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The exploration of interplanetary space and our solar bubble, the heliosphere, has made a big leap over the past two decades, due to the path-breaking observations of the two Voyager spacecraft, launched more than 44 years ago. Their in-situ particle and fields measurements were complemented by remote observations of 5.2 to 55 keV Energetic Neutral Atoms (ENA) from the Cassini mission (Ion and Neutral Camera-INCA), revealing a number of previously unanticipated heliospheric structures such as the “Belt”, a region of enhanced particle pressure inside the heliosheath. The Suprathermal Time Of Flight (HSTOF) instrument on the Solar and Heliospheric Observatory (SOHO) also provided information of 58–88 keV ENAs from the heliosphere. In this chapter we provide a brief discussion for the contribution of the Voyager 1 and 2 Low Energy Charged Particle (LECP) observations that provided “ground truth” to the ENA images from Cassini/INCA towards addressing fundamental questions for the heliosphere’s interaction with the Very Local Interstellar Medium.

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.

International audience; The Magnetospheric Multiscale (MMS) spacecraft obtained unprecedented high-time resolution multipoint particle and field measurements of an interplanetary shock event on 8 January 2018. The spacecraft encountered the supercritical forward shock of a forward/reverse shock pair in the pristine solar wind upstream of the bow shock near the subsolar point as they neared apogee at 25 RE. The high-time resolution measurements from the four spacecraft, separated by only 20 km, allowed direct measurement of particle distributions revealing evidence of electron heating and near specularly reflected ions. The cross-shock potential is calculated directly from 3-D electric field measurements. This is the first reported direct high temporal resolution (

We report "ground truth", 28-3500 keV in-situ ion and 5.2-55 keV remotely sensed ENA measurements from Voyager 2/Low Energy Charged Particle (LECP) detector and Cassini/Ion and Neutral Camera (INCA), respectively, that assess the components of the ion pressure in the heliosheath. In this process, we predict an interstellar neutral hydrogen density of ~0.12 cm-3 and an interstellar magnetic field strength of ~0.5 nT upstream of the heliopause in the direction of V2, i.e. consistent with the measured magnetic field and neutral density measurements at Voyager 1 from August 2012, when the spacecraft entered interstellar space, to date. Further, this analysis results in an estimated heliopause crossing by V2 of ~119 AU, as observed, suggesting that the parameters deduced from the pressure analysis are valid. The shape of the >5.2 keV ion energy spectra play a critical role towards determining the pressure balance and acceleration mechanisms inside the heliosheath.

This paper discusses plasma characteristics in the heliosheath region before the heliopause (HP), at the HP, and in the very local interstellar medium (VLISM). The Voyager 2 (V2) HP was a sharp boundary where the radial plasma currents went to background levels. The radial flow speeds derived from 53-85 keV (V1) and 28-43 keV (V2) ion data decreased about 2 years (8 AU) before the HP at V1 and V2. A speed decrease was not observed by the V2 plasma instrument until 160 days (1.5 AU) before the HP crossing when V2 entered the plasma boundary layer where the plasma density and 28-43 keV ion intensity increased. We determine the HP orientation based on the plasma flow and magnetic field data and show these observations are consistent with models predicting a blunt HP. Variations are observed in the currents observed in the VLISM; roll data from this region clearly show the plasma instrument observes the interstellar plasma and may be consistent with larger than expected VLISM temperatures near the HP.

Voyager 2 observations revealed that the hot solar wind ions (the so-called pickup ions) play a dominant role in the thermodynamics of the termination shock and the heliosheath. The number density and temperature of this hot population, however, have remained unknown, since the plasma instrument on board Voyager 2 can only detect the colder thermal ion component. Here we show that due to the multifluid nature of the plasma, the fast magnetosonic mode splits into a low-frequency fast mode and a high-frequency fast mode. The coupling between the two fast modes results in a quasi-stationary nonlinear wave mode, the “oscilliton,” which creates a large-amplitude trailing wave train downstream of the thermal ion shock. By fitting multifluid shock wave solutions to the shock structure observed by Voyager 2, we are able to constrain both the abundance and the temperature of the undetected pickup ions. In our three-fluid model, we take into account the nonnegligible partial pressure of suprathermal energetic electrons (0.022–1.5 MeV) observed by the Low-Energy Charged Particle Experiment instrument on board Voyager 2. The best fitting simulation suggests a pickup ion abundance of 20 ± 3%, an upstream pickup ion temperature of 13.4 ± 2 MK, and a hot electron population with an apparent temperature of ~0.83 MK. We conclude that the actual shock transition is a subcritical dispersive shock wave with low Mach number and high plasma β.

