The New Horizons (NH) spacecraft is en route to cross the termination shock (TS) as it traverses the outer reaches of our solar system and will eventually leave the heliosphere and enter interstellar space. It is currently the only operational mission in the outer heliosphere and is equipped with instrumentation allowing to study the heliospheric boundaries. Careful preparation and operations planning for the TS encounter are necessary to maximize the scientific return from in situ observations. Thus, in this study, we aim to predict the timing of the TS crossing by NH. For this, we apply three different methods to derive the TS distance based on forecasted solar wind variations over time. We consider a stationary spherical TS approximation, an analytical hydrodynamic approximation under time-dependent solar wind dynamic pressure, and a time-dependent full 3D MHD model solution. We predict the NH encounter with the TS based on its radial distance from the Sun in time. Our results indicate that NH could encounter the TS as early as 2029 or as late as 2040, depending on the model used and future solar wind activity. Our results also indicate that multiple TS crossings by NH could happen over one to several years. (c) 2026 The Authors. Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

We analyze ∼40–200 keV energetic particle observations from the Pluto Energetic Particle Spectrometer Science Investigation (PEPSSI) on board the New Horizons (NH) spacecraft from 2007 to 2024 and compare them with similar measurements from the Low Energy Charged Particle experiments on the Voyager 1 and Voyager 2 spacecraft when they were at comparable heliocentric distances (5–60 au). In both Voyager data sets, particle intensities generally decrease with increasing radial distance from the Sun, reaching a minimum in the outer heliosphere before rising again prior to their respective encounters with the heliospheric termination shock (TS). This radial behavior in the intensity-time profiles is described as a heliospheric valley. The NH/PEPSSI time series from 5 au (2007) to 60 au (2024) exhibits a similar decrease in particle fluxes with distance. Analysis of the radial dependence of energetic particle intensities from all three spacecraft normalized by observations at 1 au to account for the solar cycle effects reveals an approximate piece-wise composite power-law relationship, with a slope break (steeper decline) beyond ∼33 au. This break may reflect differences in the dominant transport and acceleration mechanisms operating in the two regions demarcated by this radial break. In addition, a radial scaling method is applied to Voyager observations to best match the NH data. This comparison provides an estimate for the NH TS crossing between 2027 (68 au) and 2034 (83 au).

Galactic Cosmic Rays (GCRs) play a critical role in space weather forecasting and radiation hazard assessment for deep space missions. Accurate modeling of GCR flux throughout the heliosphere is therefore crucial. This paper describes the physical model and the functionality of a new user-friendly web-based tool designed to rapidly estimate the energy spectrum and flux of GCRs using minimal user input such as radial distance, time, and energy. The tool is based on the force-field approximation to the Parker transport equation and incorporates solar modulation via the correlation between GCRs and the mean sunspot number. Compared to existing Monte Carlo or finite-difference-based models which include a more physically realistic model of the heliosphere, this simpler 1D spherical model offers significant advantages in computational speed and ease of use. The tool is validated against spacecraft data from Voyager 1, Voyager 2, and New Horizons, and is capable of projecting future GCR spectra and fluxes based on sunspot number predictions. The key aim is to provide a very rapid estimate of GCR properties using a physics-based model that is constructed to fit as many data sets as possible. The tool will continue to be modified as new data sets are added, with the platform also serving as a data repository. Designed for both researchers and the general public, the platform also serves as an educational resource on GCRs and heliospheric physics. (c) 2026 The Author(s). Published by Elsevier B.V. on behalf of COSPAR. This is an open access article under the CC BY license (http:// creativecommons.org/licenses/by/4.0/).

