Ion-driven plasma instability thresholds, derived from linear theory, constrain the distribution of solar observations in parameter space, defining boundaries of stable plasma parameters. Excursions beyond these thresholds result in the emission of energy, transferred from particles to coherent electromagnetic waves, acting to adjust the system toward a more stable configuration. In this Letter, we use linear Vlasov-Maxwell theory to define parametric limits for a low-β plasma that contains a drifting proton beam or helium (α-particle) population. A sufficiently fast and dense drifting population triggers an oblique drift instability (ODI). This instability decreases the velocity drift between the thermal core proton and secondary populations and prevents the ratio of core thermal to magnetic pressure β_{c} from decreasing below a minimum value by increasing the temperatures-i.e., heating-of both the core and drifting populations. Our theoretical results are of interest for Parker Solar Probe observations, as they provide an additional mechanism for perpendicular heating of ions active in the sub-Alfvénic solar wind. The ODI may explain the discrepancy between long-standing expectations of measurements of very low-β plasmas with very large ion temperature anisotropies in the near-Sun environment and in situ observations, where β is consistently measured above a few percentages and the secondary ion populations drift faster than the bulk of the proton population by no more than approximately the local Alfvén speed.

The Heavy Ion Instrument (HIS) onboard Solar Orbiter measures mass, charge, and full 3-D velocities of ions in the energy/charge range 0.5-75keV/charge. Using the data from HIS we study how interplanetary events like shocks or CME fronts create suprathermal tails in the velocity distribution and how those tails change with time. HIS observed the passage of three interplanetary shocks during the period October 2021 - May 2022. The three events were characterized by the acceleration of plasma from the solar wind energy regime (~1keV per amu/charge) to higher energies (5-75 keV), commonly referred to as suprathermal ions; later during the events, energetic particles (100keV and above) were measured by the EPD instrument. This energization process was characterized by a clear dependence upon mass/charge, and found consistent with preferential acceleration of ions present in the high energy tails of solar wind distributions, the seed population. Details of the distribution functions during the three events are presented and contrasted to each other.

The distribution of charged particles in the heliosphere covers more than 16 orders of magnitude in particle flux and more than 6 orders of magnitude in energy. While the majority of these particles are ionized hydrogen (protons) and fully ionized helium (alpha particles), heavier ions are also present. Because of the large parameter space that must be covered, different instruments are required and these instruments must be optimized to specific energy and particle flux ranges. They must also be designed to target specific ion species. To properly characterize the means by which different energy ranges are populated, the observations from these different instruments must be intercalibrated. We present initial progress intercalibrating observations from Solar Orbiter’s Heavy Ion Sensor (HIS) and Suprathermal Ion Spectrograph (SIS). HIS is a heavy ion composition experiment that targets the solar wind through the low energy range of suprathermal energies with mass and charge state resolution. SIS covers the suprathermal and low range energetic particles with high mass resolution but without charge state resolution. Together, these two sensors cover heavy ion composition from solar wind to suprathermal energies. During advantageous conditions, proton distributions across both instruments are also available. Properly intercalibrated observations across these instruments enable studies of charged particle energization across the energy ranges, which is essential for characterizing a wide range of phenomena in heliosphere.

In the corona, plasma is accelerated to hundreds of kilometers per second, and heated to temperatures hundreds of times hotter than the Sun's surface, before it escapes to form the solar wind. Decades of space-based experiments have shown that the energization process does not stop after it escapes. Instead, the solar wind continues to accelerate and it cools far more slowly than a freely-expanding adiabatic gas. Recent work suggests that fast solar wind requires additional momentum beyond what can be provided by the observed thermal pressure gradients alone whereas it is sufficient for the slowest wind. The additional acceleration for fast wind can be provided through an Alfvén wave pressure gradient. Beyond this fast-slow categorization, however, a subset of slow solar wind exhibits high Alfvénicity that suggest Alfvén waves could play a larger role in its acceleration compared to conventional slow wind outflows. Through a well-timed conjunction between Solar Orbiter and Parker Solar Probe, we trace the energetics of slow wind to compare with a neighboring Alfvénic slow solar wind stream. An analysis that integrates remote and heliospheric properties and modeling of the two distinct solar wind streams finds Alfvénic slow solar wind behaves like fast wind, where a wave pressure gradient is required to reconcile its full acceleration, while non-Alfvénic slow wind can be driven by its non-adiabatic electron and proton thermal pressure gradients. Derived coronal conditions of the source region indicate good model compatibility but extended coronal observations are required to effectively trace solar wind energetics below Parker's orbit.

