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The 2021 October 28 solar energetic particle (SEP) event was a rare ground level enhancement (GLE) event, where secondary particles from the interactions of SEPs with the Earth’s atmosphere were detected by neutron monitors on the ground. A number of papers have examined the solar signatures, neutron monitor observations, and the characteristics of the SEP protons and electrons for this event. Here we describe the heavy ion signatures, specifically O and Fe, observed by multiple spacecraft. Parker Solar Probe, Solar Terrestrial Relations Observatory-Ahead, and Advanced Composition Explorer were distributed over nearly 60° in solar longitude and 0.4 au in heliocentric distance. Despite their separations, all three spacecraft measured event-integrated O and Fe spectra, well represented by power laws, with nearly the same power-law index of approximately −1.7, which is significantly harder than most large SEP events and many GLE events. Moreover, the Fe/O abundance ratio determined from these spectra was also found to be spatially invariant over the 60° in longitude and 0.4 au in heliocentric distance. Such near uniformity is highly unusual, and only one similar occurrence was found in a previous multispacecraft. The observed Fe/O ratio of 0.39 is higher than typical for large SEP events but not unusual for GLE events.

In this study, we present the first-ever direct measurements of synchrotron-emitting heliospheric traveling shocks, intercepted by the Parker Solar Probe (PSP) during its close encounters. Given that much of our understanding of powerful astrophysical shocks is derived from synchrotron radiation, these observations by PSP provide an unprecedented opportunity to explore how shocks accelerate relativistic electrons and the conditions under which they emit radiation. The probe’s unparalleled capabilities to measure both electromagnetic fields and energetic particles with high precision in the near-Sun environment has allowed us to directly correlate the distribution of relativistic electrons with the resulting photon emissions. Our findings reveal that strong quasi-parallel shocks emit radiation at significantly higher intensities than quasi-perpendicular shocks due to the efficient acceleration of ultrarelativistic electrons. These experimental results are consistent with theory and recent observations of supernova remnant shocks and advance our understanding of shock physics across diverse space environments.

In this study, we employ the combined charged particle measurements from Integrated Science Investigation of the Sun on board the Parker Solar Probe (PSP) and Energetic Particle Detector on board the Solar Orbiter (SolO) to study the composition variation of the solar energetic particle (SEP) event occurring on 2023 May 16. During the event, SolO and PSP were located at a similar radial distance of ∼0.7 au and were separated by ∼60° in longitude. The footpoints of both PSP and SolO were west of the flare region, but the former was much closer (18° versus 80°). Such a distribution of observers is ideal for studying the longitudinal dependence of the ion composition with the minimum transport effects of particles along the radial direction. We focus on H, He, O, and Fe measured by both spacecraft in sunward and antisunward directions. Their spectra are in a double power-law shape, which is fitted best by the Band function. Notably, the event was Fe rich at PSP, where the mean Fe/O ratio at energies of 0.1–10 Mev nuc ^−1 was 0.48, higher than the average Fe/O ratio in previous large SEP events. In contrast, the mean Fe/O ratio at SolO over the same energy range was considerably lower at 0.08. The Fe/O ratio between 0.5 and 10 MeV nuc ^−1 at both spacecraft is nearly constant. Although the He/H ratio shows energy dependence, decreasing with increasing energy, the He/H ratio at PSP is still about twice as high as that at SolO. Such a strong longitudinal dependence of element abundances and the Fe-rich component in the PSP data could be attributed to the direct flare contribution. Moreover, the temporal profiles indicate that differences in the Fe/O and He/H ratios between PSP and SolO persisted throughout the entire event rather than only at the start.

The Interstellar Boundary Explorer (IBEX) has been observing the outer heliosphere and its interactions with the very local interstellar medium (VLISM) via measurements of energetic neutral atoms (ENAs) for over 14 yr. We discovered the IBEX Ribbon—a structure completely unanticipated by any prior theory or model—that almost certainly resides beyond the heliopause in the VLISM. We also characterized the other major source of heliospheric ENAs, the globally distributed flux (GDF), produced largely in the heliosheath between the termination shock and heliopause. In this study, we make three major new contributions. First, we validate, provide, and analyze the most recent 3 yr of IBEX-Hi (0.5–6 keV FWHM) data (2020–2022) for the first time. Second, we link these observations to the prior 11 yr of observations, exploring long-term variations. Finally, we provide the first IBEX team-validated Ribbon/GDF separation scheme and separated maps. Because of the uncertainty in separating different line-of-sight integrated sources, we provide not just best guess (median) maps, but also maps with upper and lower reasonable values of Ribbon and GDF fluxes, along with bounding fluxes that add the uncertainties to the upper and lower values. This allows theories and models to be compared with a range of possible values that the IBEX team believes are consistent with data. These observations, along with the reanalysis of the prior 11 yr of IBEX-Hi data, provide new insights and even further develop our detailed understanding of the heliosphere’s interaction with the local interstellar medium unlocked by IBEX.

