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.     

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.     

Observations of interplanetary shocks: Recent progress

Interplanetary shock observations since the prior Solar Terrestrial Physics Symposium in 1978 are reviewed. Since the interval coincides with the recent solar maximum, emphasis is placed on shocks associated with transient solar phenomena, including coronal transients and eruptive prominences as well as flares. A good correlation between shocks and Storm Sudden Commencements has persisted into the recent maximum. Shocks have been identified that are associated with disappearing filaments and coronal transients rather than with flares. Significant progress has been made in the indirect observation of shocks near the Sun as a result of radio wave measurements in interplanetary space and measurement of the scintillation and spectral broadening of spacecraft radio transmissions. Preliminary results regarding the thickness of interplanetary shocks have appeared. Several quasi-parallel shocks propagating more nearly along, rather than across, the magnetic field have been identified. The plasma drivers accompanying interplanetary shocks have received increased attention and distinctive features have been found in electron, ion and magnetic field data.     

Interplanetary shock waves: Ulysses observations in and out of the ecliptic plane

Between its launch in October 1990 and the end of 1993, approximately 160 fast collisionless shock waves were observed in the solar wind by the Ulysses space probe. During the in-ecliptic part of the mission, to February 1992, the observed shock waves were first caused mainly by solar transient events following the solar maximum and the reorganisation of the large scale coronal fields. With the decay in solar activity, relatively stable Corotating Interaction Regions (CIRs) were observed betwen 3 and 5.4 AU, each associated with at least one forwardreverse shock pair. During the out-of-ecliptic phase of the orbit, from February 1992 onwards, CIRs and shock pairs associated with them continued to dominate the observations. From July 1992, Ulysses encountered the fast solar wind flow from the newly developed southern polar coronal hole, and from May 1993 remained in the unipolar magnetic region associated with this coronal hole. At latitudes beyond 30°, CIRs were associated almost exclusively with reverse shocks only. A comprehensive list of shock waves identified in the magnetic field and solar wind plasma data from Ulysses is given in Table 1. The principal characteristics were determined mainly from the magnetic field data. General considerations concerning the determination of shock characteristics are outlined in the Introduction.     

The August 1972 solar-terrestrial events: Interplanetary magnetic field observations

A review is presented of the interplanetary magnetic field observations acquired in early August 1972 when four solar flares erupted in McMath Plage region 11976. Measurements of the interplanetary field were obtained by Earth satellites, HEOS-2 and Explorer 41, and by Pioneers 9 and 10 which, by good fortune, were radially aligned and only 45° east of the Earth-Sun direction. In response to the four flares, four interplanetary shocks were seen at Earth and at Pioneer 9, which was then at a heliocentric distance of 0.78 AU. However, at Pioneer 10, which was 2.2 AU from the Sun, only two forward shocks and one reverse shock were seen. The available magnetic field data acquired in the vicinity of the shocks are presented. Efforts to identify corresponding shocks at the several locations and to deduce their velocities of propagation between 0.8 and 2.2 AU are reviewed. The early studies were based on average velocities between the Sun and Pioneer 9, the Sun and Earth and the Sun and Pioneer 10. A large deceleration of the shocks between the Sun and 0.8 AU as well as between 0.8 and 2.2 AU was inferred. More recently the local velocities of the shocks at Pioneers 9 and 10 have become available. A comparison of these velocities shows little, if any, deceleration between 0.8 and 2.2 AU and implies that most or all of the deceleration actually occurred nearer the Sun. Evidence is also presented that shows a significant departure of the flare-generated shock fronts from spherical symmetry.     