The solar wind gradually slows with increasing distance from the Sun as interstellar material becomes ionized and picked up by the solar wind. When observing the solar wind in the outer heliosphere, it is difficult to differentiate temporal decreases in the speed originating at the Sun from the slowing of the solar wind with increase distance as interstellar material is picked up. In order to resolve this issue, we examine the solar wind speed in the outer heliosphere relative to the corresponding solar wind speed observed at 1 au. Additionally, we use a simulation to help discern the temporal and spatial changes in speed at New Horizons (NH) by driving the inner boundary of a heliospheric simulation with 1 au speed measurements. We conclude that the long interval of slow solar wind from 2018-2023 found at NH was caused by a combination of slow wind being directed toward NH owing to a low current sheet tilt angle keeping the streamers in the ecliptic, and the gradual slowing with increasing distance caused by additional pickup of interstellar material as the solar wind travels farther way from the Sun. When NH was between 50 and 58.5 au, the measured solar wind speed was reduced by similar to 13%-15% relative to 1 au owing to pickup of interstellar material. This reduction is a further decrease from the previously reported slowing of 5%-7% in the 30-43 au region.

We investigate the acceleration of pickup ions (PUIs) at an oblique heliospheric termination shock (HTS) using 2D kinetic hybrid simulations. The background magnetic field, B0, is set in-plane and makes an angle of Theta Bn = 50 degrees with respect to the shock normal. The shock compression ratio is r = 3.24. PUIs are reflected at the shock front and backstream upstream, while the thermal ion component is only heated across the shock wave. The backstreaming PUI component excites large-amplitude waves upstream of the shock (delta B similar to 0.2-1.0B0), propagating parallel to the background magnetic field, for which the wavenumber satisfies the resonant condition with the backstreaming PUI component. The large-amplitude wave further initiates shock rippling. PUIs form a power-law energy spectrum downstream of the shock, and the power-law index is fitted to -2.2, which is similar to the value estimated from diffusive shock acceleration (DSA) theory, Gamma=-r+2r-1 similar to-2.33 . PUIs primarily gain energy by specular reflection and the shock-drift-type acceleration mechanism, and when the gyroradius becomes much larger than the shock transition scale, such an energy gain formally becomes equivalent to the energy gain in the DSA theory, thereby explaining the consistency of the power-law distribution with the DSA power-law index. This study can help understand energetic neutral atom (ENA) distributions inferred from the polar regions, where the HTS tends to be oblique due to the configuration of the solar magnetic field. Large-scale shock obliquity is also possible in lower-latitude oblique regions of the HTS. We expect, from this study, that the ENA spectrum from the polar regions should resemble that expected for a power-law index in the DSA theory.

The Kelvin–Helmholtz (KH) instability occurs in multiple heliospheric (solar-wind stream interfaces, planetary magnetospheres, cometary tails, heliopause flanks) and interstellar (protoplanetary disks, relativistic jets, neutron star accretion disks) environments. While the KH instability has been well studied in the magnetohydrodynamic (MHD) limit, only limited studies were performed in the collisionless regime, which is conducive to the development of anisotropic pressures. Collisionless plasmas are often described using the Chew, Goldberger, and Low (CGL) equations, which feature an anisotropic pressure tensor. This paper presents a comprehensive analysis of the CGL version of the KH instability using linearized and numerical techniques. We find that the largest growth rates and the greatest incidence of magnetic effects occur in the MHD limit. In the large relaxation time CGL limit, part of the energy goes into the formation of pressure anisotropies, resulting in smaller amounts of energy being available for bending the field lines. Consequently, when we cross-compare CGL and MHD simulations that are otherwise identical, the current densities are largest in the MHD limit, and the largest magnetic islands also form in that limit. Early time and late-time formations of pressure anisotropies have also been studied. We also find that the strongest trend for forming intermittencies in the flow also occurs in the MHD limit. The paper also discusses possible consequences of our results for turbulence and reconnection in the heliosheath (the layer between the solar wind termination shock and the heliopause).