For more than five decades, the shape and interactions of the heliosphere with the local interstellar medium have been discussed in the context of two competing models, posited in 19611: a magnetosphere-like heliotail and a more symmetric bubble shape. Although past models broadly assumed the magnetosphere-like concept, the accurate heliospheric configuration remained largely undetermined due to lack of measurements. In recent years, however, Voyagers 1 and 2 (V1 and V2) crossed the termination shock — the boundary where the solar wind drops — north and south of the ecliptic plane at 94 au2,3 and 84 au4 in 2004 and 2007, respectively, and discovered the reservoir of ions and electrons that constitute the heliosheath, while Cassini remotely imaged the heliosphere5 for the first time in 2003. Here we report 5.2–55 keV energetic neutral atom (ENA) global images of the heliosphere obtained with the Cassini/Ion and Neutral Camera (INCA). We compare them with 28–53 keV ions measured within the heliosheath by the low-energy charged particle (LECP) experiment onboard V1 and V2 over an 11-year period (2003–2014). We show that the heliosheath ions are the source of ENA. These observations also demonstrate that the heliosphere responds promptly, within ~2–3 years, to outward propagating solar wind changes in both the nose and tail directions. These results, together with the V1 measurement of a ~0.5 nT interstellar magnetic field6 and the enhanced ratio between particle pressure and magnetic pressure in the heliosheath7, strongly suggest a diamagnetic bubble-like heliosphere with few substantial tail-like features. Our results are consistent with recent modelling8–11. The authors put together measurements of ions and neutral atoms from Cassini and the two Voyagers and find that the heliosphere responds quickly (with a lag of 2–3 years) to the solar cycle and that it is bubble-shaped and not tail-shaped, as usually schematized.

We have modeled an electron precipitation pattern expected on Mimas, Tethys, and Dione, using two different approaches. In the first approach, we adapt a previously developed model to compute an integrated energy flux into the surfaces of Mimas, Tethys, and Dione. This is a guiding-center, bounce-averaged model. In the second approach, we track individual particles in an electromagnetic field for an inert or slightly magnetized satellite. This second approach allows us to include the effects of electron pitch angle and gyrophase on the weathering pattern. Both methods converge on an enhanced dose pattern on each satellite’s leading hemisphere that is lens-shaped. We also present mission-averaged electron energy spectra obtained near these satellites by Cassini’s Magnetosphere Imaging Instrument (MIMI). These data are interpreted using our current understanding of both the environment and the instrument’s response. Fits to the data are integrated to find an energy flux into each satellite’s surface, as a function of longitude and latitude. Using positions on the moon accessible to energetic electrons from the modeling and the integrated energy flux based on data, we find lens patterns that fall off with increasing moon latitude. The predicted patterns are qualitatively consistent with some but not all of the optical observations made of these hemispheres.

We present solar energetic particle events observed at 1 AU from the Sun for which the proton energy spectra at energies between ~50 keV to ~1 MeV flatten during a period of at least ~12 hours prior to the passage of the associated interplanetary shock. The flattening of the proton energy spectra occurs when the source of the particles (presumably the traveling interplanetary shock) is still downwind from the spacecraft and particle intensities are still continuously increasing. The arrival of the shock at the spacecraft is then characterized by a steepening of the spectra, where low-energy proton intensities show a more pronounced enhancement than the high-energy proton intensities. We discuss the mechanisms that may result in this flattening of the spectra in terms of current models presented in the literature.

We address the origin of the enhancement of ~40 keV to 5 MeV ions at the solar wind termination shock. Using self-consistent two-dimensional hybrid simulations (kinetic proton, fluid electron) of a shock moving through a plasma similar to that observed in the outer heliosphere, we conclude that the observed ion enhancements are consistent with accelerated "core" interstellar pickup ions (those that have not previously undergone any significant energization) by the termination shock via a combination of shock drift acceleration and particle scattering in meandering magnetic fields in the vicinity of the shock. In addition to the consequences for our understanding of anomalous cosmic rays, this work is also relevant to the more-general long-standing problem of accelerating low-energy particles by shocks that move nearly normal to a mean magnetic field.

Shock waves, as shown by simulations and observations, can generate high levels of downstream vortical turbulence, including magnetic islands. We consider a combination of diffusive shock acceleration (DSA) and downstream magnetic-island-reconnection-related processes as an energization mechanism for charged particles. Observations of electron and ion distributions downstream of interplanetary shocks and the heliospheric termination shock (HTS) are frequently inconsistent with the predictions of classical DSA. We utilize a recently developed transport theory for charged particles propagating diffusively in a turbulent region filled with contracting and reconnecting plasmoids and small-scale current sheets. Particle energization associated with the anti-reconnection electric field, a consequence of magnetic island merging, and magnetic island contraction, are considered. For the former only, we find that (i) the spectrum is a hard power law in particle speed, and (ii) the downstream solution is constant. For downstream plasmoid contraction only, (i) the accelerated spectrum is a hard power law in particle speed; (ii) the particle intensity for a given energy peaks downstream of the shock, and the distance to the peak location increases with increasing particle energy, and (iii) the particle intensity amplification for a particular particle energy, f(x,c/c_0)/f(0,c/c_0), is not 1, as predicted by DSA, but increases with increasing particle energy. The general solution combines both the reconnection-induced electric field and plasmoid contraction. The observed energetic particle intensity profile observed by Voyager 2 downstream of the HTS appears to support a particle acceleration mechanism that combines both DSA and magnetic-island-reconnection-related processes.

