In the coming years, New Horizons (NH) is expected to exit the heliosphere by crossing the solar wind termination shock (TS) and make the first measurements of pick-up ions (PUIs) across the TS boundary. To date, the only working spacecraft to have crossed the TS are Voyager 1 and 2, with Voyager 1 encountering the TS on day of year (DOY) 351, 2004 at ~94 AU, and Voyager 2 undergoing multiple crossings between DOY 243 and 344, 2007 at ~83.6 AU. Although NH is approximately aligned in heliolongitude with Voyager 2, its trajectory lies near the heliographic equator, in contrast to the higher northern and southern heliolatitudes of Voyager 1 and 2, respectively.In this work, we analyze energetic particle observations (∼40–200 keV) from the Voyager Low Energy Charged Particle (LECP) instruments and the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) onboard NH to characterize radial intensity variations in the outer heliosphere. Voyager 1 and 2 observations show a systematic decrease in energetic particle intensities with increasing heliocentric distance, followed by a recovery prior to their respective TS crossings, forming a heliospheric energetic particle “valley.” NH/PEPSSI observations from 5 to 60 AU exhibit a comparable radial decline but have yet to show the expected increase on the march toward the TS crossing.To mitigate temporal variability associated with solar cycle effects, all observations are normalized using near-Earth energetic particle measurements from IMP-8/EPE and ACE/EPAM. The combined radial profiles from Voyager and NH are well described by a double power-law with a break at~33 AU. The combined radial profiles from Voyager and NH are well described by a power-law dependence with a distinct break beyond ~33 AU. This break likely reflects a transition in the dominant transport and/or acceleration mechanisms operating in the inner and outer regions separated by this radial distance. The presence of this break across multiple heliolatitudes suggests a global heliospheric feature, potentially reflecting changes in particle transport, acceleration, or local plasma conditions in the outer heliosphere. By scaling the Voyager observations to the NH measurements, we estimate a NH TS crossing between 2027 (~68 AU) and 2034 (~83 AU).

We analyze the count rates of 40-139 keV ions that were measured in situ by the Low Energy Charged Particle instrument on Voyager 1 in order to identify the suprathermal ion anisotropies beyond the termination shock and in the very local interstellar (IS) medium (VLISM). The analysis results in a region of similar to 9-10 au before the heliopause (HP) where the radial anisotropy of ions is negative, while the azimuthal ion anisotropy inside the heliosheath lies in the -T direction. In agreement to our previous analyses, we identify a positive radial anisotropy of ions up to at least similar to 30 au beyond the HP, which becomes nearly zero from 2021 up to 2023 November (for similar to 10 au). Notably, the anisotropy in the azimuthal direction is statistically zero throughout the upstream region, i.e., for similar to 40 au past the HP, showing that the 40-139 keV ion anisotropy in the VLISM is only in the radial direction and has no azimuthal component. The presence of suprathermal ions of solar origin over such long spatial scales in the VLISM, along with the inflow of ions from IS space into the heliosheath, are important constraints for characterizing the interaction of the heliosheath with the VLISM. Our observations provide indications that V1 has entered a new regime in the VLISM since (at least) the year 2021, progressively developing characteristics akin to the pristine IS medium. Alternatively, this drop to nearly zero radial anisotropies beyond 2021 could be a manifestation of a prolonged compression/shock of solar origin.

We now have the responsibility to take the steps to bring resolution to the open question concerning the origins and behavior of galactic cosmic rays (GCRs), which, we point out, is the first "space aged" scientific mystery.By ensuring the existence of a capable GCR experiment on an Interstellar Probe (ISP) mission that launches around 2036 we will have all the measurements we need to resolve the GCR puzzles by the end of the current century, cap-stoning roughly 200 years of cosmic ray science.The GCR experiment needs to be as far from the heliosphere as possible in the unperturbed local interstellar medium (LISM), where uniquely the low-energy GCR spectra are measurable and where uniquely the GCR anisotropies can be measured at low-to-high energies, and all species, including electrons.Unless we put the right experiment in the right environment, we will be unable to make the measurements that are most sensitive to the "nearby" (but non-heliospheric) interstellar sources and processing of GCRs.To not prepare to make dedicated GCR observations on a robust ISP mission to the unperturbed LISM would be analogous to trying to study deep sea currents and wildlife from the sheltered lagoon of a tropical atoll.While GCRs are not the only reason to strike out into the "deep water" of interstellar space, they provide one distinct, elusive scientific prize that cannot be snared from the "beach".

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.

We report on the energy dependence of Galactic cosmic rays (GCRs) in the very local interstellar medium (VLISM) as measured by the Low Energy Charged Particle (LECP) instrument on the Voyager 1 spacecraft. The LECP instrument includes a dual-ended solid-state detector particle telescope mechanically scanning through 360° across eight equally spaced angular sectors. As reported previously, LECP measurements showed a dramatic increase in GCR intensities for all sectors of the ≥211 MeV count rate (CH31) at the Voyager 1 heliopause (HP) crossing in 2012; however, since then the count rate data have demonstrated systematic episodes of intensity decrease for particles around 90° pitch angle. To shed light on the energy dependence of these GCR anisotropies over a wide range of energies, we use Voyager 1 LECP count rate and pulse height analyzer (PHA) data from ≥211 MeV channel together with lower-energy LECP channels. Our analysis shows that, while GCR anisotropies are present over a wide range of energies, there is a decreasing trend in the amplitude of second-order anisotropy with increasing energy during anisotropy episodes. A stronger pitch angle scattering at higher velocities is argued as a potential cause for this energy dependence. A possible cause for this velocity dependence arising from weak rigidity dependence of the scattering mean free path and resulting velocity-dominated scattering rate is discussed. This interpretation is consistent with a recently reported lack of corresponding GCR electron anisotropies.