The Kelvin-Helmholtz (KH) instability occurs in multiple heliospheric (solar-wind stream interfaces, planetary magnetospheres, cometary tails, heliopause flanks) and interstellar (protoplanetary disks, relativistic jets, neutron star accretion disks) environments. While the KH instability has been well-studied in the magnetohydrodynamic (MHD) limit, only limited studies were performed in the collisionless regime, which is conducive to development of anisotropic pressures. Collisionless plasmas are often described using the Chew Goldberger and Low (CGL) equations which feature an anisotropic pressure tensor. This paper presents a comprehensive analysis of the CGL version of the KH instability using linearised and numerical techniques. We find that the largest growth rates and the greatest incidence of magnetic effects occur in the MHD limit. In the large relaxation time CGL limit, part of the energy goes into the formation of pressure anisotropies, resulting in smaller amounts of energy being available for bending the field lines. Consequently, when we cross-compare CGL and MHD simulations that are otherwise identical, the current densities are largest in the MHD limit, and the largest magnetic islands also form in that limit. Early and late time formation of pressure anisotropies have also been studied. We also find that the strongest trend for forming intermittencies in the flow also occurs in the MHD limit. The paper also discusses possible consequences of our results for turbulence and reconnection in the heliosheath (the layer between the solar wind termination shock and the heliopause).

Interstellar dust has been detected in situ flowing through the heliosphere. Understanding the implications of this dust for the nature of interstellar dust in the very local interstellar medium requires modeling the transport of the grains as they interact with the solar wind magnetic field. The magnetic field in the sector region (SR) that contains the heliospheric current sheet is substantially different from that in the monopolar solar wind. The rapid polarity flips that occur in the SR can present an effectively very low averaged field strength to grains that have gyroradii of tens of au. We present new calculations of dust transport through the heliosphere using a model that includes the SR. We show that the SR can act as a window allowing even relatively small grains to penetrate deep into the heliosphere. The presence of the SR reduces the variation in dust density with the solar cycle (as compared to models without it), with very little concentration or dilution of the dust for grains larger than ∼ 0.1 μm for most of the solar cycle (except for a focusing overall polarity of the field at solar minimum.) While the lack of time dependence of the magnetic field during transport of grains through the heliosphere is a limitation of the model, the relative lack of variation as a function of the point in the solar cycle of the grain density in the inner heliosphere suggests that our results will not deviate dramatically from a model that fully incorporates time dependence.

Recent research raises the possibility that 2–3 and 6–7 million years ago, the Sun encountered massive clouds that shrank the heliosphere —the solar cocoon protecting our solar system— exposing Earth to its interstellar environment, in agreement with geological evidence from 60Fe and 244Pu isotopes. Here we show that during such encounters Earth was exposed to increased radiation in the form of high-energy particles. During periods of Earth’s immersion in the heliosphere, it received particle radiation that we name Heliospheric Energetic Particles (HEPs). The intensity of < 10 MeV protons was at least an order of magnitude more intense than today’s most extreme solar energetic particle (SEP) events. SEPs today last minutes to hours, but HEP exposure then lasted for extensive periods of several months, making it a prolonged external driver. During Earth’s excursion outside the heliosphere, it was exposed to a galactic cosmic ray radiation with the intensity of < 1 GeV protons at least an order of magnitude more intense than today. Therefore, the space surrounding Earth was permeated by a variable high-energy radiation. We discuss the implications for Earth’s climate and biodiversity.

Recent works suggest there are periods when the Sun encountered massive interstellar cold clouds which compressed the heliosphere to within Earth's orbit, exposing Earth to interstellar galactic cosmic rays and energetic particles of heliospheric origin. We model 10bE production in Earth's atmosphere during possible interstellar cloud encounters and supernovae. We find that, if the heliosphere is compressed to 0.2 AU (r_TS=0.12 AU), the 10Be production rate is elevated 9x above the background. To be distinguished in marine sediments (iron-manganese crusts) beyond terrestrial variability, this signal must be sustained for >0.01 Myr (>0.5 Myr). A 0.7 AU (r_TS=0.4 AU) compression increases the 10Be production rate 3x above the background, which may be detectable if the event lasts >1 Myr. We find that the 10Be peak observed by Koll et al. (2025) 10 Ma is too prolonged to be produced by a supernova, but could be attributed to an interstellar cloud crossing.