We study periods of elevated energetic particle intensities observed by STEREO-A when the partial pressure exerted by energetic ($\geq$83 keV) protons ($P_{EP}$) is larger than the pressure exerted by the interplanetary magnetic field ($P_{B}$). In the majority of cases, these periods are associated with the passage of interplanetary shocks. Periods when $P_{EP}$ exceeds $P_{B}$ by more than one order of magnitude are observed in the upstream region of fast interplanetary shocks where depressed magnetic field regions coincide with increases of the energetic particle intensities. When solar wind parameters are available, $P_{EP}$ also exceeds the pressure exerted by the solar wind thermal population ($P_{TH}$). Prolonged periods ($>$12 h) with both $P_{EP}$$>$$P_{B}$ and $P_{EP}$$>$$P_{TH}$ may also occur when energetic particles accelerated by an approaching shock encounter a region well-upstream of the shock characterized by low magnetic field magnitude and tenuous solar wind density. Quasi-exponential increases of the sum $P_{SUM}$=$P_{B}$+$P_{TH}$+$P_{EP}$ are observed in the immediate upstream region of the shocks regardless of individual changes in $P_{EP}$, $P_{B}$ and $P_{TH}$, indicating a coupling between $P_{EP}$ and the pressure of the background medium characterized by $P_{B}$ and $P_{TH}$. The quasi-exponential increase of $P_{SUM}$ implies a convected exponential radial gradient $\partial{P_{SUM}}/\partial{r}$$>$0 that results in an outward force applied to the plasma upstream of the shock. This force can be maintained by the mobile energetic particles streaming upstream of the shocks that, in the most intense events, drive electric currents able to generate diamagnetic cavities and depressed solar wind density regions., 7 figures, Submitted to The Astrophysical Journal

On or about 2012 August 25, the Voyager 1 spacecraft crossed the heliopause into the nearby interstellar plasma. In the nearly three years that the spacecraft has been in interstellar space, three notable particle and field disturbances have been observed, each apparently associated with a shock wave propagating outward from the Sun. Here, we present a detailed analysis of the third and most impressive of these disturbances, with brief comparisons to the two previous events, both of which have been previously reported. The shock responsible for the third event was first detected on 2014 February 17 by the onset of narrowband radio emissions from the approaching shock, followed on 2014 May 13 by the abrupt appearance of intense electron plasma oscillations generated by electrons streaming outward ahead of the shock. Finally, the shock arrived on 2014 August 25, as indicated by a jump in the magnetic field strength and the plasma density. Various disturbances in the intensity and anisotropy of galactic cosmic rays were also observed ahead of the shock, some of which are believed to be caused by the reflection and acceleration of cosmic rays by the magnetic field jump at the shock, and/or by interactions with upstream plasma waves. Comparisons to the two previous weaker events show somewhat similar precursor effects, although differing in certain details. Many of these effects are very similar to those observed in the region called the "foreshock" that occurs upstream of planetary bow shocks, only on a vastly larger spatial scale.

Interplanetary coronal mass ejections (ICMEs) propagate into the outer heliosphere, where they can have a significant effect on the structure, evolution, and morphology of the solar wind, particularly during times of high solar activity. They are known to play an important role in cosmic ray modulation and the acceleration of energetic particles. ICMEs are also believed to be associated with the large global transient events that swept through the heliosphere during the declining phases of solar cycles 21 and 22. But until recently, little was known about the actual behavior of ICMEs at large heliographic latitudes and large distances from the Sun. Over the past decade, the Ulysses spacecraft has provided in situ observations of ICMEs at moderate heliographic distances over a broad range of heliographic latitudes. More recently, observations of alpha particle enhancements, proton temperature depressions, and magnetic clouds at the Voyager and Pioneer spacecraft have begun to provide comparable information regarding the behavior of ICMEs at extremely large heliocentric distances. At the same time, advances in modeling have provided new insights into the dynamics and evolution of ICMEs and their effects on cosmic rays and energetic particles.

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.

We report on observations at Voyagers 1 and 2, in the outer heliosphere, of ions with energies ∼30 keV to 30 MeV. The period of interest is from 2001 through 2002 DOY 268, during which both spacecraft observe significant variations in energetic ion intensities as large-scale plasma disturbances associated with intense periods of solar activity in 2000–2002 reach the outer heliosphere.