Context. It has been recently accepted that the standard classification of the solar wind solely according to flow speed is outdated, and particular interest has been devoted to the study of the origin and evolution of so-called Alfvenic slow solar wind streams and to what extent such streams resemble or differ from fast wind. Aims. In March 2022, Solar Orbiter completed its first nominal phase perihelion passage. During this interval, it observed several Alfvenic streams, allowing for characterization of fluctuations in three slow wind intervals (AS1-AS3) and comparison with a fast wind stream (F) at almost the same heliocentric distance. Methods. This work makes use of Solar Orbiter plasma parameters from the Solar Wind Analyzer (SWA) and magnetic field measurements from the magnetometer (MAG). The magnetic connectivity to the solar sources of selected solar wind intervals was reconstructed using a ballistic extrapolation based on measured solar wind speed down to the (spherical) source surface at 2.5 Rs below which a potential field extrapolation was used to map back to the Sun. The source regions were identified using SDO/AIA observations. A spectral analysis of in situ measured magnetic field and velocity fluctuations was performed to characterize correlations, Alfvenicity, normalized cross-helicity, and residual energy in the frequency domain as well as intermittency of the fluctuations and spectral energy transfer rate estimated via mixed third-order moments. A machine learning technique was used to separate proton core, proton beam, and alpha particles and to study v-b correlations for the different ion populations in order to evaluate the role played by each population in determining the Alfvenic content of solar wind fluctuations. Results. The comparison between fast wind and Alfvenic slow wind intervals highlights the differences between the two solar wind regimes: The fast wind is characterized by larger amplitude fluctuations, and magnetic and velocity fluctuations are closer to equipartition of energy. In fact the Alfvenic slow wind streams appear to be on a spectrum of wind types, with AS1, originating from open field lines neighboring active regions and displaying similarities with the fast wind in terms of fluctuation amplitude and turbulence characteristics, but not with respect to the alpha particles and proton beams. The other two slow streams differed both in their sources as well as plasma characteristics, with AS2 coming from the expansion of a narrow coronal hole corridor and AS3 from a region straddling a pseudostreamer. The latter displayed the coldest and highest density but the slowest stream with the smallest fluctuation amplitude and greatest magnetic energy excess. It also showed the largest scatter in proton beam speeds and the greatest difference in speed between proton beam and alpha particles. Conclusions. This study shows how the old fast-slow solar wind dichotomy, already called into question by the observations of slower Alfvenic solar wind streams, should further be refined, as the Alfvenic slow wind, originating in different solar wind regions, show significant differences in density, temperature, and proton and alpha-particle properties in the inner heliosphere. The observations presented here provide the starting point for a better understanding of the origin and evolution of different solar wind streams as well as the evolving turbulence contained within.