On 2022 February 15–16, multiple spacecraft measured one of the most intense solar energetic particle (SEP) events observed so far in Solar Cycle 25. This study provides an overview of interesting observations made by multiple spacecraft during this event. Parker Solar Probe (PSP) and BepiColombo were close to each other at 0.34–0.37 au (a radial separation of ∼0.03 au) as they were impacted by the flank of the associated coronal mass ejection (CME). At about 100° in the retrograde direction and 1.5 au away from the Sun, the radiation detector on board the Curiosity surface rover observed the largest ground-level enhancement on Mars since surface measurements began. At intermediate distances (0.7–1.0 au), the presence of stream interaction regions (SIRs) during the SEP arrival time provides additional complexities regarding the analysis of the distinct contributions of CME-driven versus SIR-driven events in observations by spacecraft such as Solar Orbiter and STEREO-A, and by near-Earth spacecraft like ACE, SOHO, and WIND. The proximity of PSP and BepiColombo also enables us to directly compare their measurements and perform cross-calibration for the energetic particle instruments on board the two spacecraft. Our analysis indicates that energetic proton measurements from BepiColombo and PSP are in reasonable agreement with each other to within a factor of ∼1.35. Finally, this study introduces the various ongoing efforts that will collectively improve our understanding of this impactful, widespread SEP event.

Solar energetic particle (SEP) events have been observed by the Parker Solar Probe (PSP) spacecraft since its launch in 2018. These events include sources from solar flares and coronal mass ejections (CMEs). The IS⊙IS instrument suite on board PSP is measuring ions over energies from ∼ 20 keV nucleon ^−1 to 200 MeV nucleon ^−1 and electrons from ∼ 20 keV to 6 MeV. Previous studies sought to group CME characteristics based on their plasma conditions and arrived at general descriptions with large statistical errors, leaving open questions on how to properly group CMEs based solely on their plasma conditions. To help resolve these open questions, the plasma properties of CMEs have been examined in relation to SEPs. Here, we reexamine one plasma property, the solar wind proton temperature, and compare it to the proton SEP intensity in a region immediately downstream of a CME-driven shock for seven CMEs observed at radial distances within 1 au. We find a statistically strong correlation between proton SEP intensity and bulk proton temperature, indicating a clear relationship between SEPs and the conditions in the solar wind. Furthermore, we propose that an indirect coupling of SEP intensity to the level of turbulence and the amount of energy dissipation that results is mainly responsible for the observed correlation between SEP intensity and proton temperature. These results are key to understanding the interaction of SEPs with the bulk solar wind in CME-driven shocks and will improve our ability to model the interplay of shock evolution and particle acceleration.

Parker Solar Probe (PSP) observations of a small dispersive event on 2022 February 27 and 28 indicate scatter-free propagation as the dominant transport mechanism between the low corona and greater than 35 solar radii. The event occurred during unique orbital conditions that prevailed along specific flux tubes that PSP encountered repeatedly between 25 and 35 R _s during outbound orbit 11. This segment of the PSP orbit exhibits almost stationary angular motion relative to the rotating solar surface, such that in the rotating frame, PSP’s motion is essentially radial. The time dispersion often observed in impulsive solar energetic particle (SEP) events continues in this case down to velocities including the core solar-wind ion velocities. Especially at the onset of this event, the ^3 He content is much larger than the usual SEP abundances seen in the energy range from ∼100 keV to several MeV for helium. Later in the event, iron is enhanced. The compositional signatures suggest this to be an example of an acceleration mechanism for generating the seed energetic particles required by shock (or compression) acceleration models in SEP events to account for the enrichment of various species above solar abundances in such events. A preliminary search of similar orbital conditions over the PSP mission has not revealed additional such events, although favorable conditions (isolated impulsive acceleration and well-ordered magnetic field connection with minimal magnetic field fluctuation) that would be required are infrequently realized, given the small fraction of the PSP trajectory that meets these observation conditions.

On 2022 September 5, Parker Solar Probe (Parker) observed a large solar energetic particle (SEP) event at the unprecedented distance of only 15 R _S from the Sun. The observations from the Integrated Science Investigation of the Sun (IS⊙IS) obtained over the course of this event are remarkably rich, and an overview is presented here. IS⊙IS is capable of measuring ions from 20 keV to over 100 MeV nuc ^−1 and electrons from 30 keV to 6 MeV; here, we primarily focus on the proton and helium measurements above 80 keV. Among the surprising results are evidence of inverse velocity dispersion at energies above 1 MeV during the onset of the event, a sharp decrease in the energetic particle intensities at all energies at the interplanetary shock crossing, and repeated short durations of highly anisotropic sunward flow. Many changes in the SEP intensities, anisotropy, and spectral steepness are coincident with solar wind structure boundaries identified using the Parker solar wind magnetic field and plasma data. However, there are significant changes that are not correlated with any clearly visible solar wind variation. The observations presented here serve as an introduction to a complex event with numerous opportunities for future, more in-depth studies.

