In this study, we utilize 2.5D resistive magnetohydrodynamic (MHD) simulations to examine the generation and evolution of plasmoids resulting from the interaction between shock waves and turbulence in interplanetary space. We focus on (quasi-)perpendicular shock geometries with an out-of-plane mean magnetic field to investigate the impact of preexisting large-scale turbulent magnetic fluctuations upstream on MHD shock waves and on magnetic reconnection in the downstream region. Our findings reveal several key insights. First, the magnetic turbulence spectra exhibit three ranges: a low- k range characterized by cascading, a mid- k range potentially mediated by reconnection, and a high- k range corresponding to resistive diffusion. Importantly, these spectral features, including their characteristic breaks and slopes, remain invariant across the shock transition. Second, in all cases, the downstream occurrence of X-points and O-points increases, saturates to a peak value for a certain interval, and gradually decreases away from the shock. This behavior of X-points is consistent with the in situ measurements taken by the Magnetospheric Multiscale mission at Earth’s bow shocks. Third, the resistive simulations allow for the explicit calculation of the magnetic reconnection rate per unit area in the x–y plane, indicating a higher rate per unit area downstream than upstream. This is consistent with the increased occurrence of X-points.

The Sun’s surface vibrates in characteristic 5 minute oscillations, known as p -modes, generated by sound waves trapped within the convection zone. Although these oscillations have long been hypothesized to reach into the solar wind, direct in situ evidence has remained elusive, even during previous close encounters by Parker Solar Probe (PSP). Here, we present strong in situ evidence of 5 minute oscillations in the upper solar corona, based on observations from PSP’s three closest perihelia. In two events at 9.9 solar radii ( R _⊙ ), we identify statistically significant (∼6 σ ) 3.1–3.2 mHz peaks in the magnetic field power spectrum, each appearing as a large-amplitude, spherically polarized Alfvénic wave train lasting approximately 35 minutes. These results demonstrate that global solar oscillations can reach the solar wind.

The early evolution of fast polar coronal hole (PCH) solar wind remains largely unconstrained by in situ measurements. In March 2025, Parker Solar Probe (Parker) at its closest approach of 9.86 Solar Radii (R_⊙) measured outflow from a large equatorial coronal hole (ECH) which was also measured at 1 au and at intermediate distances by Solar Orbiter (also near its perihelion). At 1 au the stream properties are consistent with PCH properties established by Ulysses. The stream was measured by Parker substantially below the Alfvén surface, with proton temperatures in excess of 2 MK and a speed at ∼10 R_⊙ which was only ∼60% of its asymptotic value. The Solar Orbiter data indicates that the acceleration is largely complete by 60 R_⊙. Spherically-polarized fluctuations in the stream are observed to develop from near-transverse and small-angle at Parker to full reversal “switchbacks” at Solar Orbiter. Comparison of the implied acceleration profile to historical doppler-dimming measurements suggests that the stream's low coronal acceleration is similar to that of PCH flows. Consistent with previous work, this acceleration requires significantly more energy than can be provided by the observed thermal pressure gradients, with a significant contribution likely from the abundant Alfvénic fluctuation energy observed at Parker. These observations provide unique constraints on models of the radial evolution of the fastest solar wind, and indicate that these wind streams experience gradual, steady acceleration over their first few tens of solar radii of evolution.

The solar wind undergoes nonadiabatic heating as it travels away from the Sun. The velocity phase-space distribution of nonequilibrium ions in the solar wind indicates a source of free energy that could contribute significantly to this heating. Parker Solar Probe (PSP) has observed velocity distributions containing highly anisotropic, perpendicularly diffused proton beams with a distinctly constricted gap between the core and beam populations. These distributions resemble a “hammerhead” shape and were first reported in the fourth PSP encounter. Numerical simulations have reproduced the qualitative nature of hammerheads under certain initial conditions but have not convincingly captured the prevalence or extreme attributes of the observed beam. This necessitates a broad study of the occurrence conditions and the associated plasma processes, to better guide simulations. We statistically investigate the occurrences of these structures from 20 recent PSP encounters and find that hammerheads dominantly occur around the heliospheric current sheet (HCS). As the inclination of the HCS at PSP crossing points increases over the rising phase of the solar cycle, the occurrence of hammerheads is increasingly concentrated in narrow time periods around the HCS crossings. For comparison with previous work, we present statistical trends in the anisotropy of the proton beam and its connection to the density of proton beams as well as the drift speed of the beam to the core. Our study establishes a consistent occurrence pattern of hammerhead distributions around the HCS, indicating hammerheads are diagnostics of energization processes associated with the HCS and its escaping wind.

