Michael Louis Stevens

The early evolution of fast polar coronal hole (PCH) solar wind remains largely unconstrained by in situ measurements. In 2025 March, 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 indicate 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.

Coronal mass ejections (CMEs) are enormous bursts of plasma and magnetic field from the Sun that propagate explosively through the Solar System. CMEs are the primary drivers of the most severe space-weather events, posing major radiation hazards for human exploration beyond Earth’s magnetosphere, including missions to the Moon and Mars. During Europa Clipper’s cruise between Earth and Mars, its Plasma Instrument for Magnetic Sounding (PIMS) recorded unusual solar-wind conditions later identified as the wake of a complex CME. Data from an unprecedented network of 17 spacecraft across the inner Solar System revealed a hidden Earth-directed component that propagated asymmetrically and could not have been predicted without off-Sun-Earth-line observations. This component also affected Europa Clipper and Mars, demonstrating how a missed forecast of this kind could endanger a crewed mission. These findings highlight the need for multipoint observations, including planetary missions in cruise, to improve space-weather forecasting, mission planning, and operations.

Solar prominences are cool, dense stable structures routinely observed in the corona. Prominences are often ejected from the Sun via coronal mass ejections (CMEs). However, they are rarely detected in a cool, low-ionized state within CMEs measured in situ, making their evolution hard to study. We examine the thermodynamic evolution of one of these rare cases where a quiescent prominence eruption clearly preserves its low-ionized charge state as evidenced by in situ detection. We use multiviewpoint Extreme Ultraviolet observations to track and estimate the density, temperature, and speed of the prominence as it erupts. We observe that part of the prominence remains in absorption well beyond initial liftoff, indicating the bulk of the prominence experiences minimal ionization and suggesting any strong heating is balanced by radiative losses, expansion, or conduction. From its subsequent in situ passage near 1 au, charge states reveal that the prominence is composed of both cool, low-ionized ions as well as hotter plasma reflected by the presence of highly ionized iron, Fe ¹⁶⁺ . Simulated nonequilibrium ionization and recombination results using observationally derived initial conditions match the in situ multithermal state for a prominence composed of 70% cool plasma with a 1.8 MK peak temperature, and 30% hot plasma with a 4.3 MK peak temperature. This suggests that the prominence may not be heated uniformly, that parts of it cool more rapidly and that mixing in of neighboring hotter plasma with the prominence core may play a role. The complex, multithermal nature of this erupting prominence emphasizes the need for more comprehensive spectral observations of the global corona.

Calibrated measurements of electron velocity distribution functions (VDFs) are necessary to characterize fluid and kinetic processes in weakly collisional and nearly collisionless plasmas such as the solar wind. Therefore, we analyzed 3,996,051 electron VDFs observed by the Wind 3DP thermal electron detector near 1 au between 2005 January 1 and 2017 November 25. The data were calibrated for each electron VDF to produce accurate velocity moments in the solar wind. This is the first full solar cycle coverage electron velocity moment dataset in the near-Earth solar wind. Herein (Paper I) we discuss the calibration process/algorithms and the velocity moment constraints, uncertainties, and resulting public dataset. In the second paper (Paper II), we statistically analyze the electron velocity moment dataset.

Between 2005 January 1 and 2017 November 25, the Wind spacecraft measured 3,996,051 electron velocity distribution functions (VDFs) in the near-Earth solar wind. The instrument was calibrated, and velocity moments were calculated up to the third moment (i.e., electron heat flux). The magnetic field and ion velocity moments have been interpolated to the midpoint time of the VDFs, where we have 3,947,607 finite electron velocity moments (most prior studies used n _{e ,int} ∼ 5.43 cm ⁻³ , ∣ V _oe ∣ ∼ 421 km s ⁻¹ , T _{e ,tot} ∼ 12.7 eV, and ∣ q _{e ,∥} ∣ ∼4.92 μ W m ⁻² . The median values for the electron-to-proton temperature ratio is T e , tot / T p , tot ∼2.55, the electron temperature anisotropy A _e = T e , ⊥ / T e , ∥ ∼ 0.89, total electron beta β _{e ,tot} ∼ 1.27, electron–electron collision frequency ν _ee ∼ 0.18 day ⁻¹ , and electron–electron Knudsen number K _{n ,ee} ∼ 2.57. The results are mostly consistent with previous studies except that K _{n ,ee} is significantly larger than some other works, which implies that the near-Earth solar wind is less collisional than may have been inferred.