We study the variability of the heliospheric energetic proton-to-helium abundance ratios during different phases of the solar cycle. We use energetic particle, solar wind, and magnetic field data from the Ulysses, ACE and IMP-8 spacecraft to compare the H/He intensity ratio at high heliographic latitudes and in the ecliptic plane. During the first out-of-ecliptic excursion of Ulysses (1992–1996), the HI-SCALE instrument measured corotating energetic particle intensity enhancements characterized by low values (< 10) of the 0.5–1.0 MeV nucleon-1 H/He intensity ratio. During the second out-of-ecliptic excursion of Ulysses (1999–2002), the more frequent occurrence of solar energetic particle events resulted in almost continuously high (< 20) values of the H/He ratio, even at the highest heliolatitudes reached by Ulysses. Comparison with in-ecliptic measurements from an identical instrument on the ACE spacecraft showed similar H/He values at ACE and Ulysses, suggesting a remarkable uniformity of energetic particle intensities in the solar maximum heliosphere at high heliolatitudes and in the ecliptic plane. In-ecliptic observations of the H/He intensity ratio from the IMP-8 spacecraft show variations between solar maximum and solar minimum similar to those observed by Ulysses at high heliographic latitudes. We suggest that the variation of the H/He intensity ratio throughout the solar cycle is due to the different level of transient solar activity, as well as the different structure and duration that corotating solar wind structures have under solar maximum and solar minimum conditions. During solar minimum, the interactions between the two different types of solar wind streams (slow vs. fast) are strong and long-lasting, allowing for a continuous and efficient acceleration of interstellar pickup He +. During solar maximum, transient events of solar origin (characterized by high values of the H/He ratio) are able to globally fill the heliosphere. In addition, during solar maximum, the lack of strong recurrent high-speed solar wind streams, together with the dynamic character of the Sun, lead to weak and short-lived solar wind stream interactions. This results in a less efficient acceleration of pickup He +, and thus a higher value of the H/He intensity ratio.Key words. Interplanetary physics (energetic particles, interplanetary shocks; solar wind plasma)

Both a magnetosphere-like tail and a bubble model of the heliosphere were posited by E. N. Parker in 1961. Recently, we showed that heliosheath ions are the source of > 5.2 keV Energetic Neutral Atoms (ENA), whose images of the heliosphere exhibit a rough nose to anti-nose (tail) global symmetry that resembles a diamagnetic bubble. The comparison between energetic neutral atom (ENA) global images of the helioshphere obtained with the Ion and Neutral Camera (INCA) on board Cassini and ions measured in-situ by the Low Energy Charged Particle experiment (LECP) on board Voyager 1 and 2 (V1/V2) in overlapping energy bands over an 11-year period shows that the heliosphere responds promptly, within ~2-3 years, to outward propagating solar wind changes in both the nose and tail directions. Here we focus on the recovery of solar cycle 24 and the response times of > 5.2 keV ENAs to show that this ~2-3-year time delay is consistent with a "tail" of ~80-120 AU. This preliminary rough calculation is generally consistent with lower energy ENA data (E < 6 keV, from the IBEX-Lo and IBEX-Hi) and is supported by recent modelling of the heliosphere.

Voyager 2 data from 47 AU shows a delay of about 6 days between the arrival of interplanetary shock fronts and the peaks in the 0.52–1.45 MeV energetic proton flux. Similar data recorded when Voyager 2 was at 5 AU shows that the shock fronts and the peaks in the energetic proton flux are virtually coincident. If injection and acceleration is taking place at the shock fronts one would expect the peaks in the energetic proton flux and the shocks to coincide. We show that the delay observed at 47 AU can be understood if the injection and acceleration of energetic protons becomes negligibly small some time before the shocks are observed.

We discuss the structure and evolution of CIRs and their successors in the outer helio- sphere. These structures undergo significant evolution as they are convected to greater heliocentric distances. A progression of different types of structure are observed at increasing distance from the Sun. Similar structures are observed at similar heliocentric distance at different portions of the solar cycle. CIRs and their successors are associated with many important physical processes in the outer heliosphere. We discuss the relationship between these structures and recurrent phenomena such as cosmic ray variations, and review some of the associated theoretical models on the role of corotating structures and global merged interaction regions (GMIRs) in global cosmic ray modulation. We also discuss some outstanding questions related to the origin of non-dispersive quasi-periodic particle enhancements associated with CIRs and their successors in the outer heliosphere.

This report emphasizes new observational aspects of CIR ions revealed by advanced instruments launched on the Ulysses, WIND, SOHO, and ACE spacecraft, and by the unique van- tage point of Ulysses which carried out the first survey of Corotating Interaction Region (CIR) properties over a very wide range of heliolatitudes. With this more complete observational picture established, this review is the basis to consider the status of theoretical models on origin, injection, and acceleration of CIR particles reported by Scholer, Mann et al. (1999) in this volume.

The Low Energy Charged Particle (LECP) instruments on Voyagers 1 (V1) and 2 (V2) measure the differential in energy fluxes and anisotropies of low energy ions ≥30 keV and electrons ≥20 keV, differential in energy ion composition ≥200 keV/nuc, and the integral rates of cosmic ray protons >70 MeV (Krimigis et al., 1977). We discuss shock-accelerated ions and latitude-associated differences between V1 and V2 during 1991 to April 1994.