Latitudinal and radial variation of shock associated ≥30 keV ion spectra and anisotropies at Voyagers 1 and 2

The spectra and anisotropies of ions 30 keV have been measured by the Low Energy Charged Particle experiment on Voyagers 1 and 2 in the vicinity of interplanetary shocks between radial distances of 1–55 AU and heliographic latitudes 11° S-32° N. The spectra and anisotropies associated with a recent corotating (CIR) event at low latitude observed at Voyager 2 (36.6 AU, –9°) are similar to those of another event at high latitude observed at Voyager 1 (49.8 AU, 33.5°). An earlier CIR event observed at Voyager 2 (14 AU) associated with the previous solar cycle produced spectra and anisotropies remarkably similar to the more recent events. The anisotropies are used to calculate the solar wind velocity downstream of shocks where possible using the Compton-Getting effect, allowing the determination of previously unknown velocities at the locations of Voyager 1. For the large shock event observed at Voyagers 1 (38 AU, 30°) and 2 (29 AU, 3°) in mid-1989, the postshock spectra and anisotropies are well described by convected power law distributions. The Voyager 1 and 2 postshock spectra 4 days after the shock passage are nearly identical. The preshock anisotropies at low energy are similar, despite differences in the magnetic field orientation and the low energy spectrum. We find that the 30 keV ion anisotropies are generally well described by convective distributions downstream but not in the upstream region for shocks and many other shock events at Voyagers 1 and 2.     

S/WAVES: The Radio and Plasma Wave Investigation on the STEREO Mission

This paper introduces and describes the radio and plasma wave investigation on the STEREO Mission: STEREO/WAVES or S/WAVES. The S/WAVES instrument includes a suite of state-of-the-art experiments that provide comprehensive measurements of the three components of the fluctuating electric field from a fraction of a hertz up to 16 MHz, plus a single frequency channel near 30 MHz. The instrument has a direction finding or goniopolarimetry capability to perform 3D localization and tracking of radio emissions associated with streams of energetic electrons and shock waves associated with Coronal Mass Ejections (CMEs). The scientific objectives include: (i) remote observation and measurement of radio waves excited by energetic particles throughout the 3D heliosphere that are associated with the CMEs and with solar flare phenomena, and (ii) in-situ measurement of the properties of CMEs and interplanetary shocks, such as their electron density and temperature and the associated plasma waves near 1 Astronomical Unit (AU). Two companion papers provide details on specific aspects of the S/WAVES instrument, namely the electric antenna system (Bale et al., Space Sci. Rev., 2007) and the direction finding technique (Cecconi et al., Space Sci. Rev., 2007).     

The Electric Antennas for the STEREO/WAVES Experiment

The STEREO/WAVES experiment is designed to measure the electric component of radio emission from interplanetary radio bursts and in situ plasma waves and fluctuations in the solar wind. Interplanetary radio bursts are generated from electron beams at interplanetary shocks and solar flares and are observed from near the Sun to 1 AU, corresponding to frequencies of approximately 16 MHz to 10 kHz. In situ plasma waves occur in a range of wavelengths larger than the Debye length in the solar wind plasma λ _D ≈10 m and appear Doppler-shifted into the frequency regime down to a fraction of a Hertz. These phenomena are measured by STEREO/WAVES with a set of three orthogonal electric monopole antennas. This paper describes the electrical and mechanical design of the antenna system and discusses efforts to model the antenna pattern and response and methods for in-flight calibration.     

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.     

The August 1972 solar-terrestrial events: Solar wind plasma observations

Solar wind observations associated with the enhanced levels of solar activity in August 1972 are reviewed with an emphasis on recent analyses which more unambiguously characterize the changes in the interplanetary medium. Observations from Pioneer 9 at 0.8 AU, Pioneer 10 at 2.2 AU, and in the vicinity of Earth are reviewed and reinterpreted in the light of new data. Recent calculations of local shock velocities are reviewed and their implications discussed. These results indicate that contrary to previously published observations there is no significant deceleration of the interplanetary shocks between 0.8 AU and 2.2 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 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.     

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.     