Global ideal magnetohydrodynamic models of the heliosphere typically predict a greatly exaggerated magnetic field pile-up in the inner heliosheath (IHS), the region between the termination shock and heliopause. However, Voyager 1 and 2 observations show only a gradual increase throughout this region. This mismatch is largely attributed to the simplified assumption of a unipolar solar magnetic field in many global models, which neglects the complex, folded structure of the heliospheric current sheet (HCS). The IHS, especially at low heliolatitudes, contains these compressed sector boundaries, widely considered prime locations for magnetic dissipation via reconnection. To align global model simulations with observations without incurring the prohibitive computational cost of resolving the kinetic-scale current sheet, this work introduces a phenomenological term into the magnetic field induction equation. This term captures the macroscopic effect of magnetic energy dissipation due to unresolved HCS dynamics. It is designed to mitigate the artificial magnetic pile-up, preserve the topological integrity of the magnetic field lines, and avoid explicit magnetic diffusion. This study demonstrates that incorporating a phenomenological dissipation term into global heliospheric models helps to resolve the longstanding discrepancy between simulated and observed magnetic field profiles in the IHS. The proposed mechanism reduces exaggerated magnetic energy (converts it into thermal energy), aligns model output with Voyager measurements of both magnetic field and proton density, and produces the outward shift in termination shock position and a reduction of the IHS thickness. We found that the characteristic time for magnetic field dissipation of about 6 years provides improved agreement with Voyager data in the IHS.

The outer heliosphere model in the Space Weather Modeling Framework describes the dynamics of the thermal solar wind, pickup ions, electrons, and neutrals. We have extended this model with turbulence transport and a kinetic description of the pickup ions (PUIs) to better capture the interaction of the solar wind with the interstellar medium. The turbulence energy is enhanced via the isotropization of newly born PUIs. This increased turbulence can further heat the thermal protons and electrons. Additionally, the PUIs are nonthermal. We assume that the PUIs are fast pitch-angle scattered into isotropic distribution through wave-particle interaction. Hence, we simulate the PUI velocity distribution function by solving the PUI transport equations under the assumption that the distribution is isotropic and depends only on the velocity magnitude in the solar wind rest frame. In the PUI transport equations, we include velocity-space advection contributions. We also have additional source terms containing charge exchange. This is the first 3D magnetohydrodynamic model to treat the full kinetic distribution of PUIs. The model, extended with turbulence, is validated against Voyager 2 data. The results are within the range of the observations in the supersonic solar wind.

The large-scale dynamics of the heliosphere is driven by solar activity and variable solar wind (SW) conditions. In situ SW observations inform time-dependent heliosphere modeling efforts but are only available up until the present day, thus restricting informed predictions to the very near future. We developed and implemented a method to provide long-term forecasting of SW parameters at 1 au. Such forecasting supports modeling efforts of the time-dependent global heliosphere by providing realistic boundary conditions for heliospheric models at the upwind model-domain boundary ahead of time in order to understand the global dynamic processes in the heliosphere such as the evolution of the termination shock (TS), heliosheath, and heliopause. Such forecasting capabilities are needed to better constrain the timing of New Horizons’ TS encounter and predict energetic neutral atom flux variation for IMAP. We analyzed SW direct measurement time series for periodicities at various timescales using Lomb–Scargle periodogram analysis. Based on identified prominent periodicities, we construct quasiperiodic functions for the SW parameters. By extrapolating these functions forward in time, we obtain a prediction of the SW evolution over the next two solar cycles. Our results indicate that the next pulse in the SW dynamic pressure, which controls the global heliosphere, will occur around 2035.