The slow solar wind is typically characterized as having low Alfv\'enicity. However, Parker Solar Probe (PSP) observed predominately Alfv\'enic slow solar wind during several of its initial encounters. From its first encounter observations, about 55.3\% of the slow solar wind inside 0.25 au is highly Alfv\'enic ($|\sigma_C| > 0.7$) at current solar minimum, which is much higher than the fraction of quiet-Sun-associated highly Alfv\'enic slow wind observed at solar maximum at 1 au. Intervals of slow solar wind with different Alfv\'enicities seem to show similar plasma characteristics and temperature anisotropy distributions. Some low Alfv\'enicity slow wind intervals even show high temperature anisotropies, because the slow wind may experience perpendicular heating as fast wind does when close to the Sun. This signature is confirmed by Wind spacecraft measurements as we track PSP observations to 1 au. Further, with nearly 15 years of Wind measurements, we find that the distributions of plasma characteristics, temperature anisotropy and helium abundance ratio ($N_\alpha/N_p$) are similar in slow winds with different Alfv\'enicities, but the distributions are different from those in the fast solar wind. Highly Alfv\'enic slow solar wind contains both helium-rich ($N_\alpha/N_p\sim0.045$) and helium-poor ($N_\alpha/N_p\sim0.015$) populations, implying it may originate from multiple source regions. These results suggest that highly Alfv\'enic slow solar wind shares similar temperature anisotropy and helium abundance properties with regular slow solar winds, and they thus should have multiple origins.

The two-state solar wind paradigm is based on observations showing that slow and fast solar wind have distinct properties like helium abundances, kinetic signatures, elemental composition, and charge-state ratios. Nominally, the fast wind originates from solar sources that are continuously magnetically open to the heliosphere like coronal holes while the slow wind is from solar sources that are only intermittently open to the heliosphere like helmet streamers and pseudostreamers. The Alfvénic slow wind is an emerging third class of solar wind that challenges the two-state fast/slow paradigm. It has slow wind speeds but is highly Alfvénic, i.e., has a high correlation between velocity and magnetic field fluctuations along with low compressibility typical of Alfvén waves, which is typically observed in fast wind. Its other properties are also more similar to the fast than slow wind. From 28 yr of Wind observations at 1 au, we derive the solar wind helium abundance ( A _He ), Alfvénicity (∣ σ _c ∣), and solar wind speed ( v _sw ). Characterizing v _sw as a function of ∣ σ _c ∣ and A _He , we show that the maximum solar wind speed for plasma accelerated in source regions that are intermittently open is faster than the minimum solar wind speed for plasma accelerated in continuously open regions. We infer that the Alfvénic slow wind is likely solar wind originating from open field regions with speeds below the maximum solar wind speed for plasma from intermittently open regions. We then discuss possible implications for solar wind acceleration. Finally, we utilize the combination of helium abundance and normalized cross helicity to present a novel solar wind categorization scheme that illustrates the transition in observations of solar wind at 1 au from magnetically closed to magnetically open sources.

Context. The solar wind is typically categorized as fast and slow based on the measured speed (vsw). The separation between these two regimes is often set between 400 and 600 km s(-1) without a rigorous definition. Observations with vsw above this threshold are considered "fast" and are typically considered to come from polar regions, that is, coronal holes. Observations with vsw below this threshold speed are considered "slow" wind and typically considered to originate outside of coronal holes. Observations of the solar wind's kinetic signatures, chemical makeup, charge state properties, and Alfv & eacute;nicity suggest that such a two-state model may be insufficiently nuanced to capture the relationship between the solar wind and its solar sources. As heavy ion composition ratios are unchanged once the solar wind leaves the Sun, they serve as a key tool for connecting in situ observations to their solar sources. Helium (He) is the most abundant solar wind ion heavier than hydrogen (H). Long-duration observations from the Wind Solar Wind Experiment (SWE) Faraday cups show that the solar wind helium abundance has two distinct gradients at speeds above and below similar to 400 km s(-1). This is a key motivator for identifying the separation between fast and slow wind at such a speed. Aims. 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. Methods. We study the variation in the gradients of the helium and heavy ion abundances as a function of the solar wind speed and characterize how the gradient of each abundance changes in fast and slow wind. We calculate vsw as the proton or hydrogen bulk speed. The work uses Advanced Composition Explorer (ACE) heavy ion observations collected by the Solar Wind Ion Composition Spectrometer (SWICS) from 1998 to 2011. We compare the helium abundance observed by ACE/SWICS to the helium abundance observed by Wind/SWE to show that the results are consistent with prior work. Results. 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. Conclusions. We infer that (1) identifying slow solar wind as having a speed of vsw less than or similar to 400 km s(-1) 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.