Observation of Injection and Pre-Acceleration Processes in the Slow Solar Wind

Knowledge of injection and pre-acceleration mechanisms of ions is of fundamental importance for understanding particle acceleration that takes place in various astrophysical settings. The heliosphere offers the best chance to study these poorly understood processes experimentally. We examine ion injection and pre-acceleration using measurements of the bulk and suprathermal solar wind, and pickup ions. Our most puzzling observation is that high-velocity tails, extending to at least 60 keV/e - the upper limit of measurements -, are omnipresent in the slow, in-ecliptic solar wind; these tails exist even in the absence of any shocks. The cause of these tails is unknown. In the disturbed solar wind inside CIRs and downstream of shocks and waves these high-speed tails in the distributions of H⁺, He⁺ and He⁺⁺ become more pronounced and more complex, but with the shapes of the tails showing the same dependence on ion speed for the different species. Pickup hydrogen and helium are found to be readily injected for subsequent acceleration to MeV energies, and thus are the dominant source of CIR-accelerated energetic ions. Competing sources of MeV ions heavier than He are: (1) heated suprathermal solar wind observed downstream of CIR shocks, (2) interstellar N, O and Ne, and (3) the newly discovered heavy pickup ions from an extended inner source inside 1 AU. Our main conclusion is that mechanisms other than the traditional first-order shock acceleration process produce most of the modestly accelerated ions seen in the slow solar wind. This revised version was published online in August 2006 with corrections to the Cover Date.     

Shocks: Commonalities in Solar-Terrestrial Chains

Shocks are found throughout the heliosphere, wherever supersonic (or super-magnetosonic) flows encounter obstacles or other, slowly moving, media. Although some of the physical parameters are in different regimes, all shocks heat and decelerate the media incident upon them. Most shocks must propagate in a collisionless plasma, thereby adding importance to the particle interactions with the electromagnetic fields, and enabling some particles to be accelerated to high energies. This paper explores the commonalities, and differences, in shocks throughout the heliosphere, and concentrates on the role of shock microstructure in effecting the shock transition and in governing the resulting energy partition amongst the constituent species. Shocks play a significant role in the solar-terrestrial chain.     

Anomalous and Galactic Cosmic Rays at 1 AU During the Cycle 23/24 Solar Minimum

Anomalous cosmic ray (ACR) intensities at 1 AU at solar minimum generally track galactic cosmic ray (GCR) intensities such as those measured by neutron monitors, albeit with differences between solar polarity cycles. The unusual cycle 23/24 solar minimum was long-lasting with very low sunspot numbers and significantly reduced interplanetary magnetic field strength and solar wind dynamic pressure and turbulence, but also featured a heliospheric current sheet tilt that remained high for an extended period. Peak ACR intensities did not recover to the maximum values reached during the last two A>0 solar minima and just barely reached the last A<0 levels. However, GCR intensities in 2009 (neutron monitor rates and also at ～200 MeV/nucleon) were the highest recorded during the last 50 years, indicating their intensities were not as heavily modulated during their transport from the outer heliosphere. This unexpected difference in the behavior of ACRs and GCRs remains unexplained, but suggests that either the ACR source intensity may have weakened since the last A<0 epoch, or perhaps that ACR intensities at 1 AU in the ecliptic may be more sensitive than GCRs to the higher tilt angle. This seems plausible if the ACR source intensity is greater at low latitudes during A<0 cycles, while the GCR distribution at the heliospheric boundary is more uniform in latitude. Shortly after an abrupt increase in the current sheet tilt angle in late 2009, both ACR and GCR intensities showed dramatic decreases, marking the end of solar minimum modulation conditions for this cycle.     