The radially outward flow of plasma from the Sun is expected to be deflected when it meets the flow of interstellar plasma through which the Solar System moves, but the spacecraft Voyager 1 unexpectedly finds that the deflected, meridional, flow is consistent with zero within the transition region. The Voyager 1 and 2 spacecraft are now deep into the heliosheath, the most distant layer of the heliosphere, where the solar wind (a stream of charged particles ejected from the Sun) is still evident but is much slowed by the pressure of interstellar gas. Both Vikings are still sending back data and this paper reports the results of recent manoeuvres in which — after holding a steady course for 25 years — Voyager 1 was periodically rotated through 70° to allow the probe's charged-particle detectors to test predictions from magnetohydrodynamic models that the heliosheath's initially radial flow was being deflected polewards, towards meridional flow. Five such rolls have been completed, the last one on 30 January this year, and the results of the experiment are surprising. The meridional-flow velocity is low — almost zero — suggesting that Voyager 1 is not yet close to the heliopause, the theorectical point where the solar wind slows to a standstill. Over a two-year period, Voyager 1 observed a gradual slowing-down of radial plasma flow in the heliosheath to near-zero velocity1 after April 2010 at a distance of 113.5 astronomical units from the Sun (1 astronomical unit equals 1.5 × 108 kilometres). Voyager 1 was then about 20 astronomical units beyond the shock that terminates the free expansion of the solar wind and was immersed in the heated non-thermal plasma region called the heliosheath. The expectation from contemporary simulations2,3 was that the heliosheath plasma would be deflected from radial flow to meridional flow (in solar heliospheric coordinates), which at Voyager 1 would lie mainly on the (locally spherical) surface called the heliopause. This surface is supposed to separate the heliosheath plasma, which is of solar origin, from the interstellar plasma, which is of local Galactic origin. In 2011, the Voyager project began occasional temporary re-orientations of the spacecraft (totalling about 10–25 hours every 2 months) to re-align the Low-Energy Charged Particle instrument on board Voyager 1 so that it could measure meridional flow. Here we report that, contrary to expectations, these observations yielded a meridional flow velocity of +3 ± 11 km s−1, that is, one consistent with zero within statistical uncertainties.

We have analyzed space weather throughout the heliosphere using the three-dimensional (3D) timedependent magnetohydrodynamic (MHD) Hybrid Heliospheric Modeling System with Pickup Protons (HHMS-PI) [1] out to Voyager 2 (V2) and beyond by comparing the HHMS-PI model results with the available spacecraft data. We also have analyzed space weather throughout the heliosphere through in-depth analyses of the available simultaneous data from a number of instruments on spacecraft at various locations. In this paper we focus on our HHMS-PI modeling (starting at the Sun) of the Halloween 2003 solar events by comparing the model results with spacecraft data at ACE and Ulysses. For the Halloween 2003 solar events we also summarize our inter-comparisons of the in-situ V2 data from many of the V2 instruments. These analyses of the comparisons ("benchmarking") of HHMS-PI simulations and the various spacecraft data and of our in-depth analyses of the V2 particle and field data indicate that particle acceleration and other important physical processes are associated with the heliospheric propagation of these large solar cycle 23 space weather events. We conclude that space weather, originating at the Sun, can have important affects throughout the heliosphere to distances as great as 73 AU and beyond.

In December 2004, the Voyager 1 spacecraft began to leave the Solar System, crossing the solar wind termination shock and entering the heliosheath, a region where the solar wind (a stream of ionized particles) slows down as it begins to interact with the interstellar medium. The Voyager instruments are still monitoring the bulk velocity of the heliosheath plasma, and the latest news is that plasma velocity has been decreasing almost linearly during the past three years from 70 kilometres per second to close to zero, where it has remained for the past 8 months. This means that Voyager 1 may be close to the heliopause, where what remains of the solar wind matches the pressure exerted by interstellar space. Theorists had predicted a sharp discontinuity at the heliopause, so the gradual nature of Voyager's Solar System exit comes as something of a surprise. Voyager 1 has been in the reservoir of energetic ions and electrons that constitutes the heliosheath since it crossed the solar wind termination shock1,2,3 on 16 December 2004 at a distance from the Sun of 94 astronomical units (1 au = 1.5 × 108 km). It is now ∼22 au past the termination shock crossing4. The bulk velocity of the plasma in the radial–transverse plane has been determined5 using measurements of the anisotropy of the convected energetic ion distribution6. Here we report that the radial component of the velocity has been decreasing almost linearly over the past three years, from ∼70 km s−1 to ∼0 km s−1, where it has remained for the past eight months. It now seems that Voyager 1 has entered a finite transition layer of zero-radial-velocity plasma flow, indicating that the spacecraft may be close to the heliopause, the border between the heliosheath and the interstellar plasma. The existence of a flow transition layer in the heliosheath contradicts current predictions7—generally assumed by conceptual models—of a sharp discontinuity at the heliopause.

The acceleration of superthermal ions is investigated when a planar shock that is on average nearly perpendicular propagates through a plasma in which the magnetic field is the superposition of a constant uniform component plus a random field of transverse hydromagnetic fluctuations. The importance of the broadband nature of the transverse magnetic fluctuations in mediating ion acceleration at nearly perpendicular shocks is pointed out. Specifically, the fluctuations are composed of short-wavelength components which scatter ions in pitch angle and long-wavelength components which are responsible for a spatial meandering of field lines about the mean field. At nearly perpendicular shocks the field line meandering produces a distribution of transient loops along the shock. As an application of this model, the acceleration of a superthermal monoenergetic population of seed protons at a perpendicular shock is investigated by integrating along the exact phase-space orbits.