Over decades of solar wind observations, heavy ions have been observed to have a higher temperature and flow faster than protons in the solar corona and heliosphere. Remote observations have largely been limited to the low corona (<4 R _⊙ ), while in situ observations for heavy ions ( Z > 2) have only been sampled at 1 au and beyond. As a result, theories that address heavy ion heating and acceleration remain largely unconstrained. With the launch of Solar Orbiter, heavy ion kinetics can be probed closer to the Sun, as close as the orbit of Mercury (65 R _⊙ ), to examine their radial behavior. Through a statistical analysis of O ^6+ , this work provides a comprehensive analysis of the velocity and temperature of O ^6+ from 0.3 to 1 au. The study finds that the O ^6+ relative drift, normalized to the local Alfvén speed, and its temperature compared to protons both decrease with distance from the Sun and show some speed dependence. The O ^6+ temperature is well fit by a single temperature adiabatic profile across all wind speeds, suggesting that there is no significant heating at these heliocentric distances, which is in contrast to what is observed for protons and He ^2+ . Alfvénic fluctuations, some with full 180 ^∘ field rotation, create momentary negative differential streaming where the speed of O ^6+ trails the protons. The amount of negative differential streaming gradually increases at larger distances. These results provide critical constraints on the proposed mechanisms seeking to describe ion heating and acceleration in the solar wind.

We report on the annual variation of quiet-time suprathermal heavy ion spectral indices for C through Fe in the energy range 0.3–1.28 MeV nuc ^−1 during Solar Cycle 23's rising phase through Solar Cycle 24's declining phase. These Advanced Composition Explorer/Ultra-Low Energy Isotope Spectrometer measurements cover 1998–2019. We show that the average quiet-time suprathermal spectral index across species is γ = 2.5 ± 0.3. Such observations may imply that quiet-time suprathermals are the result of a superposition of various underlying acceleration and transport processes that accelerate suprathermal ions. As such, they may be remnants of particles from discrete events like large and impulsive solar energetic particle events along with corotating interaction regions that have decayed in intensity.

Since Parker Solar Probe's (Parker's) first perihelion pass at the Sun, large-amplitude Alfv & eacute;n waves grouped in patches have been observed near the Sun throughout the mission. Several formation processes for these magnetic switchback patches have been suggested with no definitive consensus. To provide insight into their formation, we examine the heavy ion properties of several adjacent magnetic switchback patches around Parker's 11th perihelion pass, capitalizing on a spacecraft lineup with Solar Orbiter where each samples the same solar wind streams over a large range of longitudes. Heavy ion properties (Fe/O, C6+/C5+, O7+/O6+) related to the wind's coronal origin, measured with Solar Orbiter, can be linked to switchback patch structures identified near the Sun with Parker. We find that switchback patches do not contain distinctive ion and elemental compositional signatures different from the surrounding nonswitchback solar wind. Both the patches and ambient wind exhibit a range of fast and slow wind qualities, indicating coronal sources with open and closed field lines in close proximity. These observations and modeling indicate switchback patches form in coronal hole boundary wind and with a range of source region magnetic and thermal properties. Furthermore, the heavy ion signatures suggest interchange reconnection and/or shear-driven processes may play a role in their creation.