High-energy neutral solar radiation in the form of γ -rays and neutrons is produced as secondary products in solar flares. The characteristics of this emission can provide key information regarding the energization of charged particles, particularly when primary particles remain trapped in the corona. The Integrated Science Investigation of the Sun (IS⊙IS) suite on Parker Solar Probe is composed of instruments primarily intended to measure energetic charged particles. However, the High Energy Telescope (HET) in IS⊙IS was also designed with a supplementary neutral mode intended to measure γ -rays and neutrons. HET observed its first clear solar γ -ray event in connection with a hard X-ray flare, the eruption of a coronal mass ejection, and a solar energetic particle event on 2022 September 5. The X-ray spectral shape was observed to harden over the course of the event, culminating with the observation of γ -rays by HET. A coincident enhancement in the lower-energy Energetic Particle Instrument (EPI-Lo) was also observed, likely produced by incident solar γ -rays despite the EPI-Lo instrument not having any special neutral measurement capabilities. We use Monte Carlo modeling to reconstruct the incident γ -ray spectrum based on the measured spectrum to demonstrate that the combination of IS⊙IS instruments can measure hard X-rays and γ -rays from ∼60 keV–7 MeV. Despite the fact that this is a supplemental science goal of the mission, the capability of the IS⊙IS instruments to measure γ -rays is important for the study of this population due to the very limited instruments currently observing the Sun in γ -rays.

We present an event observed by Parker Solar Probe (PSP) at ∼0.2 au on 2022 March 2 in which imaging and in situ measurements coincide. During this event, PSP passed through structures on the flank of a streamer blowout coronal mass ejection (CME) including an isolated flux tube in front of the CME, a turbulent sheath, and the CME itself. Imaging observations and in situ helicity and principal variance signatures consistently show the presence of flux ropes internal to the CME. In both the sheath and the CME interval, the distributions are more isotropic, the spectra are softer, and the abundance ratios of Fe/O and He/H are lower than those in the isolated flux tube, and yet elevated relative to typical plasma and solar energetic particle abundances. These signatures in the sheath and the CME indicate that both flare populations and those from the plasma are accelerated to form the observed energetic particle enhancements. In contrast, the isolated flux tube shows large streaming, hard spectra, and large Fe/O and He/H ratios, indicating flare sources. Energetic particle fluxes are most enhanced within the CME interval from suprathermal through energetic particle energies (∼keV to >10 MeV), indicating particle acceleration, as well as confinement local to the closed magnetic structure. The flux-rope morphology of the CME helps to enable local modulation and trapping of energetic particles, in particular along helicity channels and other plasma boundaries. Thus, the CME acts to build up energetic particle populations, allowing them to be fed into subsequent higher-energy particle acceleration throughout the inner heliosphere where a compression or shock forms on the CME front.

The paper shows the thermodynamic nature of the evolution of the pickup ion (PUI) distributions through their incorporation and subsequent expansion as the solar wind moves outward through the heliosphere. In particular, the PUI expansive cooling is connected to thermodynamic polytropic processes and the thermodynamic kappa parameter. Previously, the characterization of the cooling was phenomenologically given by a “cooling index” α , which is the exponent involved in the power-law relationship between PUI speed and position. Here, we develop the relationship between the cooling and polytropic indices. Then, we show the connection between the cooling index and the thermodynamic parameter kappa. Finally, we verify the derived thermodynamic relations with direct heliospheric observations over varying distances from the Sun. Going forward, we suggest that studies of PUIs seeking to understand the underlying physics of these important particles rely on the thermodynamic parameter of kappa, and its association with the polytropic index, and not on an ad hoc cooling index.

We investigate the long-term persistence of solar energetic particle (SEP) time series by means of four different methods: Hurst rescaled range R / S analysis, detrended fluctuation analysis, centered moving average analysis, and the fluctuation of natural time under the time reversal method. For these analyses, we use data sets from the Integrated Science Investigation of the Sun instrument suite on board NASA's Parker Solar Probe. Background systematic noise is modeled using cross-correlation analysis between different SEP energy channels and subtracted from the original data. The use of these four methods for deriving the time-series persistence allows us to (i) differentiate between quiet- and active-Sun periods based on the values of the corresponding self-similarity exponents alone; (ii) identify the onset of an ongoing activity well before it reaches its maximum SEP flux; (iii) reveal an interesting fine structure when activity is observed; and (iv) provide, for the first time, an estimate of the maximum SEP flux of a future storm based on the entropy change of natural time under time reversal.