We perform a detailed study of the energetics for a well-observed solar eruption and flare that occurred on 2021 October 28. This event included a GOES class X1.0 flare, a global extreme-UV (EUV) wave, and a coronal mass ejection (CME) that reached speeds of >2000 km s(-1). The event was observed from a variety of spacecraft in NASA's Heliophysics System Observatory, including multiple missions near Earth, STEREO-A off the Sun-Earth line, and Solar Orbiter, near the Sun-Earth line at about 0.8 au. Using remote sensing, in situ observations, and in some cases scaling laws based on previous observations, we characterize the following quantities: free magnetic energy, energy in nonthermal electrons, energy in nonthermal ions, bolometric energy, energy deposited in the chromosphere, thermal energy radiated in the flare loops, energy dissipated by the EUV wave, CME kinetic and gravitational potential energy, CME energy flux in the heliosphere, and the energy partition in the CME shock. We find that the total energy released during the event is consistent with estimates of the pre-event stored magnetic energy, and the CME kinetic + potential energy dominates the energy partition.

Collisionless electron–ion shocks are fundamental to astrophysical plasmas, yet their behavior in strong magnetic fields remains poorly understood. Using particle-in-cell simulations with the SHARP-1D3V code, we investigate the role of the ion magnetization parameter σ _i in parallel shock transitions. Strongly magnetized converging flows ( σ _i > 1) exhibit lower-density compression ratios ( R ∼ 2), smaller entropy jumps, and suppressed particle acceleration while maintaining pressure anisotropy stability due to conserved perpendicular temperatures across the transition region, alongside increased parallel temperatures. In contrast, weakly magnetized shocks drive downstream mirror and firehose instabilities due to ion temperature anisotropy, which are suppressed in strongly magnetized cases. Additionally, weakly magnetized shocks exhibit the onset of a suprathermal population induced by shock-drift acceleration, with most of the upstream kinetic energy thermalized for both electrons and ions in the downstream region. Our results demonstrate that perpendicular temperatures for both species are conserved in weakly and strongly magnetized cases and highlight deviations from standard ideal magnetohydrodynamic behavior in strongly magnetized cases. These findings provide critical insights into the role of magnetic fields in parallel collisionless astrophysical shocks.

Current multi-spacecraft in situ measurements allow for the investigation of the time evolution of energetic particles at interplanetary shocks (IPs) at small (≲0.1 au) heliocentric distances. The energy spectrum of accelerated particles at IPs was shown by a previous 1D transport model that includes both self-excited plus preexisting turbulence and a term representing the escape of particles from the system to gradually steepen as a result of a finite acceleration-to-escape timescales ratio; such a model was found in excellent agreement with the entire sample of the ground-level enhancement spectra of solar cycle 23. We solve the time-dependent case of such a model in the case of diffusion dominated by preexisting turbulence. The average timescale for particle acceleration at various heliocentric distances, from 1 au down to the inner heliosphere (<0.1 au), is shorter than in the no-escape case, as higher energy particles have a shorter time to accelerate before completely leaving the system into the upstream medium. A simple scaling with time of the time-dependent spectrum is provided. We compare the “nose” structure at a few ∼100s keV protons first measured in situ by Parker Solar Probe in crossing the very fast 2022 September 5 shock at 0.07 au; we find that the nose is reasonably well explained by a lack of the highest energy particles not yet produced by the young shock by both our model and the no-escape version.

We investigate the generation and stability of ion acoustic waves (IAWs) in the ramp region of perpendicular Earth's bow shock. Using a fluid model, we derive the dispersion relation of IAWs, assuming a flat‐top electron velocity distribution function typically observed at interplanetary and Earth's bow shocks, along with jumps in plasma parameters. Our findings show that these electrostatic modes are non‐dispersive within the shock ramp, which is in agreement with in‐situ observations from the Magnetospheric Multiscale Mission (MMS). The calculated frequencies and phase velocities align closely with MMS measurements. Furthermore, we find that the growth rate of IAWs is more strongly affected by the ion temperature jump across the shock than by the electron temperature jump. The methodology developed in this work can be extended to investigate the generation and stability of other wave modes in diverse space and astrophysical plasma environments.