Context.In the solar wind (SW), the particle distribution functions are generally not Gaussian. They present nonthermal features that are related to underlying acceleration and heating processes. These processes are critical in the overall dynamics of this expanding astrophysical fluid.Aims.The Proton Alpha Sensor (PAS) on board Solar Orbiter commonly observes skewed proton distributions, with a more populated high-energy side in the magnetic field direction than the Gaussian distribution. Our objectives are: (1) to identify a theoretical statistical function that adequately models the observed distributions and (2) to use its statistical interpretation to constrain the acceleration and heating processes.Methods.We analyzed the 3D velocity distribution functions (VDFs) measured by PAS and compared them to model statistical functions.Results.We show that the normal inverse Gaussian (NIG), a type of hyperbolic statistical distribution, provides excellent fits of skewed and leptokurtic proton distributions. NIG can model both the core distribution and the beam, if present. We propose an interpretation that is inspired by the mathematical formulation of the NIG. It assumes that the acceleration or heating mechanism can be modeled as a drifting diffusion process in velocity space, controlled (or subordinated) by the time of interaction of the particles with “accelerating structures”. The probability function of the interaction time is an inverse Gaussian (IG), obtained by considering a random drift across structures of a given size. The control of the diffusion by interaction times that follow an IG probability function formally defines the NIG distribution. Following this model, we show that skewness and kurtosis can be used to estimate the kinetic and thermal energy gains provided by the interaction with structures. For example, in the case studies presented here, the analyzed populations would have gained kinetic energy representing approximately two to four times their thermal energy, with an increase in velocity – due to acceleration – of from one-tenth to one-third of the observed flow velocity. We also show that the model constrains the initial temperature of the populations.Conclusions.Overall, the NIG model offers excellent fits of the observed proton distributions. Combining the skewness and the kurtosis, it also leads to constraints in the part of acceleration and heating due to the interactions with structures in the formation of the proton populations. We suggest that these effects add to the classical thermal evolution of the bulk velocity and temperature resulting from SW expansion.

The current operational dimension available for space weather analysis and operations is not suitable for deep space exploration.As NASA plans for missions beyond the Low Earth Orbit (LEO), new advancements in modeling, observations, and communications are needed to establish a suitable monitoring and protection environment for the missions and the crew.The initial step is to establish multiple observational points that will improve the current analysis/modeling capabilities and extend them to deep space exploration missions.We summarize the value of multiple observational points, outline the current gaps (with examples) in providing operational space weather support for deep space exploration, and propose ideas for missions and international collaborations that will address these existing gaps.These proposed missions and collaborations will be essential to ensure a successful future for deep space exploration.

The middle corona, the region roughly spanning heliocentric distances from 1.5 to 6 solar radii, encompasses almost all of the influential physical transitions and processes that govern the behavior of coronal outflow into the heliosphere. The solar wind, eruptions, and flows pass through the region, and they are shaped by it. Importantly, the region also modulates inflow from above that can drive dynamic changes at lower heights in the inner corona. Consequently, the middle corona is essential for comprehensively connecting the corona to the heliosphere and for developing corresponding global models. Nonetheless, because it is challenging to observe, the region has been poorly studied by both major solar remote-sensing and in-situ missions and instruments, extending back to the Solar and Heliospheric Observatory (SOHO) era. Thanks to recent advances in instrumentation, observational processing techniques, and a realization of the importance of the region, interest in the middle corona has increased. Although the region cannot be intrinsically separated from other regions of the solar atmosphere, there has emerged a need to define the region in terms of its location and extension in the solar atmosphere, its composition, the physical transitions that it covers, and the underlying physics believed to shape the region. This article aims to define the middle corona, its physical characteristics, and give an overview of the processes that occur there.

Recognition for All: A Way Forward to Enhance Diversity, Equity and Inclusion in Space Physics M. Fraz Bashir,1 Amy M. Keesee,2, 3 Seth G. Claudepierre,4 Michael D. Hartinger,5 Elizabeth A. MacDonald,6 and Allison Jaynes7 Department of Earth, Planetary and Space Sciences, UCLA, Los Angeles, CA, USA∗ Department of Physics and Astronomy, University of New Hampshire, Durham, NH, USA Space Science Center, University of New Hampshire, Durham, NH, USA Department of Atmospheric and Oceanic Sciences, UCLA, Los Angeles, CA, USA Center for Space Plasma Physics, Space Science Institute, Boulder, CO, USA NASA Goddard Space Flight Center, Greenbelt, MD, USA Department of Physics Astronomy, University of Iowa, IA, USA

Motivation ➔ Understanding the creation and early evolution of solar wind is fundamental to Heliophysics as a whole.➔ To understand the solar wind, it is critical to reliably track individual packets of solar wind from the low corona to 1 AU.Strategy ➔ In order to accomplish this, we need coronal and solar wind plasma measurements of: ◆ Magnetic fields ◆ Composition ◆ Particle density ◆ Velocities ➔ Coverage must be 360˚, high-resolution, and high-cadence. Main Takeaway➔ These are bottom-line requisites to advance fundamental science, space weather modeling, and operational forecasting.