We report 40–139 keV in situ ion measurements from the Voyager 1/Low Energy Charged Particle (LECP) instrument, over the time period from 2000 to the first 3 months of 2020, toward assessing energetic ion flow properties inside the heliosheath (HS) and upstream of the heliopause (HP). We identify an average radial outflow of ions at ∼46, 67, and 109 keV of ∼2.33 × 10−3, ∼1.37 × 10−3, and ∼7.27 × 10−4 (cm2srskeV) –1, respectively, from the beginning of 2013 up to 2020, i.e., over a spatial range of ∼28 au past the HP. The raw 40–139 keV rates, uncorrected for background, show distinct intensity dropouts for particles with 90° pitch angles, in concert with the >211 MeV Galactic cosmic-ray anisotropy events, reported after Voyager 1 exited the HS in 2012 August. The intensity spectrum is consistent with a power-law form in energy with a spectral index of ∼−1.4, similar to energy spectra measured by both Voyager 1 and 2/LECP inside the HS, providing strong indication that these particles correspond to an ion population leaking from the HS into interstellar space, most likely due to the flux tube interchange instability at the boundary. Future modeling efforts should incorporate this ion population, which provides the framework for establishing a communication between the HS and the very local interstellar medium.

We report a unique combination of ∼10 eV to ∼344 MeV in situ ion measurements from the Plasma Science (PLS), Low Energy Charged Particle (LECP), and Cosmic Ray Subsystem (CRS) experiments on the Voyager 2 (V2) spacecraft, and remotely sensed ∼110 eV to ∼55 keV energetic neutral atom (ENA) measurements from the Interstellar Boundary Explorer (IBEX) mission and Ion and Neutral Camera (INCA) on the Cassini mission. This combination is done over the time period from 2009 to the end of 2016, along the V2 trajectory, toward assessing the properties of the ion energy spectra inside the heliosheath. The combined energy spectra exhibit a series of softening and hardening breaks, providing important insights on the various ion acceleration processes inside the heliosheath. Ions in the <6 keV energy range dominate the total pressure distribution inside the heliosheath but the ion distributions at higher energies (>5.2 keV) provide a significant contribution to the total pressure. With the assumption that all ENAs (∼110 eV to 55 keV) are created by charge-exchange interactions inside the heliosheath, we estimate that the magnetic field upstream at the heliopause required to balance the pressure from the heliosheath in the direction of V2 is ∼0.67 nT. This number is consistent with the measured magnetic field at V2 from 2018 November, when the spacecraft entered interstellar space.

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.

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.

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 R E . 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 (<1 s) observation at an interplanetary shock of near specularly reflected ions. Calculation of the cross‐shock potential yields a potential jump significant enough to reflect at least some of the protons from the incident solar wind beam. The cross‐shock potential calculated here is consistent with previous estimations based on particle measurements and numerical/analytical simulations. The ambipolar contribution to the cross‐shock potential calculated from the four‐spacecraft divergence of the electron pressure tensor is somewhat higher than that inferred form the Liouville‐mapped electron energy gain across the shock. Furthermore, the high‐time‐resolution 3‐D electric field measurements reported here reveal small‐scale nonlinear structures embedded in the shock layer that contribute to the nonmonotonic shock transition.

Abstract We report “ground truth,” 28‐ to 3,500‐keV in situ ion and 5.2‐ to 55‐keV remotely sensed ENA measurements from Voyager 2/Low Energy Charged Particle detector and Cassini/Ion and Neutral Camera, 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, that is, 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 toward determining the pressure balance and acceleration mechanisms inside the heliosheath.

This paper provides the latest data from Voyager 2 on plasma characteristics in the heliosheath including the observations of pressure waves in the plasma and particle data. Models and observations show that solar transients drive pressure waves through the heliosphere. Pressure pulses that could drive heliosheath waves are observed near the previous solar maximum upstream of the termination shock. We show that the most recent data is consistent with the presence of pressure waves and compare the heliosheath waves with the pressure increases in the heliosheath. The magnetic field is better correlated with density and galactic cosmic ray intensities in the supersonic solar wind than in the heliosheath. The galactic cosmic rays are correlated with the plasma and particles with a ∼30-day lag in both the supersonic wind and heliosheath.

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 (0.022-1.5MeV) observed by the Low-Energy Charged Particle Experiment instrument on board Voyager 2. The best fitting simulation suggests a pickup ion abundance of 203%, an upstream pickup ion temperature of 13.42MK, and a hot electron population with an apparent temperature of similar to 0.83MK. We conclude that the actual shock transition is a subcritical dispersive shock wave with low Mach number and high plasma .

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.

We have analyzed space weather throughout the heliosphere using the three-dimensional (3D) time-dependent 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 insitu 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.

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.

Voyager 1's long goodbye 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.

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” of Gurnett and Kurth (2008). Maxwellian fits to HEI detections indicate the HEIs are moving radially anti‐Sunward with a proton speed of 600 km/s, a density of 10−4 cm−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−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.