In the latter moments of 17 July 2023, the solar active region 13363, near the southwestern face of the Sun, was undergoing considerable evolution, which resulted in a significant solar energetic particle (SEP) event measured by Parker Solar Probe's Integrated Science Investigation of the Sun (ISOIS) and near-Earth spacecraft. Remote observations from GOES and CHASE captured two M5.0+ solar flares that peaked at 23:34 and 00:06 UT from the source region. In tandem, STEREO COR2 first recorded a small, narrow coronal mass ejection (CME) emerging at 22:54 UT and then saw a major halo CME emerge at 23:43 UT with a bright, rapidly expanding core and CME-driven magnetic shock with an estimated speed of ∼1400 kms^-1. Parker Solar Probe was positioned at 0.65 au, near-perfectly on the nominal Parker spiral magnetic field line which connected Earth and the active region for a 537 kms^-1 ambient solar wind speed at L1. This fortuitous alignment provided the opportunity to examine how the SEP velocity dispersion, energy spectra, elemental composition, and fluence varied from 0.65 to 1 au along a shared magnetic connection to the Sun. We find a strong radial gradient, which is best characterized for H and He as r^-4.0 and most surprisingly is stronger for O and Fe which is better described by r^-5.7.

The AXIS Community Science Book represents the collective effort of 592 scientists worldwide to define the transformative science enabled by the Advanced X-ray Imaging Satellite (AXIS), a next-generation X-ray mission selected by NASA's Astrophysics Probe Program for Phase A study. AXIS will advance the legacy of high-angular-resolution X-ray astronomy with 1.5” imaging over a wide 24' field of view and an order of magnitude greater collecting area than Chandra in the 0.3-12 keV band. Combining sharp imaging, high throughput, and rapid response capabilities, AXIS will open new windows on virtually every aspect of modern astrophysics, exploring the birth and growth of supermassive black holes, the feedback processes that shape galaxies, the life cycles of stars and exoplanet environments, and the nature of compact stellar remnants, supernova remnants, and explosive transients. This book compiles 138 community-contributed science cases developed by five Science Working Groups focused on AGN and supermassive black holes, galaxy evolution and feedback, compact objects and supernova remnants, stellar physics and exoplanets, and time-domain and multi-messenger astrophysics. Together, these studies establish the scientific foundation for next-generation X-ray exploration in the 2030s and highlight strong synergies with facilities of the 2030s, such as JWST, Roman, Rubin/LSST, SKA, ALMA, ngVLA, and next-generation gravitational-wave and neutrino networks.

The geometry of a star’s Alfvén surface determines stellar angular momentum loss, separates a causally distinct “corona” and stellar wind, and potentially affects exoplanetary habitability. The solar Alfvén surface is the only such structure that is directly measurable and, since 2021, has been routinely measured in situ by NASA’s Parker Solar Probe (Parker). We use these unique measurements in concert with Solar Orbiter and L1 in situ data spanning the first half of solar cycle 25 in time and from 0.045 to 1 au in heliocentric distance to develop a radial scaling technique to estimate the morphology of the Alfvén surface from measurements of the solar wind speed and local Alfvén speed. We show that accounting for solar wind acceleration and mass flux is necessary to achieve reasonable agreement between the scaled location of the Alfvén surface and the locations of direct crossings measured by Parker. We produce continuous 2D equatorial cuts of the Alfvén surface over half a solar cycle (ascending phase and maximum). Parker’s earliest crossings clipped outward extrusions, many of which are likely transient-related, while more recently, Parker has unambiguously sampled deep sub-Alfvénic flows. We analyze the average altitude, departure from spherical symmetry, and surface roughness, finding that all are positively correlated to solar activity. For the current modest solar cycle, the height varies up to 30%, which corresponds to a near doubling in angular momentum loss per unit mass loss.

Gamma-ray emission in the GeV–TeV range from the solar disk is likely to arise from collisions of galactic cosmic rays (GCRs) with solar atmospheric plasma. In a previous study, we demonstrated that closed turbulent magnetic arcades trap GCRs efficiently, leading to a gamma-ray flux consistent with the Fermi-HAWC observations (from ∼0.1 GeV to ∼1 TeV). Here, we model a synthetic magnetic field with a static, laminar structure of open field lines in the chromosphere, increasingly braided near the solar surface, with a scale height of ∼10 ^−2 R _⊙ . The height-dependent increase in magnetic field line braiding is modulated by an exponential scalar function, mimicking the bending of the photo- and chromospheric magnetic field revealed by polarimetric observations and reproduced by MHD simulations. Employing 3D test-particle numerical simulations, we investigate how distorted magnetic field lines affect the gamma-ray production by injecting GeV–TeV protons into both magnetically laminar and braided regions. We find that with the chosen spatial resolution, this synthetic magnetic field can account for the >10 GeV gamma-ray spectrum observed by Fermi/HAWC. A rebrightening between approximately 30 and 100 GeV (following a ∼30 GeV spectral dip) suggests an enhanced confinement within the photo-/chromospheric layer by stronger braiding.

