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Abstract The Kelvin–Helmholtz instability (KHI) can occur when there is a relative motion between two adjacent fluids. In the case of magnetized plasma, the shear velocity must exceed the local Alfvén speed for the instability to develop. The KHI produces nonlinear waves that eventually roll up into vortices and contribute to turbulence and dissipation. In the solar atmosphere, KHI has been detected in coronal mass ejections (CMEs), jets, and prominences, mainly in the low corona. Only a few studies have reported the KHI in the upper corona, and its vortex development there has not been previously observed. We report an event with large-scale KHI waves observed from ∼6 to 14R_⊙on 2024 February 16 using Solar and Heliospheric Observatory/Large Angle and Spectrometric Coronagraph and STEREO-A coronagraphs. A KHI appeared during the passage of a fast CME and evolved into the nonlinear stage showing evidence of vortices. A closely timed subsequent CME in the same region further developed the fully nonlinear KHI waves along its flank. We find that the radial speed of the CMEs exceeds the estimated local Alfvén speed obtained from in-situ Parker Solar Probe magnetic field data at perihelia. We propose that such events are rare because the fast CME created specific conditions favorable for instability growth in its trailing edge, including radial elongation of magnetic-field lines, reduced plasma density, and enhanced velocity and magnetic-field shear along the developing interface. The observed growth rate of the KHI wave is in qualitative agreement with the theoretical predictions. 

Abstract Analysis of ion-kinetic instabilities in solar wind plasmas is crucial for understanding energetics and dynamics throughout the heliosphere, as evident from spacecraft observations of complex ion velocity distribution functions (VDFs) and ubiquitous ion-scale kinetic waves. In this work, we explore machine learning (ML) and deep learning (DL) classification models to identify unstable cases of ion VDFs driving kinetic waves. Using 34 hybrid particle-in-cell simulations of kinetic protons andα-particles initialized using plasma parameters derived from solar wind (SW) observations, we prepare a data set of nearly 1600 VDFs representing stable/unstable cases and associated plasma and wave properties. We compare feature-based classifiers applied to VDF moments, such as support vector machine and random forest (RF), with DL convolutional neural networks (CNNs) applied directly to VDFs as images in the gyrotropic velocity plane. The best-performing classifier, RF, has an accuracy of 0.96 ± 0.01, and a true skill score of 0.89 ± 0.03, with the majority of missed predictions made near stability thresholds. We study how the variations of the temporal derivative thresholds of anisotropies and magnetic energies, and sampling strategies for simulation runs, affect classification. CNN-based models have the highest accuracy of 0.88 ± 0.18 among all considered if evaluated on the runs entirely not used during the model training. The addition of theE_⊥power spectrum as an input for the ML models leads to the improvement of instability analysis for some cases. The results demonstrate the potential of ML and DL for the detection of ion-scale kinetic instabilities using spacecraft observations of SW and magnetospheric plasmas. 

Abstract Recent observations of the solar wind ions by the SPAN-I instruments on board the Parker Solar Probe (PSP) spacecraft at solar perihelia (Encounters) 4 and closer find ample evidence of complex anisotropic non-Maxwellian velocity distributions that consist of core, beam, and “hammerhead” (i.e., anisotropic beam) populations. The proton core populations are anisotropic, withT_⊥/T_∥ > 1, and the beams have super-Alfvénic speed relative to the core (we provide an example from Encounter 17). Theα-particle population shows similar features to the protons. These unstable velocity distribution functions (VDFs) are associated with enhanced, right-hand (RH) and left-hand (LH) polarized ion-scale kinetic wave activity, detected by the FIELDS instrument. Motivated by PSP observations, we employ nonlinear hybrid models to investigate the evolution of the anisotropic hot-beam VDFs and model the growth and the nonlinear stage of ion kinetic instabilities in several linearly unstable cases. The models are initialized with ion VDFs motivated by the observational parameters. We find rapidly growing (in terms of proton gyroperiods) combined ion-cyclotron and magnetosonic instabilities, which produce LH and RH ion-scale wave spectra, respectively. The modeled ion VDFs in the nonlinear stage of the evolution are qualitatively in agreement with PSP observations of the anisotropic core and “hammerhead” velocity distributions, quantifying the effect of the ion kinetic instabilities on wind plasma heating close to the Sun. We conclude that the wave–particle interactions play an important role in the energy transfer between the magnetic energy (waves) and random particle motion, leading to anisotropic solar wind plasma heating. 

Abstract The variability of helium abundance in the solar corona and the solar wind is an important signature of solar activity, solar cycle, and solar wind sources, as well as coronal heating processes. Motivated by recently reported remote-sensing UV imaging observations by Helium Resonance Scattering in the Corona and Heliosphere payload sounding rocket of helium abundance in the inner corona on 2009 September 14 near solar minimum, we present the results of the first three-dimensional three-fluid (electrons, protons, and alpha particles) model of tilted coronal streamer belt and slow solar wind that illustrates the various processes leading to helium abundance differentiation and variability. We find good qualitative agreement between the three-fluid model and the coronal helium abundance variability deduced from UV observations of streamers, providing insight on the effects of the physical processes, such as heating, gravitational settling, and interspecies Coulomb friction in the outflowing solar wind that produce the observed features. The study impacts our understanding of the origins of the slow solar wind. 