We have found in the Voyager 2 (V2) plasma science data in the heliosheath (HS) near the termination shock (TS) high-energy ions (HEIs) in addition to the bulk plasma convective flow ions. The HEI detections temporally coincide with increased V2 plasma wave subsystem (PWS) activity in "event A"h of Gurnett and Kurth. Maxwellian fits to HEI detections indicate the HEIs are moving radially anti -Sunward with a proton speed of 600 km/s, a density of 10(exp -4) (exp -3), and a thermal speed of 10 km/s. The heliosheath bulk convective protons have a speed of 204 km/s, a density of 0.0029 cm(exp -3), and a thermal speed of 26.7 km/s. The HEI flux and ram pressure are approximately 10% and 30% of those of the bulk HS flow. Since the HEI speed is both close to twice the solar wind speed and independent of the heliosheath bulk plasma speed, the HEIs may be detections of pickup protons formed in the solar wind and convected through the TS. The HEIs also are reminiscent of the pickup protons upstream of the Mars bow shock where their energy also was independent of the bulk plasma speed and attributed to multiple reflections off the Mars bow shock. Gurnett and Kurth 's (2008) event A enhanced PWS activity may be generated by a two ]stream instability from the interaction of these HEIs with the heliosheath bulk plasma ions. We present our findings, discuss their implications, and also present alternative interpretations.

We present results on the recurrence of High Energy Ions (HEIs) in the V2 data about 90 days after the initial sunward‐moving termination shock (TS) crossings of V2 in Aug.–Sept. 2007. We associate the HEIs in Nov.–Dec. 2007 with the outward motion of the TS or with a ripple in the TS so that the TS is again near V2. Comparisons of the timings of the recurrence of the HEI detections and the simultaneous V2 convective plasma, energetic particle, and magnetic field data indicate that the variations in all these V2 data sets are consistent with the TS re‐approach to V2 in Nov.–Dec. 2007. We use our three‐dimensional (3D) kinematic HAFSS model to investigate whether the timing of the arrival at V2 of the increase in solar wind dynamic pressure associated with the December 2006 solar events could have been responsible for the dynamic pressure pulse that moved the TS outward toward V2 ∼90 days after the initial TS crossings of V2. This explanation or, alternatively, a 3D solar wind inhomogeneity‐caused TS ripple (on the scale of ∼1 AU) could be viable scenarios to explain the recurrence of the HEIs in the V2 data in Nov.–Dec. 2007.

Collisionless shocks accelerate charged particles throughout heliosphere, from low in the solar corona to the outer edge of the supersonic solar wind. Particles can gain energy while interacting with shocks in different ways, depending on conditions at the shock and on particle species and energy. Different acceleration processes generally produce distinct features in energetic particle anisotropies, energy spectra, and time intensity profiles. Thus, a study of in‐situ interplanetary shocks and particle distributions in their vicinity provides the only meaningful way to test our theoretical understanding of shock acceleration. During solar cycle 23, advanced instrumentation from the ACE spacecraft detected over 300 interplanetary shocks. More than half of these shocks produced signatures (so called energetic storm particle (ESP) events) in the low energy ion intensities. Of these ESP events, only one event did exhibit signatures that agree with steady‐state diffusive shock‐acceleration theory. Generally, ...

Voyager 2 (V2) is now 20 AU deep into the heliosheath; if the heliosheath width were similar to that in the Voyager 1 (V1) direction, then V2 is 2/3 of the way to the heliopause. We present recent V2 observations, compare with observations from V1, and compare with model predictions. The speed of the flows observed by V2 in the heliosheath are, on average, remarkably constant at 150 km/s. The flow angle has changed dramatically, however, and is now over 60° from radial, with more of the turning occurring in the RT than RN planes. These flows are very different than those at V1, where the speed was always below 100 km/s and decreased across the heliosheath. Models predict VR at V2 well, but the the flow angle predictions differ from the observations. The average density and temperature of the plasma decreased by a factor of two in 2008, recovered in 2011, and have not changed significantly since 2011.

The most recent data from Voyager 1 (V1) show that a second event (TS2), apparently associated with the termination shock (TS), is in progress, with spectral characteristics similar to the energetic particle increase observed from 2002.4–2003.1 (TS1). We concentrate on the pressure, composition, and anisotropy profiles of TS1. The magnetic field pressure is significantly smaller than the particle pressure perpendicular to the interplanetary magnetic field (IMF) in the 40–4000 keV range. The composition during the interplanetary shock event (ISE) observed by V1 during 1991 is drastically different from that during TS1 (C/O ∼0.2 for ISE, ∼0.02 for TS1). The dominant anisotropy during TS1 is azimuthally in the outward direction for a Parker spiral field, suggesting a source inward of the spacecraft, while the radial anisotropy is consistent with zero (−0.024 ± 0.02), implying a slow (

We combined daily averages of magnetic field vector data and 0.6–1.8 MeV proton angular intensity data to construct 32 pitch angle distributions (PADs) for measurements made by Voyager 1 (V1) at 85–87 AU. The PADs were observed during the period 2002.6–2003.1, when energetic particle instruments on V1 measured unusually high intensities. The angular data show large, mainly unidirectional beaming of protons most often in the −T direction, i.e., away from the sun in the sense of a spiral magnetic field. The mean anisotropy amplitude based on the 32 samples is 0.55±0.21.

