acceleration and transport of suprathermal ions and anomalous cosmic rays at the termination shock and in the heliosheath, charge-exchange interactions in the outer heliosphere including mass and momentum loading of the solar wind, the transport of interstellar pickup ions, and the production and anticipated intensities of energetic neutral atoms (ENAs) in the heliosphere.     

Voyager Observations of the Magnetic Field in the Distant Heliosphere

The latitudinal structure of the heliospheric magnetic field during much of the solar cycle is determined by a "sector zone", in which both positive and negative magnetic polarities are observed, and by the unipolar regions above and below the sector zone. Distinct corotating streams and interactions regions are found primarily in the sector zone during the declining phase of the solar cycle. Within a few AU, the streams and interaction regions are distinct and are related to solar features. A restructuring of the solar wind occurs between 1 AU and 15 AU, in which the isolated streams, interaction regions and shocks merge to form compound streams and merged interaction regions ("MIRs"). Memory of the source conditions is lost in this process. In the region between 30 AU and the termination shock (the "distant heliosphere"), the pressure of interstellar pickup protons dominates that of the magnetic field and solar wind particles and largely controls the dynamical processes. During 1983 and 1994, corotating streams and corotating interaction regions were observed at 1 AU. Merged interaction regions were observed at 15 AU in 1983, but not at 45 AU during 1994. This result suggests a further restructuring of the solar wind in the distant heliosphere, but variations from one solar cycle to the next might also contribute to the result. Approaching solar minimum in 1996, the latitudinal extent of the sector zone decreased, and Voyager 2 gradually entered the unipolar region below it. The speed was lower in the sector zone than below it. At Voyagers 1 and 2, the change in cosmic ray intensity is related to the magnetic field strength during each year from 1983 through 1996. The magnetic field strength has a multifractal distribution throughout the heliosphere. This fundamental symmetry of the heliosphere has not been incorporated explicitly in cosmic ray propagation models.     

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.     

The Solar Wind in the Outer Heliosphere

The solar wind evolves as it moves outward due to interactions with both itself and with the circum-heliospheric interstellar medium. The speed is, on average, constant out to 30 AU, then starts a slow decrease due to the pickup of interstellar neutrals. These neutrals reduce the solar wind speed by about 20% before the termination shock (TS). The pickup ions heat the thermal plasma so that the solar wind temperature increases outside 20–30 AU. Solar cycle effects are important; the solar wind pressure changes by a factor of 2 over a solar cycle and the structure of the solar wind is modified by interplanetary coronal mass ejections (ICMEs) near solar maximum. The first direct evidences of the TS were the observations of streaming energetic particles by both Voyagers 1 and 2 beginning about 2 years before their respective TS crossings. The second evidence was a slowdown in solar wind speed commencing 80 days before Voyager 2 crossed the TS. The TS was a weak, quasi-perpendicular shock which transferred the solar wind flow energy mainly to the pickup ions. The heliosheath has large fluctuations in the plasma and magnetic field on time scales of minutes to days.     

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.     

Pioneer and Voyager observations of large-scale spatial and temporal variations in the solar wind

The Pioneer 10, Pioneer 11, and Voyager 2 spacecraft were launched in 1972, 1974, and 1977, respectively. While these three spacecraft are all at compartively low heliographic latitudes compared with Ulysses, their observation span almost two solar cycles, a range of heliocentric distances from 1 to 57 AU, and provide a unique insight into the long-term variability of the global structure of the solar wind. We examine the spatial and temporal variation of average solar wind parameters and fluxes. Our obsevations suggest that the global structure of the outer heliosphere during the declining phase of the solar cycle at heliographic latitudes up to 17.5°N was charaterized by two competing phenomena: 1) a large-scale increase of solar wind density, temperature, mass flux, dynamic pressure, kinetic energy flux, and thermal enery flux with heliographic latitude, similar to the large-scale latitudinal gradient of velocity seen in IPS observations, 2) a small-scale decrease in velocity and temperature, and increase in density near the heliospheric current sheet, which is associated with a band of low speed, low temperature, and high density solar wind similar to that observed in the inner heliosphere.     

Emission mechanism for low-frequency radiation in the outer heliosphere

The question of how low-frequency radio emissions in the outer heliosphere might be generated is considered. It is argued that the free energy contained in an electron beam distribution is first transformed into electrostatic Langmuir waves. The nonlinear interactions of these waves which can produce electromagnetic waves are then treated in the semi-classical formalism. Comparison of the results of the discussed model with electromagnetic radiation coming from upstream of the Earth's bow shock shows that the model adequately explains the generation of plasma waves at planetary shocks. By analogy, this model can provide a quantitative explanation of intensity of radio emissions at 2 to 3 kHz detected by the Voyager plasma wave instrument in the outer heliosphere provided that the electron beams generating Langmuir waves exist also in the postshock plasma due to secondary shocks in the compressed solar wind beyond the termination shock. The field strength of Langmuir waves required to generate the second harmonic emissions are approximately of 100–200 V m^–1 for the primary and 50–100 V m^–1 for the secondary foreshocks. However, only in the secondary foreshock the expected density is consistent with the observed frequency.     