We develop the kappa-tail fitting technique, which analyzes observations of power-law tails of distributions and energy flux spectra, and connects them to theoretical modeling of kappa distributions, to determine the thermodynamics of the examined space plasma. In particular, we (i) construct the associated mathematical formulation; (ii) prove its decisive lead for determining whether the observed power-law is associated with kappa distributions; and (iii) provide a validation of the technique using pseudo-observations of typical input plasma parameters. Then, we apply this technique to a case study by determining the thermodynamics of solar energetic particle (SEP) protons, for an SEP event observed on 2021 April 17, by the Parker Solar Probe (PSP)/Integrated Science Investigation of the Sun instrument suite on board PSP. The results show SEP temperatures and densities of the order of ∼1 MeV and ∼5 × 10 ^−7 cm ^−3 , respectively.

In this paper, we model the high-energy tail of observed solar energetic proton energy distributions with a kappa distribution function. We employ a technique for deriving the thermodynamic parameters of solar energetic proton populations measured by the Parker Solar Probe Integrated Science Investigation of the Sun EPI-Hi high-energy telescope, over energies from 10 to 60 MeV. With this technique, we explore, for the first time, the characteristic thermodynamic properties of the solar energetic protons associated with an interplanetary coronal mass ejection (ICME) and its driven shock. We find that: (1) the spectral index or, equivalently, the thermodynamic parameter kappa of solar energetic protons ( κ _EP ) gradually increases, starting from the pre-ICME region (upstream of the CME-driven shock), reaching a maximum in the CME ejecta ( κ _EP ≈ 3.5), followed by a gradual decrease throughout the trailing portion of the CME; (2) the solar energetic proton temperature and density ( T _EP and n _EP ) appear anticorrelated, a behavior consistent with subisothermal polytropic processes; and (3) values of T _EP and κ _EP appear to be positively correlated, indicating an increasing entropy with time. Therefore, these proton populations are characterized by a complex and evolving thermodynamic behavior, consisting of multiple subisothermal polytropic processes, and a large-scale trend of increasing temperature, kappa, and entropy. This study and its companion study by Livadiotis et al. open up a new set of procedures for investigating the thermodynamic behavior of energetic particles and their shared thermal properties.

Interstellar Boundary Explorer (IBEX) observations of energetic neutral atom (ENA) fluxes from the heliosphere have greatly enriched our understanding of the interaction of the solar wind (SW) with the local interstellar medium (LISM). However, there has been recent controversy surrounding the inability of most ENA models to produce as high an intensity of ∼0.5–6 keV ENAs as IBEX observes at 1 au, especially as a function of time. In our previous study (E. J. Zirnstein et al.), we introduced a new model that utilizes a data-driven magnetohydrodynamic simulation of the SW–LISM interaction to propagate pickup ions through the heliosheath (HS) after they are nonadiabatically heated at the heliospheric termination shock. E. J. Zirnstein et al. only simulated and analyzed IBEX observations from the direction of Voyager 2. In this study, we expand our model to include fluxes from the direction of Voyager 1, as well as in the low-latitude part (middle) of the ribbon (10° below the ecliptic plane). We show that the model results at Voyager 1 are consistent with E. J. Zirnstein et al.’s results at Voyager 2 in terms of a secondary ENA source contribution of ≲20% from both directions. Our results in the middle of the ribbon also reproduce the data, when including a time-dependent secondary ENA source. Finally, we demonstrate with our simulation that three large pressure waves likely merged in the VLISM and were observed by Voyager 1 as “pf2,” while at least one of the wave’s effects in the HS was observed by IBEX as a brief enhancement in ENA flux in early 2016.

We investigate the long-term persistence of the time series of energetic neutral atom (ENA) fluxes recorded by the Interstellar Boundary Explorer (IBEX). ENAs provide global information about the outer heliosphere and its interactions with the very local interstellar medium. To avoid the IBEX Ribbon, here, we focus our analysis solely on the polar regions N60°–N90° and S60°–S90°. Each year, IBEX takes two measurements of every pixel in the sky. We make use of the whole set of 14 yr of IBEX data and adhere to the correct time order for the construction of the flux time series. We examine in detail both the trend and the fluctuations of these time series. Using modern methods of time series analysis and persistence characterization, we show that the time series (i) trend is influenced by the solar cycle, (ii) persistence can be established independently of the presence of this trend, and (iii) statistical properties of the fluctuations differs between north and south, pointing to the existence of anisotropy and thus a north–south asymmetry.