The solar wind travels supersonically in the solar system until it reaches the termination shock, where itis slowed down due to the interplay between the interstellar medium (ISM) and heliosphere. The regionof slowed down solar wind is referred to as the heliosheath, where a great number of mysteries remainunsolved. Within the SHIELD NASA Drive Center, investigating the physical processes within theheliosheath and their consequences is a fundamental goal in understanding both the shape of theheliosphere and also how it protects the solar system from harmful galactic cosmic rays. Here, wehighlight some of the findings of SHIELD: (1) a Rayleigh-Taylor like instability develops in theheliosheath due to charge exchange, which in turn allows for mixing between the solar wind and ISMplasma yielding a short, “croissant-like” heliotail; (2) via magnetohydrodynamic modeling, we cancapture this mixing region, which has potential implications for particle acceleration; (3) currentenergetic neutral atom (ENA) observations appear to be insufficient for distinguishing the true shape ofthe heliotail, but future ENA-focused missions, such as IMAP, will have the capability to determine theshape of the heliotail; (4) ENA observations of the heliotail and Lyman-alpha observations may also beable to reveal the properties of the interstellar magnetic field

As the protective cocoon of solar wind enveloping the solar system, the heliosphere shields Earth from low‐energy Galactic Cosmic Rays (GCRs) in the interstellar medium. Recent work that mapped the trajectory of the Sun found that the solar system may have passed through a massive interstellar cold cloud–the Local Lynx of Cold Clouds–with density 3,000 two to three million years ago. Such crossing would have compressed the heliosphere and directly exposed Earth to the interstellar flux of GCRs, which will react with and molecules to produce ion pairs in the atmosphere. Using a 2D atmospheric chemistry and dynamics model, we simulate how the increased low‐energy GCR flux affects Earth's climate. We study two cases: the removal of heliospheric shielding, and the concurrent effect of exposure to interstellar GCRs and a geomagnetic pole reversal, such as the Gauss‐Matuyama pole reversal which occurred 2.58 Mya. Without heliospheric shielding, we find a 5%–15% increase in polar stratospheric when compared to pre‐industrial solar minimum conditions. This produces a 1%–2% decrease in polar stratospheric ozone which decreases stratospheric temperatures by 0.1–0.2 K. In the case that a geomagnetic pole reversal would occur during the cold cloud crossing, we see almost no shielding of cosmic rays at the equator and mid‐latitudes; this produces over 50% enhancement, 14% polar stratospheric ozone depletion, and up to 1 K decrease in stratospheric temperature. Changes in stratospheric temperatures could have localized effects on surface temperatures, and ozone depletion could increase UV exposure on Earth's surface.

The shape and structure of the heliosphere remain subjects of ongoing debate, with current models differing on how far the heliospheric jets that form the two-lobe structure extend down the heliotail and whether the surrounding interstellar medium can penetrate the region between the lobes. Rayleigh–Taylor (RT) and Kelvin–Helmholtz (KH) instabilities along the axis of the heliospheric jets have been proposed as key drivers of turbulence in the heliosheath (HS). In this work, we present results from 225 yr MHD simulations that reveal periodic variations in solar wind speed and magnetic field within the HS, coinciding with the cyclic growth of RT and KH instabilities. At the onset of each cycle, the instabilities initially develop simultaneously in the direction normal to the plane of the heliospheric jet, with an average timescale of ∼4.2 yr. As the system evolves over ∼11.1 yr, RT and KH modes decouple, with KH modes subsequently dominating beyond the high-density region and leading to a reduced growth rate, corresponding to a timescale of ∼12.9 yr. These findings suggest that RT and KH instabilities can coevolve and reoccur periodically, contributing to turbulence generation in the HS. Their nonlinear development may play a fundamental role in shaping the large-scale structure of the heliosphere, particularly in the formation and evolution of the open heliospheric tail.