Composition of Interstellar Neutrals and the Origin of Anomalous Cosmic Rays

Knowledge of the elemental composition of the interstellar gas is of fundamental importance for understanding galactic chemical evolution. In addition to spectroscopic determinations of certain element abundance ratios, measurements of the composition of interstellar pickup ions and Anomalous Cosmic Rays (ACRs) have provided the principal means to obtain this critical information. Recent advances in our understanding of particle acceleration processes in the heliosphere and measurements by the Voyagers of the energy spectra and composition of energetic particles in the heliosheath provide us with another means of determining the abundance of the neutral components of the local interstellar gas. Here we compare the composition at the termination shock of six elements obtained from measurements of (a) pickup ions at ～5 AU, (b) ACRs in the heliosphere at ～70 AU, and (c) energetic particles as well as (d) ACRs in the heliosheath at ～100 AU. We find consistency among these four sets of derived neutral abundances. The average interstellar neutral densities at the termination shock for H, N, O, Ne and Ar are found to be 0.055±0.021 cm^?3, (1.44±0.45)×10^?5 cm^?3, (6.46±1.89)×10^?5 cm^?3, (8.5±3.3)×10^?6 cm^?3, and (1.08±0.49)×10^?7 cm^?3, respectively, assuming the He density is 0.0148±0.002 cm^?3.     

IMF connection for energetic protons observed at Ulysses via mid-latitude solar wind rarefaction regions

As Ulysses moved inward and southward from mid-1992 to early 1994 we noticed the occasional occurrence of inter-events, lasting about 10 days and falling between the recurrent events, observed at proton energies of 0.48–97 MeV, associated with Corotating Interaction Regions (CIR). These inter-events were present for several sequences of two or more solar rotations at intensity levels around 1% of those of the neighbouring main events. When we compared the Ulysses events with those measured on IMP-8 at 1 AU we saw that the inter-events appeared at Ulysses after the extended emission (>10 days) of large fluxes of solar protons of the same energy that lasted at least one solar rotation at 1 AU. The inter-events fell completely within the rarefaction regions (dv/dt<0) of the recurrent solar wind streams. The interplanetary magnetic field (IMF) lines in the rarefactions map back to the narrow range of longitudes at the Sun which mark the eastern edge of the source region of the high speed stream. Thus the inter-events are propagating at mid-latitudes to Ulysses along field lines free from stream-stream interactions. They are seen in the 0.39–1.28 MeV/nucleon He, which exhibit a faster decay, but almost never in the 38–53 keV electrons. We show that the inter-events are unlikely to be accelerated by reverse shocks associated with the CIRs and that they are more likely to be accelerated by sequences of solar events and transported along the IMF in the rarefactions of the solar wind streams.     

Oxygen and Iron Ions at High Heliolatitudes

The fluxes of O and Fe ions at high heliolatitudes measured by the HiScale instrument on Ulysses reflect the dynamical processes that affect the charged particle populations in the heliosphere. Both the O and Fe ions show more latitude dependence in the first (solar minimum) orbit to high southern heliolatitudes than during the second (solar maximum) orbit. The ion fluxes are larger during the solar minimum orbit; the flux levels are influenced by the occurrence of corotating interaction regions. The Fe/O abundance ratios are found to be similar at 1 AU and at high heliolatitudes. This revised version was published online in August 2006 with corrections to the Cover Date.     

Interstellar Dust in the Solar System

The Ulysses spacecraft has been orbiting the Sun on a highly inclined ellipse almost perpendicular to the ecliptic plane (inclination 79°, perihelion distance 1.3 AU, aphelion distance 5.4 AU) since it encountered Jupiter in 1992. The in situ dust detector on board continuously measured interstellar dust grains with masses up to 10⁻¹³ kg, penetrating deep into the solar system. The flow direction is close to the mean apex of the Sun’s motion through the solar system and the grains act as tracers of the physical conditions in the local interstellar cloud (LIC). While Ulysses monitored the interstellar dust stream at high ecliptic latitudes between 3 and 5 AU, interstellar impactors were also measured with the in situ dust detectors on board Cassini, Galileo and Helios, covering a heliocentric distance range between 0.3 and 3 AU in the ecliptic plane. The interstellar dust stream in the inner solar system is altered by the solar radiation pressure force, gravitational focussing and interaction of charged grains with the time varying interplanetary magnetic field. We review the results from in situ interstellar dust measurements in the solar system and present Ulysses’ latest interstellar dust data. These data indicate a 30° shift in the impact direction of interstellar grains w.r.t. the interstellar helium flow direction, the reason of which is presently unknown.