Context.Though the solar wind is characterized by spatial and temporal variability across a wide range of scales, long-term averages of in situ measurements have revealed clear radial trends: changes in average values of basic plasma parameters (e.g., density, temperature, and speed) and a magnetic field with a distance from the Sun.Aims.To establish our current understanding of the solar wind's average expansion through the heliosphere, data from multiple spacecraft needed to be combined and standardized into a single dataset.Methods.In this study, data from twelve heliospheric and planetary spacecraft - Parker Solar Probe (PSP), Helios 1 and 2, Mariner 2 and 10, Ulysses, Cassini, Pioneer 10 and 11, New Horizons, and Voyager 1 and 2 - were compiled into a dataset spanning over three orders of magnitude in heliocentric distance. To avoid introducing artifacts into this composite dataset, special attention was given to the solar cycle, spacecraft heliocentric elevation, and instrument calibration.Results.The radial trend in each parameter was found to be generally well described by a power-law fit, though up to two break points were identified in each fit.Conclusions.These radial trends are publicly released here to benefit research groups in the validation of global heliospheric simulations and in the development of new deep-space missions such as Interstellar Probe.

The interaction of our protective heliosphere and the Very Local Interstellar Medium (VLISM) is the least explored and most rewarding frontier of space physics.New evidence amplifies the central role of the heliosphere in the evolution of the solar system along its 4.6billion-year journey around the galaxy.In addition to the dense clouds of plasma, gas and dust seeding the early proto solar nebula, recent supernovae have left the entire solar system exposed to extreme fluxes of interstellar material and cosmic radiation with far-reaching implications.Our current knowledge lacks the direct measurements necessary to understand how our star upholds its vast heliosphere and its potentially game-changing role in the evolution of our galactic home.Interstellar Probe provides new, required measurements over more than a solar cycle to uncover the physical processes starting near the Sun responsible for creating our dynamic heliosphere.In April 2022, the pragmatic Interstellar Probe Mission Concept Study was completed after four years, detailing a Large Strategic heliophysics mission that would transect the heliosphere from 1 au to the VLISM.Its journey provides rich science for generations across heliophysics and presents an opportunity to push the frontier of space exploration farther than ever done before.Modest crossdivisional investments enable high-value planetary science and astrophysics, deepening our understanding of the emergence of our habitable planetary system.A trajectory through the forward hemisphere of the heliosphere would be accomplished by a launch in the 2036-2042 timeframe using conventional chemical propulsion and a heavy-lift launch vehicle, such as the Space Launch System (SLS).A Jupiter Gravity Assist could propel an 860-kg spacecraft with an 87-kg payload of ten instruments delivering a unified view of the global heliosphere, reaching the VLISM after 16 years.The spacecraft is designed to a 50-year nominal lifetime using modern-day technology based on successful missions like New Horizons.Two next-generation Radioisotope Thermal Generators (RTGs) would ensure 300 We at end of nominal mission at 375 au and could enable exploration even beyond 500 au.

Ion mass-per-charge and shock geometry determine both shock injection and the number of times a charged particle is reflected across a shock. As such, they govern charged particle acceleration and heating at shock. Solar Obiter’s Heavy Ion Sensor (HIS) observed a quasi-parallel CME-driven shock on March 11, 2022. HIS has sufficient time, mass, and charge resolution that it measured individual distributions of iron 8+ through 12+ on the variable timescale of 2 to 5 minutes. Using these 1D velocity distribution functions (VDFs), we report that the thermal portion of the Fe distribution heats across the shock, that this heating increases with Q/M, and the heating increases with distance downstream from the shock.