Prior to the Voyagers’ heliopause crossings, models and the community expected the magnetic field to show major rotations across the boundary. Surprisingly, the field showed no significant change in direction from the heliospheric Parker Spiral at either Voyager location. Meanwhile, a major result from the IBEX mission is the derived magnitude and direction of the interstellar field far from the Sun (∼1000 au) beyond the influence of the heliosphere. Using a self-consistent model fit to IBEX ribbon data, Zirnstein et al. reported that this “pristine” local interstellar magnetic field has a magnitude of 0.293 nT and direction of 227° in ecliptic longitude and 34.°6 in ecliptic latitude. These values differ by 27% (51%) and 44° (12°) from what Voyager 1 (2) currently observes (as of ∼2022.75). While differences are to be expected as the field undrapes away from the heliosphere, the global structure of the draping across hundreds of astronimcal units has not been reconciled. This leads to several questions: How are these distinct sets of observations reconcilable? What is the interstellar magnetic field’s large-scale structure? How far out would a future mission need to go to sample the unperturbed field? Here, we show that if realistic errors are included for the difficult-to-calibrate radial field component, the measured transverse field is consistent with that predicted by IBEX, allowing us to answer these questions through a unified picture of the behavior of the local interstellar magnetic field from its draping around the heliopause to its unfolding into the pristine interstellar medium.

Since its discovery in 2009, the IBEX energetic neutral atom (ENA) Ribbon has been a subject of numerous studies. It appears at energies ∼0.5–6 keV and is most pronounced at ∼1–3 keV. It is almost circular, ∼20°–40° wide, and its center lies near the pristine local interstellar magnetic field direction, whose field lines are draped around the heliosphere. The Ribbon intensity is enhanced above the more diffuse, globally distributed flux (GDF) and varies on timescales that are delayed compared to the underlying and slowly varying GDF. We present a novel method to infer the Ribbon boundaries and transverse profile of the Ribbon using sequential time variations of ENA fluxes, with minimal modeling assumptions involved. The method utilizes the difference in temporal evolution between the total Ribbon content and GDF fluxes. We then use the inferred Ribbon transverse profile to statistically quantify the GDF contribution to the observed peak Ribbon intensity to be ∼32.23% ± 3.15% in 2009–2011. This Ribbon separation method works best during times of gradual changes in solar wind output, and with high angular resolution and ENA counting statistics; results thus provide a proof of concept for the upcoming Interstellar Mapping and Acceleration Probe ENA measurements.

During Encounter 11, Parker Solar Probe observed a low-energy dispersive ions event of solar origin. The event was observed in the SPAN-I and IS⊙IS EPI-Lo sensors. The event started at a few MeV energy in the EPI-Lo sensor and progressed down in energy to ≈1 keV and merged with the bulk of the solar wind. This event is substantially different from typical solar energetic particles because the energetic population shows a distinct peak in the energy spectrum that descends in energy (not a power-law tail). In this Letter, we explore this event’s nature, origin, and characteristics.

A central problem of space plasma physics is how protons and electrons are heated in a turbulent, magnetized plasma. The differential heating of charged species due to dissipation of turbulent fluctuations plays a key role in solar wind evolution. Measurements from previous heliophysics missions have provided estimates of proton and electron heating rates beyond 0.27 au. Using Parker Solar Probe (PSP) data accumulated during the first 10 encounters, we extend the evaluation of the individual rates of heat deposition for protons and electrons to a distance of 0.063 au (13.5 R _⊙ ) in the newly formed solar wind. The PSP data in the near-Sun environment show different behavior of the electron heat conduction flux from what was predicted from previous fits to Helios and Ulysses data. Consequently, the empirically derived proton and electron heating rates exhibit significantly different behavior than previous reports, with the proton heating becoming increasingly dominant over electron heating at decreasing heliocentric distances. We find that the protons receive about 80% of the total plasma heating at ≈13 R _⊙ , slightly higher than the near-Earth values. This empirically derived heating partition between protons and electrons will help to constrain theoretical models of solar wind heating.

The role of pickup ions (PUIs) in the solar wind interaction with the local interstellar medium is investigated with 3D, multifluid simulations. The flow of the mixture of all charged particles is described by the ideal MHD equations, with the source terms responsible for charge exchange between ions and neutral atoms. The thermodynamically distinct populations of neutrals are governed by individual sets of gas dynamics Euler equations. PUIs are treated as a separate, comoving fluid. Because the anisotropic behavior of PUIs at the heliospheric termination shocks is not described by the standard conservation laws (a.k.a. the Rankine–Hugoniot relations), we derived boundary conditions for them, which are obtained from the dedicated kinetic simulations of collisionless shocks. It is demonstrated that this approach to treating PUIs makes the computation results more consistent with observational data. In particular, the PUI pressure in the inner heliosheath (IHS) becomes higher by ∼40%–50% in the new model, as compared with the solutions where no special boundary conditions are applied. Hotter PUIs eventually lead to charge-exchange-driven cooling of the IHS plasma, which reduces the IHS width by ∼15% (∼8–10 au) in the upwind direction, and even more in the other directions. The density of secondary neutral atoms born in the IHS decreases by ∼30%, while their temperature increases by ∼60%. Simulation results are validated with New Horizons data at distances between 11 and 47 au.