The heliosphere is influenced by the interaction of neutral hydrogen atoms from the interstellar medium with protons in the heliosheath (HS). The confinement by the solar magnetic field of the HS plasma forms distinct north and south heliospheric jets. Previous global MHD simulations of the heliosphere reveal that instabilities can develop in the heliospheric jets and make them become unstable, giving rise to large-scale turbulence. In this study, we show that there is a low-speed region of solar wind in the HS that provides a precondition for instabilities to develop. The low-speed region is formed due to charge exchange and magnetic tension. This region allows sufficient time for the instabilities to develop and induces a shear flow inside the HS leading to the development of a Kelvin-Helmholtz (K-H) instability. The estimated growth timescale of the K-H instability based on the simulation results is around 5-7 yr. Understanding the development of the instabilities in the HS is crucial for comprehending the dynamic processes within the HS and the structure of the heliosphere.

We use a state-of-the-art 3D MHD simulation to show that large-scale thermal pressure gradients in the heliosheath drive suprathermal particles (pickup ions with 1–5 keV) to stream along magnetic field lines faster than the cold bulk solar wind. Large-scale pressure gradients between the nose and flank of the heliopause stem from neutrals flowing in from the nose direction. Hybrid simulations suggest that these gradients will also drive field-aligned flows at higher energies (5–30 keV). The field-aligned flows are stronger within 10 au of the termination shock, where the magnetic field lines in the upwind hemisphere have shorter path lengths in the heliosheath before connecting back to the termination shock. This mechanism explains the large field-aligned flows seen in the data in the first 2.5 yr of heliosheath data when Voyager 2 was above the heliospheric current sheet. This work has implications for how the bulk flows are derived in Voyager 1 from the Compton–Getting effect, where particles at all energies are assumed to move with the same speed.

We present new evidence of active magnetic reconnection observed by Voyager-2 in the heliosheath region beyond the termination shock in the outer heliosphere. Multiple cases have been identified in which significant plasma jets are present during heliospheric current sheet crossings, or sector reversals. Using a combination of the plasma and magnetic field measurements from Voyager-2, candidate events are identified by rotating the magnetic field and plasma velocity vector data into a minimum-variance, LMN-coordinate frame, prior to checking consistency with the Walen relation on the corresponding plasma jet. In the LMN frame, the candidate events are selected based on consistency with the expected geometry of current sheets undergoing magnetic reconnection. The Walen relation further supports consistency with the physics of magnetic reconnection, in which the outflow jets of reconnected field and plasma are ejected from the reconnection site at the Alfvén speed. Error analysis based on limitations of the Voyager-2 data is accounted for throughout the process. In this presentation, we detail the event identification and walk through an example case before presenting several other cases. All combined, confirmation of active magnetic reconnection ongoing in the heliosheath has important implications concerning the heating of plasma in the outer heliosphere and the acceleration of pickup ions and possibly even anomalous cosmic rays.

We revisit previous hybrid simulations of the heating and acceleration of interstellar pickup ions (PUIs) at the solar wind termination shock. In previous simulations, a relatively cold initial distribution of PUIs was assumed; and while the resulting shock-heated distribution was consistent with Voyager 2 LECP measurements at about 30 keV, the intensity of the distribution downstream of the shock in the ~1–10 keV energy range was lower than predictions based on analysis of energetic neutral atoms (ENAs) from the Interstellar Boundary Explorer-Hi and Cassini's Ion and Neutral Camera. Here we perform new simulations with more realistic initial PUI distributions. We assume the distribution is a partially filled spherical shell in velocity space with a radius that varies from 320 to 640 km s −1 . We then use the distributions downstream of the shock from these new simulations to estimate the ENA flux spectrum and compare with observations. We find that the predicted ENA spectrum from the new simulations much better matches the observations over a broad range of energies. We conclude that the hybrid simulations provide reasonable predictions for the distribution of charged particles in the energy range from ~0.5 to 50 keV.