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The heating of ions downstream of the x‐line during magnetic reconnection is explored using full‐particle simulations, test particle simulations, and analytic analysis. Large‐scale particle simulations reveal that the ion temperature increases sharply across the boundary layer that separates the upstream plasma from the Alfvénic outflow. This boundary layer, however, does not take the form of a classical switch‐off shock as discussed in the Petschek reconnection model, so the particle heating cannot be calculated from the magnetohydrodynamic, slow‐shock prediction. Test particle trajectories in the fields from the simulations reveal that ions crossing the narrow boundary into the exhaust instead behave like pickup particles: they gain both a directed outflow and an effective thermal speed given by the flow speed v0 of the exhaust. The detailed dynamics of these particles are explored by taking 1‐D cuts of the simulation data across the exhaust, transforming to the deHoffman‐Teller frame, and calculating explicitly the increment in the temperature, miv02/3, with mi, the ion mass. We compare the model predictions with the temperature increment in solar wind exhausts measured by the ACE and Wind spacecraft, confirming that the temperature increment is proportional to the ion mass. The Wind data from 22 high‐shear exhaust encounters confirm the scaling of the proton temperature increment with the square of the exhaust velocity. However, the temperature increments are consistently lower than the model prediction. Implications for understanding the production of high‐energy ions in flares and the broader universe are discussed.

Connecting the solar wind observed throughout the heliosphere to its origins in the solar corona is one of the central aims of heliophysics. The variability in the magnetic field, bulk plasma, and heavy ion composition properties of the slow wind are thought to result from magnetic reconnection processes in the solar corona. We identify regions of enhanced variability and composition in the solar wind from 2003 April 15 to May 13 (Carrington Rotation 2002), observed by the Wind and Advanced Composition Explorer spacecraft, and demonstrate their relationship to the separatrix–web (hereafter, S-Web) structures describing the corona’s large-scale magnetic topology. There are four pseudostreamer (PS) wind intervals and two helmet streamer (HS) heliospheric current sheet/plasma sheet crossings (and an interplanetary coronal mass ejection), which all exhibit enhanced alpha-to-proton ratios and/or elevated ionic charge states of carbon, oxygen, and iron. We apply the magnetic helicity–partial variance of increments (H m –PVI) procedure to identify coherent magnetic structures and quantify their properties during each interval. The mean duration of these structures are ∼1 hr in both the HS and PS wind. We find a modest enhancement above the power-law fit to the PVI waiting-time distribution in the HS-associated wind at the 1.5–2 hr timescales that is absent from the PS intervals. We discuss our results in the context of previous observations of the ∼90 minutes periodic density structures in the slow solar wind, further development of the dynamic S-Web model, and future Parker Solar Probe and Solar Orbiter joint observational campaigns.

Upcoming imaging missions—NASA's LEXI and ESA/CAS's SMILE—will target solar wind charge exchange X‐ray (SWCX) emission from Earth's magnetosheath. This emission is generated by highly charged ions colliding with neutrals in Earth's exosphere. Accurate SWCX models require data on exospheric neutral densities, as well as solar wind flux and composition. The Advanced Composition Explorer (ACE) Solar Wind Ionic Composition Spectrometer (SWICS) provided the needed solar wind composition data from 1998 until an instrument anomaly in 2011 limited its outputs. To address this, we developed empirical functions using ion ratios (O7+/O6+,O8+/O6+,C6+/C5+${\\mathrm{O}}^{7+}/{\\mathrm{O}}^{6+},{\\mathrm{O}}^{8+}/{\\mathrm{O}}^{6+},{\\mathrm{C}}^{6+}/{\\mathrm{C}}^{5+}$ ) still available from ACE, partially compensating for missing composition data. The results underscore the need for a new mission to measure solar wind composition and support future SWCX analysis efforts. Plain Language Summary Upcoming missions, NASA's LEXI and ESA/CAS's SMILE, will study unique X‐ray emissions from interactions between solar wind particles and Earth's atmospheric neutrals. This process, known as solar wind charge exchange (SWCX), emits faint X‐rays that reveal how solar activity affects Earth's magnetic shield. Accurate X‐ray analysis requires detailed solar wind data, particularly for highly charged particles. While NASA's ACE spacecraft provided this data for over a decade, a 2011 anomaly limited its output. In response, new empirical methods were developed to estimate missing data, based on specific oxygen and carbon ion ratios. To fully support LEXI and SMILE, a new mission is essential to directly measure solar wind composition, ensuring comprehensive data to support modeling efforts. Key Points Solar wind composition data are crucial for accurate predictions of soft X‐ray emission from charge exchange in the Earth's magnetosheath We derive empirical functions based on charge‐state ratios to compensate the lack of abundance data in the ACE SWICS database after 2011 A new solar wind composition mission near Earth is needed to support upcoming missions such as SMILE and LEXI for magnetospheric soft X‐ray imaging