Fields and plasmas in the outer solar system

The most significant information about fields and plasmas in the outer solar system, based on observations by Pioneer 10 and 11 investigations, is reviewed. The characteristic evolution of solar wind streams beyond 1 AU has been observed. The region within which the velocity increases continuously near 1 AU is replaced at larger distances by a thick interaction region with abrupt jumps in the solar wind speed at the leading and trailing edges. These abrupt increases, accompanied by corresponding jumps in the field magnitude and in the solar wind density and temperature, consist typically of a forward and a reverse shock. The existence of two distinct corotating regions, separated by sharp boundaries, is a characteristic feature of the interplanetary medium in the outer solar system. Within the interaction regions, compression effects are dominant and the field strength, plasma density, plasma temperature and the level of fluctuations are enhanced. Within the intervening quiet regions, rarefaction effects dominate and the field magnitude, solar wind density and fluctuation level are very low. These changes in the structure of interplanetary space have significant consequences for the many energetic particles propagating through the medium. The interaction regions control the access to the inner solar system of relativistic electrons from Jupiter's magnetosphere. The interaction regions and shocks appear to be associated with an acceleration of solar protons to MeV energies. Flare-generated shocks are observed to be propagating through the outer solar system with constant speed, implying that the previously recognized deceleration of flare shocks takes place principally near the Sun. Radial gradients in the solar wind and interplanetary field parameters have been determined. The solar wind speed is nearly constant between 1 and 5 AU with only a slight deceleration of 30 km s⁺¹ on the average. The proton flux follows an r ⁺² dependence reasonably well, however, the proton density shows a larger departure from this dependence. The proton temperature decreases steadily from 1 to 5 AU and the solar wind protons are slightly hotter than anticipated for an adiabatic expansion. The radial component of the interplanetary field falls off like r ⁺² and, on the average, the magnitude and spiral angle also agree reasonably well with theory. However, there is evidence, principally within quiet regions, of a significant departure of the azimuthal field component and the field magnitude from simple theoretical models. Pioneer 11 has obtained information up to heliographic latitudes of 16°. Observations of the interplanetary sector structure show that the polarity of the field becomes gradually more positive, corresponding to outward-directed fields at the Sun, and at the highest latitudes the sector structure disappears. These results confirm a prior suspicion that magnetic sectors are associated with an interplanetary current sheet surrounding the Sun which is inclined slightly to the solar equator.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

MHD processes in the outer heliosphere

Conclusion Much has been learned about the structure and dynamics of the outer heliosphere during the last decade as a result observations from the Voyager and Pioneer spacecraft. The large scale of the observations forces one to consider the heliosphere from a new perspective, to think of new dynamical processes, and to introduce new concepts. The early studies of isolated gas dynamic flows must be replaced by MHD dynamics of interacting flows and flow systems. The simple deterministic models that have been dominant in early studies of the solar wind are now seen to have limited applicability, and statistical approaches are being developed. New concepts that have been introduced, such as inverse cascades, filtering, entrainment, etc., must be further explored and clarified, to make them more precise and quantitative. MHD turbulence is probably very important in solar wind dynamics, but the subject is poorly developed from a theoretical point of view. The statistical analysis of solar wind parameters has scarcely begun, but it is clearly necessary for an understanding of complex, large-scale flows. The multitude of possible interactions among shocks and flows of various types needs to be explored systematically with observations, models and analytical theory. Voyagers 1 and 2 and Pioneers 10 and 11 are continuing to move through the outer heliosphere and gather data. The lengthy data reduction procedures require even more care in dealing with the low field strengths, densities and temperatures at large heliocentric distances, and the analysis of the complex flows and fields in the outer heliosphere becomes increasingly difficult. Thus one can expect continued growth of our knowledge of the heliosphere, but comprehensive understanding of the data will take some time. If this review stimulates the specialists in solar wind physics to think critically about the results presented and to remedy the deficiencies of current knowledge of the heliosphere, then it will have served its purpose. It is also hoped that this review will serve to encourage specialists in other fields to bring their talents to bear on heliospheric problems and to transfer results of heliospheric physics to their fields.     