[1] We reexamine traveling convection vortices (TCVs) seen by the Magnetometer Array for Cusp and Cleft Studies on 9 November 1993. IMP-8 energetic ion observations confirm that the solar wind pressure variations previously associated with these TCVs were generated by kinetic processes within the Earth's foreshock. As expected during this interval of spiral IMF orientation, fast mode waves launched by the pressure variations first arrived in the equatorial ionosphere near dusk and propagated dawnward. We derive a model for the field-aligned currents generated by transient compressions of the magnetopause and show that it accounts for the number of TCVs seen in the prenoon ionosphere, their sense of rotation, the latitude at which they occur, and their absence in the postnoon ionosphere.

Taken together, the Voyager 1 and 2 (V1 and V2) spacecraft have collected over 11 yr of data in the heliosheath. Despite extensive study, energetic particles and magnetic fields measured in the heliosheath have not been reconciled by existing models. In particular, the differences between the energetic particle intensity variations at V1 and V2 are unexplained. While energetic particle intensities at V1 change gradually over 7 yr in the heliosheath, those at V2 vary by a factor ~10 in 1 yr. Energetic particle intensities at V2 show temporally coherent variations over a broad range of species and energies: from suprathermal ions (10s of keV) to galactic cosmic rays (>1 GeV), as well as electrons from 10s of keV to >100 MeV, corresponding to a range ~104 in particle gyroradii. Here we suggest that many of the intensity variations of energetic particle populations in the heliosheath are organized by their proximity to two fundamentally different regions—the unipolar heliosheath (UHS) and the sectored heliosheath (SHS). The SHS is a region of enhanced particle intensities, wherein particle transport, acceleration, and magnetic connectivity differ from those in the UHS. The SHS may serve as either a reservoir of energetic particles or as a region of enhanced transport, depending on the particle species and energy. Comparatively, particle intensities in the UHS are greatly reduced. We propose that the boundary between the SHS and UHS plays as important a role in the physics of heliosheath particles and fields as do the termination shock and heliopause.

This report emphasizes new observational aspects of CIR ions revealed by advanced instruments launched on the Ulysses, WIND, SOHO, and ACE spacecraft, and by the unique vantage point of Ulysses which carried out the first survey of Corotating Interaction Region (CIR) properties over a very wide range of heliolatitudes. With this more complete observational picture established, this review is the basis to consider the status of theoretical models on origin, injection, and acceleration of CIR particles reported by Scholer, Mann et al. (1999) in this volume.

The corotating particle events give us a unique opportunity to probe the three-dimensional structures of the heliosphere. This is especially true if we have observations over a period of extreme stability of the CIRs, such as existed over the recent solar minimum. We discuss how the observations fit into the context of current heliospheric magnetic field models. The energetic particle signatures of CIRs throughout the regions of the heliosphere covered by the deep-space missions are reviewed. The CIRs accelerate these particles and at the same time modulate both the high energy galactic cosmic rays and the anomalous cosmic rays.

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 ∼4days after the shock passage are nearly identical. The preshock anisotropics at low energyl described by convected power law distributions. The Voyager 1 and 2 postshock spectra ∼4 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 this shock and many other shock events at Voyagers 1 and 2.

Voyager 1(V1) and Voyager 2(V2) have observed heliosheath plasma since 2005 December and 2007 August, respectively. The observed speed profiles are very different at the two spacecrafts. Speeds at V1 decreased to zero in 2010 while the average speed at V2 is a constant 150 km s{sup -1} with the direction rotating tailward. The magnetic flux is expected to be constant in these heliosheath flows. We show that the flux is constant at V2 but decreases by an order of magnitude at V1, even after accounting for divergence of the flows and changes in the solar field. If reconnection were responsible for this decrease, the magnetic field would lose 70% of its free energy to reconnection and the energy density released would be 0.6 eV cm{sup -3}.

We review basic aspects of shock drift acceleration at fast‐mode collisionless shocks, and describe recent modeling efforts that incorporate such effects as shock structure, shock curvature, magnetic loops, rippled shocks, and magnetic turbulence.

[1] The energetic storm particle event of October 20, 1989 has often been cited as an example of high-energy (≳500 MeV) proton acceleration by CME-driven shocks near 1 AU. We examine high-time resolution solar wind and magnetic field data from the IMP-8 spacecraft and energetic particle data from the IMP-8 and GOES-7 spacecraft. We show that the high-energy particle population in this event is not a locally shock-accelerated population, but rather a population of particles confined to a plasma structure with depressed magnetic field and solar wind density. This structure was bounded by enhanced densities and strong magnetic fields. Energetic protons within this structure supplied the pressure needed to prevent the structure from collapsing. The intensity and evolution of this energetic storm particle event were shaped mainly by this spatial structure rather than by the CME-driven shocks.