The contribution of solar-wind ions exchanging electrons with helium and hydrogen near the Sun is shown to be only about 40 per cent of the 1/4-keV X-ray flux observed in the Galactic plane; this supports the existence of a local ‘hot bubble’ filled with X-ray-emitting gas, accounting for the rest of the flux. Dual source for local X-rays Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-keV X-rays, coupled with the discovery that interstellar space within about a hundred parsecs of the Sun is almost completely devoid of cool absorbing gas, have led to a picture of a local cavity, or hot bubble, filled with X-ray-emitting hot gas. Suggestions that the emission could instead be produced within the Solar System by charge exchange between heavy solar-wind ions and neutral hydrogen and helium have raised questions over this model. Massimiliano Galeazzi et al . report observations showing that solar wind charge exchange contributes about 40% of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports models that include a million-degree hot bubble extending about a hundred parsecs from the Sun. The solar neighbourhood is the closest and most easily studied sample of the Galactic interstellar medium, an understanding of which is essential for models of star formation and galaxy evolution. Observations of an unexpectedly intense diffuse flux of easily absorbed 1/4-kiloelectronvolt X-rays 1 , 2 , coupled with the discovery that interstellar space within about a hundred parsecs of the Sun is almost completely devoid of cool absorbing gas 3 , led to a picture of a ‘local cavity’ filled with X-ray-emitting hot gas, dubbed the local hot bubble 4 , 5 , 6 . This model was recently challenged by suggestions that the emission could instead be readily produced within the Solar System by heavy solar-wind ions exchanging electrons with neutral H and He in interplanetary space 7 , 8 , 9 , 10 , 11 , potentially removing the major piece of evidence for the local existence of million-degree gas within the Galactic disk 12 , 13 , 14 , 15 . Here we report observations showing that the total solar-wind charge-exchange contribution is approximately 40 per cent of the 1/4-keV flux in the Galactic plane. The fact that the measured flux is not dominated by charge exchange supports the notion of a million-degree hot bubble extending about a hundred parsecs from the Sun.

The Solar Orbiter spacecraft crossed Comet Leonard’s ion tail on 2021 December 17 near 1 au. In and near the ion tail, significant amounts of singly charged oxygen (O+) ions were detected by the Heavy Ion Sensor on board the spacecraft. These ions are likely the result of outgassed water molecules from the comet that became ionized and disassociated into protons and O+ ions and that were then picked up by the interplanetary magnetic field and advected outward with the solar wind. At this time, the spacecraft was also located amidst the denser parts of the interstellar helium (He) neutrals that are focused here by their gravitational interaction with the Sun. Pickup He+ ions in greater numbers are also found in this region and form when neutrals interact with solar photons. Newly ionized ions can generate waves that propagate mainly along the background magnetic field before the waves scatter the ions toward isotropy. Spectral magnetic field analyses are employed to find mainly elliptically polarized waves associated with O+ and He+ from ring-beam instabilities. Waves associated with He+ are identified, but O+ waves are not seen. Visibility is concluded to be affected by the relative amplitude of the waves to turbulence, and the visibility increases when the sampling direction is more aligned with the background magnetic field.

Charged particle acceleration and heating in the heliosphere requires detailed observa-tions of the velocity distribution functions (VDFs) of these particles across a range of mass and charge states. Such particle energization is often observed concurrent with transient phenom-ena. Proper characterization of such energization processes and their impact on charged particles requires careful consideration of instrument observation timing and the timing at which a tran-sient phenomenon is observed. Solar Orbiter’s Heavy Ion Sensor (HIS) observes the composition of the solar wind along with suprathermal and pickup ions. As such, it provides the observations necessary to resolve the mass and charge states appropriate for properly characterizing charged particle energization. In this work, we describe the current status of data aggregation at vari-able time intervals and how that enables proper characterization of charged particle energization concurrent with transient phenomena.