In situ observations by Parker Solar Probe (PSP) suggest that the heliospheric current sheet (HCS) undergoes near-continuous magnetic reconnection close to the Sun, in stark contrast to scarce observations of this phenomenon in the HCS at 1 au. Situated at the boundary between sectors of opposite interplanetary magnetic field polarity, reconnection in the HCS has important consequences for magnetic topology and plasma dynamics in the slow solar wind. We report observations of a reconnection outflow in the HCS near the Alfvén transition region in PSP’s 17th solar encounter, featuring plasma jetting, proton temperature enhancement, and electron heat flux dropout. Embedded within the exhaust is a non-force-free flux rope plasmoid exhibiting counterstreaming strahl electrons, indicating connection at both ends to the Sun in an otherwise disconnected region of the magnetic field. The flux rope features diminished isotropic proton temperature and lower bulk speed compared to the remainder of the HCS exhaust. Its oblique orientation and different plasma properties imply that the flux rope originates from a different reconnection site to the HCS exhaust, suggesting PSP has intercepted a flux-rope-like streamer blob produced at the helmet streamer. Remote observations show several comparable blobs traveling in a distant coronal ray, demonstrating the possibility that the in situ flux rope is a streamer blob. The combination of in situ and remote observations demonstrates the role of magnetic reconnection in HCS dynamics, contributing to a growing understanding of this fundamental mechanism and its impact on the young solar wind.

Parker Solar Probe (PSP) observes abundant circularly polarized ion-scale waves throughout the inner heliosphere. These waves are a signature of the interplay between plasma microinstabilities and turbulent dissipation. We perform a mission-wide statistical survey of ion-scale waves observed by PSP, which are stored in a publicly available repository, investigating whether the waves correspond to specific free energy sources in the measured proton velocity distributions. We find that left-handed waves (LHWs) are frequently observed, with the fraction of time they are observed increasing closer to the Sun, reaching ∼30%. Right-handed waves (RHWs) are less frequently observed, with the associated time fraction decreasing closer to the Sun. The observed LHWs are generally consistent with parallel-propagating ion cyclotron wave storms that occur continuously for extended periods of time. Turbulent energy spectra are consistently steeper when LHW storms are observed; these wave storms mediate the spatial transport of the free energy associated with temperature anisotropy. The observed RHWs are generally consistent with oblique and parallel fast magnetosonic waves (FMWs), and their observation is well correlated with enhanced proton parallel heat flux, which quantifies the presence of secondary proton populations. Using observations and the SAVIC machine learning instability identification algorithm, we identify a threshold on the proton heat flux beyond which FMWs are likely to be driven unstable by the proton beams. We are thus able to associate trends in the observed ion-scale waves with known sources of free energy for Encounters 3–24 of the PSP’s prime science phase.

We examine the statistics of Alfvénic deflections in sub- and super-Alfvénic solar wind with particular focus on the magnetic deflection angle θ . Our findings are in general agreement with earlier studies suggesting that θ > 90° rarely occurs in sub-Alfvénic regimes. We find the upper limit of θ exhibits an identifiable trend with the Alfvén Mach number M _a , suggesting that gradual steepening of Alfvénic deflections with increasing M _a is a plausible mechanism controlling deflection angles in the young solar wind. Further analysis reveals that large velocity fluctuations ( δv / v > 1) tend to be important in the largest sub-Alfvénic deflections with increasing contributions from δv _∥ approaching M _a = 1, while few deflections in the super-Alfvénic regime exhibit such large velocity perturbations. We also determine the local ratio of radial Poynting flux S _R to kinetic energy flux K _R and find that large sub-Alfvénic deflection angles tend to be dominated by S _R , while super-Alfvénic deflections are eventually dominated by the K _R associated with the radial solar wind flow. Our results show that near the Alfvén surface (where M _a = 1), there is a critical region of parameter space where δv ∼ v _a and K _R / S _R ∼ 1. We refer to this region (where ∣ log 10 ( M a ) ∣ < 0.2 ) as the conversion layer. The conversion layer may play a significant role in the evolution of magnetic defections by providing the medium for converting magnetic energy to particle energy and likely driving the formation of magnetic switchbacks in super-Alfvénic solar wind.

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.