In the sharply curved magnetic field lines of Jupiter's magnetodisk, energetic ions follow nonadiabatic orbits violating the first and second adiabatic invariants. Even in the corotating reference frame, the energetic ion distribution function is in general anisotropic, as is shown by numerically computed orbits in the Goertz et al. (1976) magnetodisk model. Pitch angle scattering that can be induced by magnetic field irregularities or plasma waves is included in the calculations. This numerically calculated anisotropy of nonadiabatic ions can account for the anisotropy of energetic ions measured by Voyager in the Jovian magnetodisk, depending strongly on the radial distribution of particles injected into the current sheet but not strongly on the scattering rate. The measured anisotropy has previously been interpreted as indicating approximate corotation in the outer Jovian magnetodisk, but the present calculations show that the measured anisotropy can be consistent with a substantial corotation lag.

Observations of shock-associated ion enhancements at energies of not less than 30 keV are presented from data gathered by Voyagers 1 and 2. Observations include examples of energetic storm particle events associated with flare-produced shocks and examples of corotating particle events (CPE) associated with forward and reverse shocks that bound corotating interaction regions in the outer heliosphere. Most of the CPE have recurrent double-peaked intensity enhancements showing little or no velocity dispersion at peak intensities, time durations of several days, and soft energy spectra extending up to 5 MeV/nucleon. The lowest energy ion enhancements are confined mainly downstream of the corotating interaction regions with the magnitude and duration of the upstream enhancements increasing with increasing ion energy. The observations are consistent with the dynamical and kinematical effects expected when low energy ions are accelerated at quasi-perpendicular shocks.

Attention is given to the initial results of a model designed to study the modification of the scatter-free, shock drift acceleration of energetic test particles by wave activity in the vicinity of a quasi-perpendicular, fast-mode MHD shock. It is emphasized that the concept of magnetic moment conservation is a valid approximation only in the perpendicular and nearly perpendicular regimes, when the angle theta-Bn between the shock normal and the upstream magnetic field vector is in the range from 70 deg to 90 deg. The present investigation is concerned with one step in a program which is being developed to combine the shock drift and diffusive processes at a shock of arbitrary theta-Bn.

A numerical simulation is used to predict the interaction between the earth's bow shock and lithium to be released in the solar wind during the Active Magnetospheric Particle Tracer Explorers program in 1984. Based on the simulation results for a release near the subsolar point, it is recommended that a favorable release condition is when the garden-hose angle of the interplanetary magnetic field exceeds about 60 to 70 deg so that the transmission of Li(+) ions through the bow shock is optimized. The model also predicts that the Li(+) is 'shock-drift' accelerated at the bow shock. The energy gain of Li(+) ions is on the average less for the transmitted particles (below a factor of about 4) than for the reflected particles (below a factor of about 16).

A numerical simulation has been developed to study the formation in the intensities of ≲0.1- to ∼10.0-MeV protons of the long-lived energetic storm particle (ESP) events observed in association with the passage of flare-produced fast mode MHD shocks. Protons are traced numerically backward in time from a given observation point (i.e., particle detector), through their adiabatic scatter-free motion along the assumed laminar spiral interplanetary magnetic field (IMF), and finally through the shock discontinuity to their pre-shock interaction state. An ambient proton can undergo at most one shock encounter, during which, as seen in the shock frame, the proton is accelerated by its effective ‘grad-B drift’ along the induced electric field each time it crosses the shock. The time-reversed calculation reveals which of the observed protons interacted with the shock, where the shock interaction occurred, and by how much each interacting proton's kinetic energy was increased at the shock. Time profiles of the omnidirectional and unidirectional protons fluxes are formed from the trajectories by invoking Liouville's theorem and an ambient proton unidirectional flux j ∼T−γ, where T is the kinetic energy and γ the spectral exponent. Simulation predictions are shown for different observed proton energies under various assumptions concerning the shock speed and strength, the slope of the ambient proton energy spectrum, the variation of the angle β1 between the shock surface and the upstream IMF vector, and the heliocentric radial position r of the observation point. The simulation model correctly recovers the gross features of the intensity variations, flux anisotropies, and energy spectra time evolutions of many observed ESP events. Representative simulation results predict shock-induced enhancements in low-energy proton fluxes that are characterized by (1) long, steady preshock or upstream rises to peaks before the shock arrival and abrupt declines following the shock passage; (2) peak enhancement amplitudes that increase for lower-energy protons, stronger and faster shocks, softer ambient proton spectra, decreased β1 and increased r, (3) large, highly field-aligned preshock flux anisotropies directed away from the sun along the IMF, (4) small postshock flux anisotropies nearly perpendicular to the magnetic field and directed towards the sun along the IMF, and (5) steadily softening preshock energy spectra that are steepest at the peak flux enhancement and slightly softer than the ambient spectra after the shock passage.