The National Science Foundation (NSF) Daniel K. Inouye Solar Telescope (DKIST) has started operations at the summit of Haleakalā (Hawai’i). DKIST joins the nominal science phases of the NASA and ESA Parker Solar Probe and Solar Orbiter encounter missions. By combining in-situ measurements of the near-Sun plasma environment and detailed remote observations of multiple layers of the Sun, the three observatories form an unprecedented multi-messenger constellation to study the magnetic connectivity in the solar system. This work outlines the synergistic science that this multi-messenger suite enables.

When observed at 1 AU, slow solar wind is typically considered to have originated in source regions with magnetic topologies that are intermittently open to the heliosphere. Fast wind is typically considered to have originated in source regions that are continuously open to the heliosphere eg coronal holes. The evolution of the solar wind helium abundance (AHe) with solar activity is likely driven by the evolution of different solar wind source regions. Separating the solar wind into fast and slow for each element based on its characteristic speed derived in Alterman et al. (2025) we quantify the evolution of helium and heavy element abundances \\((X/H):(X/H)_photo\\) with solar activity. We show that AHe strongly correlates with sunspot number; in slow and fast wind the average non-transient solar wind AHe is limited to 51% of its photospheric value; slow wind heavy element abundances evolve significantly with solar activity; fast wind heavy element abundances do not; the correlation coefficient with sunspot number of elemental abundances for species heavier than He monotonically increases with increasing mass; and the correlation coefficients between the in situ observations and the normalized sunspot number are stronger than those using the unnormalized sunspot number. We infer that the sunspot number is a clock timing the solar cycle but not the driver of the physical process underlying the evolution of these abundances with solar activity; this underlying process is likely related to the energy available to accelerate the solar plasma from the chromosphere and transition region or low corona into the solar wind; and differences between the evolution of slow and fast solar wind abundances are similarly related to the energy available to accelerate the elements at these heights above the Suns surface.

The solar wind is typically categorized as fast and slow based on the measured speed (\\(v_sw\\)). The separation between these two regimes is often set between 400 and 600 km/s without a rigorous definition. Observations of the solar wind's kinetic signatures, chemical makeup, charge state properties, and Alfvénicity suggest that such a two-state model may be insufficiently nuanced to capture the relationship between the solar wind and its solar sources. We test this two-state fast/slow solar wind paradigm with heavy ion abundances (X/H) and characterize how the transition between fast and slow wind states impacts heavy ion in the solar wind. We show that (1) the speed at which heavy ion abundances indicate a change between fast and slow solar wind as a function of speed is slower than the speed indicated by the helium abundance; (2) this speed is independent of heavy ion mass and charge state; (3) the abundance at which heavy ions indicate the transition between fast and slow wind is consistent with prior observations of fast wind abundances; (4) and there may be a mass or charge-state dependent fractionation process present in fast wind heavy ion abundances. We infer that (1) identifying slow solar wind as having a speed \\(v_sw \\) 400 km/s may mix solar wind from polar and equatorial sources; (2) He may be impacted by the acceleration necessary for the solar wind to reach the asymptotic fast, non-transient values observed at 1 AU; and (3) heavy ions are fractionated in the fast wind by a yet-to-be-determined mechanism.

Connecting the solar wind observed throughout the heliosphere to its origins in the solar corona is one of the central aims of heliophysics. The variability in the magnetic field, bulk plasma, and heavy ion composition properties of the slow wind are thought to result from magnetic reconnection processes in the solar corona. We identify regions of enhanced variability and composition in the solar wind from 2003 April 15 to May 13 (Carrington Rotation 2002), observed by the Wind and Advanced Composition Explorer spacecraft, and demonstrate their relationship to the Separatrix-Web (S-Web) structures describing the corona's large-scale magnetic topology. There are four pseudostreamer (PS) wind intervals and two helmet streamer (HS) heliospheric current sheet/plasma sheet crossings (and an ICME) which all exhibit enhanced alpha-to-proton ratios and/or elevated ionic charge states of carbon, oxygen, and iron. We apply the magnetic helicity-partial variance of increments (\\(H_m\\)-PVI) procedure to identify coherent magnetic structures and quantify their properties during each interval. The mean duration of these structures are \\(\\)1 hr in both the HS and PS wind. We find a modest enhancement above the power-law fit to the PVI waiting time distribution in the HS-associated wind at the 1.5-2 hr timescales that is absent from the PS intervals. We discuss our results in context of previous observations of the \\(\\)90 min periodic density structures in the slow solar wind, further development of the dynamic S-Web model, and future Parker Solar Probe and Solar Orbiter joint observational campaigns.