Energetic charged particles are pervasive throughout the heliosphere with contributions from solar energetic particle events, stream and corotating interaction regions, galactic cosmic rays, anomalous cosmic rays, and suprathermal ions. The Integrated Science Investigation of the Sun (IS⊙IS) on board the Parker Solar Probe is a suite of energetic particle detectors covering the energy range ∼20 keV–200 MeV nuc ^−1 . IS⊙IS measures energetic particles closer to the Sun than any instrument suite in history, providing a singular view of the energetic particle population in a previously unexplored region. To enable the global research community to efficiently use IS⊙IS data, we have developed an online living catalog of energetic particle enhancements observed by the IS⊙IS instruments. Event identification methodology, information on accessing the catalog, highlights of several events, and a summary of the overall trends are presented. Also included is a summary Event Catalog showing many of the key event parameters for IS⊙IS events to the time of writing.

The IBEX-Lo instrument on the Interstellar Boundary Explorer (IBEX) mission measures interstellar neutral (ISN) helium atoms. The detection of helium atoms is made through negative hydrogen (H ^− ) ions sputtered by helium atoms from the IBEX-Lo’s conversion surface. The energy spectrum of ions sputtered by ISN helium atoms is broad and overlaps the four lowest IBEX-Lo electrostatic analyzer (ESA) steps. Consequently, the energy response function for helium atoms does not correspond to the nominal energy step transmission. Moreover, laboratory calibration is incomplete because it is difficult to produce narrow-energy neutral atom beams that are expected for ISN helium atoms. Here, we analyze the ISN helium observations in ESA steps 1–4 to derive the relative in-flight response of IBEX-Lo to helium atoms. We compare the ratios of the observed count rates as a function of the mean ISN helium atom energy estimated using the Warsaw Test Particle Model (WTPM). The WTPM uses a global heliosphere model to calculate charge exchange gains and losses to estimate the secondary ISN helium population. We find that the modeled mean energies of ISN helium atoms, unlike their modeled fluxes, are not very sensitive to the very local interstellar medium parameters. The obtained relative responses supplement the laboratory calibration and enable more detailed quantitative studies of the ISN helium signal. A similar procedure that we applied to the IBEX-Lo observations may be used to complement laboratory calibration of the next-generation IMAP-Lo instrument on the Interstellar Mapping and Acceleration Probe (IMAP) mission.

We use Parker Solar Probe (PSP) observations to report the first direct measurements of the particle and field environments while crossing the leg of a coronal mass ejection (CME) very close to the Sun (∼14 Rs). An analysis that combines imaging from 1 au and PSP with a CME model, predicts an encounter time and duration that correspond to an unusual, complete dropout in low-energy solar energetic ions from H–Fe, observed by the Integrated Science Investigation of the Sun (IS⊙IS). The surrounding regions are populated with low-intensity protons and heavy ions from 10s to 100 keV, typical of some quiet times close in to the Sun. In contrast, the magnetic field and solar wind plasma show no similarly abrupt changes at the boundaries of the dropout. Together, the IS⊙IS energetic particle observations, combined with remote sensing of the CME and a dearth of other “typical” CME signatures, indicate that this CME leg is significantly different from the magnetic and plasma structure normally assumed for CMEs near the Sun and observed in interplanetary CMEs farther out in the solar wind. The dropout in low-energy energetic ions may be due to the cooling of suprathermal ions at the base of the CME leg flux tube, owing to the rapid outward expansion during the release of the CME.

We present the first estimation of the energy cascade rate in Jupiter’s magnetosheath (MS). We use in situ observations from the Jovian Auroral Distributions Experiment and the magnetometer investigation instruments on board the Juno spacecraft, in concert with two recent compressible models, to investigate the cascade rate in the magnetohydrodynamic (MHD) scales. While a high level of compressible density fluctuations is observed in the Jovian MS, a constant energy flux exists in the MHD inertial range. The compressible isothermal and polytropic energy cascade rates increase in the MHD range when density fluctuations are present. We find that the energy cascade rate in Jupiter’s magnetosheath is at least 2 orders of magnitude (100 times) smaller than the corresponding typical value in the Earth’s magnetosheath.