The solar wind: A turbulent magnetohydrodynamic medium

MHD turbulence properties in the solar wind are briefly reviewed. The evolution of fluctuations of alfvénic type in near-ecliptic regions of the inner heliosphere is described. The role of interplanetary sources and the influence of interactions with structures convected by the solar wind are discussed. Turbulence features at high latitudes and in the outermost regions of the heliosphere are finally highlighted.     

ICMEs in the Inner Heliosphere: Origin, Evolution and Propagation Effects

This report assesses the current status of research relating the origin at the Sun, the evolution through the inner heliosphere and the effects on the inner heliosphere of the interplanetary counterparts of coronal mass ejections (ICMEs). The signatures of ICMEs measured by in-situ spacecraft are determined both by the physical processes associated with their origin in the low corona, as observed by space-borne coronagraphs, and by the physical processes occurring as the ICMEs propagate out through the inner heliosphere, interacting with the ambient solar wind. The solar and in-situ observations are discussed as are efforts to model the evolution of ICMEs from the Sun out to 1 AU.     

Radio emissions from the outer heliosphere

For nearly fifteen years the Voyager 1 and 2 spacecraft have been detecting an unusual radio emission in the outer heliosphere in the frequency range from about 2 to 3 kHz, Two major events have been observed, the first in 1983–84 and the second in 1992–93. In both cases the onset of the radio emission occurred about 400 days after a period of intense solar activity, the first in mid-July 1982, and the second in May–June 1991. These two periods of solar activity produced the two deepest cosmic ray Forbush decreases ever observed. Forbush decreases are indicative of a system of strong shocks and associated disturbances propagating outward through the heliosphere. The radio emission is believed to have been produced when this system of shocks and disturbances interacted with one of the outer boundaries of the heliosphere, most likely in the vicinity of the the heliopause. The emission is believed to be generated by the shock-driven Langmuir-wave mode conversion mechanism, which produces radiation at the plasma frequency (f _p ) and at twice the plasma frequency (2f _p ). From the 400-day travel time and the known speed of the shocks, the distance to the interaction region can be computed, and is estimated to be in the range from about 110 to 160 AU.Abbreviations PWS Plasma Wave Subsystem - AU Astronomical Unit - DSN Deep Space Network - NASA National Aeronautics and Space Administration - GMIR Global Merged Interaction Region - MHD Magnetohydrodynamic - CME coronal mass ejection - f _p plasma frequency - R radial distance - AGC automatic gain control     

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.     

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.     

Interplanetary Shocks and Solar Wind Structure Approaching Solar Maximum: Helios,IMP-8 and Voyager Observations

We studied solar wind observations of five different spacecraft: Helios 1, Helios 2, IMP-8, Voyager 1 and Voyager 2, from November 1977 to February 1978. In this period the large-scale dynamics of the solar wind near of the ecliptic plane was characterized by transient forward shocks (TFSs), ejecta, unstable corotating interaction regions (CIRs), and complex and variable magnetic sector structures. We identified 12 forward shock events of different origin. We did not find any clear tendency of the shock parameters with heliocentric distance nor longitudinal angle, but comparing the observations of each shock event we found local variations in the shock strength and the mean propagation velocities from one spacecraft to another. These unsystematic variations indicate that there were local deformations of the shock fronts, which we attribute to the inhomogenuos solar wind structure that affects the shock propagation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Solar Imprint on ICMEs, Their Magnetic Connectivity, and Heliospheric Evolution

Interplanetary outflows from coronal mass ejections (ICMEs) are structures shaped by their magnetic fields. Sometimes these fields are highly ordered and reflect properties of the solar magnetic field. Field lines emerging in CMEs are presumably connected to the Sun at both ends, but about half lose their connection at one end by the time they are observed in ICMEs. All must eventually lose one connection in order to prevent a build-up of flux in the heliosphere; but since little change is observed between 1 AU and 5 AU, this process may take months to years to complete. As ICMEs propagate out into the heliosphere, they kinematically elongate in angular extent, expand from higher pressure within, distort owing to inhomogeneous solar wind structure, and can compress the ambient solar wind, depending upon their relative speed. Their magnetic fields may reconnect with solar wind fields or those of other ICMEs with which they interact, creating complicated signatures in spacecraft data.     

The Solar Wind - Inner Heliosphere

The solar wind in the inner heliosphere, inside ~ 5 AU, has been almost fully characterized by the addition of the high heliographic latitude Ulysses mission to the many low latitude inner heliosphere missions that preceded it. The two major omissions are the high latitude solar wind at solar maximum, which will be measured during the second Ulysses polar passages, and the solar wind near the Sun, which could be analyzed by a Solar Probe mission. Here, existing knowledge of the global solar wind in the inner heliosphere is summarized in the context of the new results from Ulysses.