In the latter moments of 2023 July 17, the solar active region (AR) 13363, near the southwestern face of the Sun, was undergoing considerable evolution, which resulted in a significant solar energetic particle (SEP) event measured by Parker Solar Probe’s Integrated Science Investigation of the Sun (IS⊙IS) and near-Earth spacecraft. Remote observations from GOES and CHASE captured two M5.0+ solar flares that peaked at 23:34 and 00:06 UT from the source region. In tandem, STEREO COR2 first recorded a small, narrow coronal mass ejection (CME) emerging at 22:54 UT and then saw a major halo CME emerge at 23:43 UT with a bright, rapidly expanding core and CME-driven magnetic shock with an estimated speed of ∼1400 km s ^−1 . Parker Solar Probe was positioned at 0.65 au, near-perfectly on the nominal Parker spiral magnetic field line, which connected Earth and the AR for a 537 km s ^−1 ambient solar wind speed at L1. This fortuitous alignment provided the opportunity to examine how the SEP velocity dispersion, energy spectra, elemental composition, and fluence varied from 0.65 to 1 au along a shared magnetic connection to the Sun. We find a strong radial gradient, which is best characterized for H and He as r ^−4.0 , and most surprisingly, is stronger for O and Fe, which is better described by r ^−5.7 .

The anisotropy of energetic particles provides essential information to help resolve the underlying fundamental physics of their spatial distributions, injection, acceleration, and transport processes. In this work, we report an energetic ion enhancement that is characterized by very large and long-lasting anisotropies observed by STEREO A and Solar Orbiter, which are nearly aligned along the same nominal Parker spiral. This ion enhancement appears at the rising phase of a widespread solar energetic particle event that was associated with the farside coronal mass ejection on 2022 February 15. According to our analysis, the long-lasting anisotropy resulted from the continuous injection of energetic ions from a well-connected particle source located beyond the STEREO A’s orbit. Solar Orbiter also observed an interval of very large anisotropy dominated exclusively by sunward streaming ions but with the additional implication that it detected the very early phase of ion injections onto magnetic field lines that newly connected to the particle source, which is likely the first reported event of this kind. These results further illustrate how energetic particle anisotropy information, in particular from multiple observer locations, can be used to disentangle the sources and transport processes of energetic ions, even when their heliospheric context is not simple.

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.

On 2022 February 15, an impressive filament eruption was observed off the solar eastern limb from three remote-sensing viewpoints, namely, Earth, STEREO-A, and Solar Orbiter. In addition to representing the most-distant observed filament at extreme ultraviolet wavelengths—captured by Solar Orbiter's field of view extending to above 6 R _⊙ —this event was also associated with the release of a fast (∼2200 km s ^−1 ) coronal mass ejection (CME) that was directed toward BepiColombo and Parker Solar Probe. These two probes were separated by 2° in latitude, 4° in longitude, and 0.03 au in radial distance around the time of the CME-driven shock arrival in situ. The relative proximity of the two probes to each other and the Sun (∼0.35 au) allows us to study the mesoscale structure of CMEs at Mercury's orbit for the first time. We analyze similarities and differences in the main CME-related structures measured at the two locations, namely, the interplanetary shock, the sheath region, and the magnetic ejecta. We find that, despite the separation between the two spacecraft being well within the typical uncertainties associated with determination of CME geometric parameters from remote-sensing observations, the two sets of in situ measurements display some profound differences that make understanding the overall 3D CME structure particularly challenging. Finally, we discuss our findings within the context of space weather at Mercury's distance and in terms of the need to investigate solar transients via spacecraft constellations with small separations, which has been gaining significant attention during recent years.

Ultracool dwarf stars are abundant, long-lived and uniquely suited to enable the atmospheric study of transiting terrestrial companions with the JWST. Among them, the most prominent is the M8.5V star TRAPPIST-1 and its seven planets. While JWST Cycle 1 observations have started to yield preliminary insights into the planets, they have also revealed that their atmospheric exploration requires a better understanding of their host star. Here we propose a roadmap to characterize the TRAPPIST-1 system - and others like it - in an efficient and robust manner with JWST. We notably recommend that - although more challenging to schedule - multi-transit windows be prioritized to mitigate the effects of stellar activity and gather up to twice more transits per JWST hour spent. We conclude that, for such systems, planets cannot be studied in isolation by small programmes but rather need large-scale, joint space- and ground-based initiatives to fully exploit the capabilities of JWST for the exploration of terrestrial planets. The JWST has the potential to increase our understanding of terrestrial exoplanets and their atmospheres, but the various signal contaminations need to be isolated and quantified. Using JWST Cycle 1 observations of TRAPPIST-1 as a benchmark, this Perspective proposes a series of steps to use future JWST data efficiently for this purpose.