A “Ribbon” of enhanced energetic neutral atom (ENA) emissions was discovered by the Interstellar Boundary Explorer in 2009, redefining our understanding of the heliosphere boundaries and the physical processes occurring at the interstellar interface. The Ribbon signal is intertwined with that of a globally distributed flux (GDF) that spans the entire sky. To a certain extent, Ribbon separation methods enabled examining its evolution independent of the underlying GDF. Observations over a full solar cycle revealed the Ribbon’s evolving nature, with intensity variations closely tracking those of the solar wind (SW) structure after a few years delay, accounting for the SW–ENA recycling process. In this work, we examine the Ribbon structure, namely its ENA fluxes, angular extent, width, and circularity properties for two years, 2009 and 2019, representative of the declining phases of two adjacent solar cycles. We find that, (i) the Ribbon ENA fluxes have recovered in the nose direction and south of it down to ∼25° (for energies below 1.7 keV) and not at mid and high ecliptic latitudes; (ii) the Ribbon width exhibits significant variability as a function of azimuthal angle; (iii) circularity analysis suggests that the 2019 Ribbon exhibits a statistically consistent radius with that in 2009. The Ribbon’s partial recovery is aligned with the consensus of a heliosphere with its closest point being southward of the nose region. The large variability of the Ribbon width as a function of azimuth in 2019 compared to 2009 is likely indicative of small-scale processes within the Ribbon.

In this paper, we develop the transport equation of kappa, the fundamental thermodynamic parameter that labels kappa distributions of particle velocities. Using the recently developed concept of entropy defect, we are able to formulate the transport equation of kappa as a function of a general, positive or negative, rate of entropy change. Then, we derive the particular case of exchanging plasma ions with low-dimensionality, newly born pickup protons, which interact and decrease the entropy of the flow of otherwise kappa-distributed plasma protons. Finally, we apply the transport equation of kappa to the solar wind plasma protons, which leads to the radial profile of kappa values, as well as the evolution of the kappa distributions through the heliosphere. The results show that the solar wind kappa decreases with increasing heliocentric distance, corresponding to plasmas residing in stationary states far from classical thermal equilibrium. Moreover, in the outer heliosphere and the heliosheath, kappa reaches its lowest values and is spread across the far-equilibrium region of 1.5 < κ < 2.5, which coincides with independent observations provided by NASA’s Interstellar Boundary Explorer mission.

We study the thermodynamics of the plasma protons in the polar regions of the inner heliosheath (IHS) and its connection with solar activity over solar cycle 24. First, we express the thermodynamic parameters of this plasma with respect to the year of energetic neutral atom (ENA) creation and perform a statistical analysis of temperatures, in order to provide a more precise characterization of the thermodynamics of IHS. Then, we perform an autocorrelation between the IHS temperature and the solar activity, using the proxies of sunspot number and fractional area of the polar coronal holes. We show that there is (i) high correlation between the time series of IHS proton temperatures and sunspot number, which is maximized for a time delay of τ ∼ 2.5 yr for both the north and south polar regions combined; (ii) high negative correlation between the temperature of the proton plasma in the north and south with the coronal hole fractional areas, where the time delay for the two poles combined is τ ∼ 2.71 ± 0.15 yr; and (iii) an asymmetry of a time-delay difference between the poles ∼0.22 yr, indicating that the southern polar ENA source region is ∼19 au closer than the northern one for a solar wind plasma protons of bulk speed of ∼400 km s ^−1 . The findings demonstrate a connection between the IHS thermodynamics and solar activity through the solar wind, primarily manifested by the coronal holes expanding near solar minimum, which drives the expansion of fast solar wind over larger angles from high down to middle latitudes in the IHS.

In situ observations of interstellar neutral (ISN) helium atoms by the IBEX-Lo instrument on board the Interstellar Boundary Explorer (IBEX) mission are used to determine the velocity and temperature of the pristine very local interstellar medium (VLISM). Most ISN helium atoms penetrating the heliosphere, known as the primary population, originate in the pristine VLISM. As the primary atoms travel through the outer heliosheath, they charge exchange with He ^+ ions in slowed and compressed plasma, creating the secondary population. With more than 2.4 million ISN helium atoms being sampled by IBEX during ISN seasons 2009–2020, we compare the observations with the predictions of a parameterized model of ISN helium transport in the heliosphere. We account for the filtration of ISN helium atoms at the heliospheric boundaries by charge-exchange and elastic collisions. We examine the sensitivity of the ISN helium fluxes to the interstellar conditions described by the pristine VLISM velocity, temperature, magnetic field, and composition. We show that comprehensive modeling of the filtration processes is critical for interpreting ISN helium observations, as the change in the derived VLISM conditions exceeds the statistical uncertainties when accounting for these effects. The pristine VLISM parameters found by this analysis are the flow speed (26.6 km s ^−1 ), inflow direction in ecliptic coordinates (255.°7, 5.°04), temperature (7350 K), and B − V plane inclination to the ecliptic plane (53.°7). The derived pristine VLISM He ^+ density is 9.7 × 10 ^−3 cm ^−3 . Additionally, we show a strong correlation between the interstellar plasma density and magnetic field strength deduced from these observations.

The paper derives the one-to-one connecting relationships between plasma heating and its polytropic index, and addresses the consequences through the transport equation of temperature. Thermodynamic polytropic processes are classified in accordance to their polytropic index, the exponent of the power-law relationship of thermal pressure expressed with respect to density. These processes generalize the adiabatic one, where no heating is exchanged between the system and its environment. We show that, in addition to heating terms, the transport equation of temperature depends on the adiabatic index, instead of a general, nonadiabatic polytropic index, even when the plasma follows nonadiabatic processes. This is because all the information regarding the system's polytropic index is contained in the heating term, even for a nonconstant polytropic index. Moreover, the paper (i) defines the role of the polytropic index in the context of heating; (ii) clarifies the role of the nonadiabatic polytropic index in the transport equation of temperature; (iii) provides an alternative method for deriving the turbulent heating through the comparably simpler polytropic index path; and, finally, (iv) shows a one-component plasma proof-of-concept of this method and discusses the implications of such derived connecting relationships in the solar wind plasma in the heliosphere.

Direct-sampling observations of interstellar neutral (ISN) species and their secondary populations give information about the physical state of the local interstellar medium and processes occurring in the outer heliosheath. Such observations are performed from Earth’s orbit by the IBEX-Lo experiment on board the Interstellar Boundary Explorer (IBEX) mission. IBEX ISN viewing is restricted to directions close to perpendicular to the Earth–Sun line, which limits the observations of interstellar species to several months during the year. A greatly improved data set will be possible for the upcoming Interstellar Mapping and Acceleration Probe (IMAP) mission due to a novel concept of putting the IMAP ISN detector, IMAP-Lo, on a pivot platform that varies the angle of observation relative to the Sun–Earth line and the detector boresight. Here, we suggest a 2 yr scenario for varying the viewing angle in such a way that all the necessary atom components can be observed sufficiently well to achieve the science goals of the nominal IMAP mission. This scenario facilitates, among others, removal of the correlation of the inflow parameters of interstellar gas, unambiguous analysis of the primary and secondary populations of interstellar helium (He), neon (Ne), and oxygen (O), and determination of the ionization rates of He and Ne free of possible calibration bias. The scheme is operationally simple, provides good counting statistics, and synergizes observations of interstellar species and heliospheric energetic neutral atoms.

We present analyses of 0.05–2 MeV ions from the 2022 February 16 energetic storm particle event observed by Parker Solar Probe's (PSP) IS⊙IS/EPI-Lo instrument at 0.35 au from the Sun. This event was characterized by an enhancement in ion fluxes from a quiet background, increasing gradually with time with a nearly flat spectrum, rising sharply near the arrival of the coronal mass ejection (CME)–driven shock, becoming nearly a power-law spectrum, then decaying exponentially afterward, with a rate that was independent of energy. From the observed fluxes, we determine diffusion coefficients, finding that far upstream of the shock the diffusion coefficients are nearly independent of energy, with a value of 10 ^20 cm ^2 s ^−1 . Near the shock, the diffusion coefficients are more than 1 order of magnitude smaller and increase nearly linearly with energy. We also determine the source of energetic particles, by comparing ratios of the intensities at the shock to estimates of the quiet-time intensity to predictions from diffusive shock acceleration theory. We conclude that the source of energetic ions is mostly the solar wind for this event. We also present potential interpretations of the near-exponential decay of the intensity behind the shock. One possibility we suggest is that the shock was overexpanding when it crossed PSP and the energetic particle intensity decreased behind the shock to fill the expanding volume. Overexpanding CMEs could well be more common closer to the Sun, and this is an example of such a case.

Angular scattering (AS) in charge exchange and elastic collisions between interstellar ions and neutral (ISN) atoms has been assumed to be negligible in previous studies. Here, we investigate the momentum transfer associated with the AS of H atoms using Monte Carlo calculations to simulate their transport through the outer heliosheath. We consider two cases where charge exchange and elastic collisions between ISN H atoms and protons occur with and without AS in the outer heliosheath. We show that considering AS decelerates and heats primary ISN H, reducing the effect of selective charge exchange in the outer heliosheath. Secondary ISN H atoms, on the other hand, are not significantly affected by AS. We then simulate the transport of ISN H atoms inside the heliosphere to simulate count rates observed in the lowest energy bin of IBEX-Lo. We study the effect of radiation pressure on the ISN H measurements for the cases with and without AS and compare them with our previous findings. We find an effective radiation parameter ( μ _eff , which represents force associated with radiation pressure relative to gravity) for the years 2009–2018 based on the longitudinal shift of the ISN H signal. The two cases with and without AS reproduce the longitudinal shift in accordance with variations in solar activity, in agreement with our previous results, and they result in similar values for the μ _eff , which is ∼22%–23% larger than the value found based on solar Ly α profile observations.