Abstract The abundance of helium (A_He) in the solar wind exhibits variations typically in the range from 2% to 5% with respect to solar cycle activity and solar wind velocity. However, there are instances where the observedA_Heis exceptionally low (<1%). These low-A_Heoccurrences are detected both near the Sun and at 1 au. The low-A_Heevents are generally observed near the heliospheric current sheet. We analyzed 28 low-A_Heevents observed by the Wind spacecraft and 4 by Parker Solar Probe to understand their origin. In this work, we make use of the ADAPT-WSA model to derive the sources of our events at the base of the solar corona. The modeling suggests that the low-A_Heevents originated from the boundaries of coronal holes, primarily from large quiescent helmet streamers. We argue that the cusp above the core of the streamer can produce such very low helium abundance events. The streamer core serves as an ideal location for gravitational settling to occur as demonstrated by previous models, leading to the release of this plasma through reconnection near the cusp, resulting in low-A_Heevents. Furthermore, observations from Ulysses provide direct evidence that these events originated from coronal streamers. 

Abstract Recent in situ observations from Parker Solar Probe (PSP) near perihelia reveal ion beams, temperature anisotropies, and kinetic wave activity. These features are likely linked to solar wind heating and acceleration. During PSP Encounter 17 (at 11.4R_s) on 2023 September 26, the PSP/FIELDS instrument detected enhanced ion-scale wave activity associated with deviations from local thermodynamic equilibrium in ion velocity distribution functions (VDFs) observed by the PSP/Solar Probe Analyzers-Ion. Dense beams (secondary populations) were present in the proton VDFs during this wave activity. Using bi-Maxwellian fits to the proton VDFs, we found that the density of the proton beam population increased during the wave activity and, unexpectedly, surpassed the core population at certain intervals. Interestingly, the wave power was reduced during the intervals when the beam population density exceeded the core density. The drift velocity of the beams decreases from 0.9 to 0.7 of the Alfvén speed, and the proton core shows a higher temperature anisotropy (T_⊥/T_∥ > 2.5) during these intervals. We conclude that the observations during these intervals are consistent with a reconnection event during a heliospheric current sheet crossing. During this event,α-particle parameters (density, velocity, and temperature anisotropy) remained nearly constant. Using linear analysis, we examined how the proton beam drives instability or wave dissipation. Furthermore, we investigated the nonlinear evolution of ion kinetic instabilities using hybrid kinetic simulations. This study provides direct clues about energy transfer between particles and waves in the young solar wind. 

Abstract Characterizing the plasma state in the near-Sun environment is essential to constrain the mechanisms that heat and accelerate the solar wind. In this study, we use Parker Solar Probe observations from Encounters 1 through 24 to investigate the radial evolution of solar wind plasma and magnetic field properties in this region. Using intervals with high field-of-view (>85%) coverage, we derive the radial profiles of magnetic field strength (∣B∣), proton density (N), bulk speed (V), total proton temperature (T), parallel (T_∥) and perpendicular (T_⊥) temperatures, temperature anisotropy (T_⊥/T_∥), plasma beta (β), Alfvén Mach number (M_A), and magnetic field fluctuations (δB/B) for sub and super-Alfvénic regions. In super-Alfvénic regions, power laws of ∣B∣,N,V, andTas a function of the heliocentric distance are broadly consistent with previous Helios results at >0.3 au. The radial evolution of the components of the temperature tensor reveals distinct behavior:T_⊥decreases monotonically with distance, whereasT_∥exhibits a nonmonotonic trend—decreasing in the sub-Alfvénic region, increasing just beyond the Alfvén surface. We interpret the increase inT_∥as a proxy for proton beam occurrence. We further examine the evolution of magnetic field fluctuations, finding decreasing radial/parallel fluctuations but enhanced tangential/normal/perpendicular fluctuations in the sunward direction. These fluctuations may provide free energy for beam generation and particle heating via wave–particle interactions. 

In this paper, we share the experience of the U.S. National Science Foundation (NSF) National Solar Observatory (NSO) scientists, educators, and public outreach officers in organizing an eclipse viewing party within a stadium at a sports complex on the US/Mexico border in Eagle Pass, TX in collaboration with educators from the Eagle Pass and the Uvalde areas. We describe the reasons we chose Eagle Pass, contacts we established with the local community, preparations for and activities set up during the eclipse viewing party, the eclipse day on April 8, 2024, and lessons learned from organizing our event. 

This white paper is on the HMCS Firefly mission concept study. Firefly focuses on the global structure and dynamics of the Sun's interior, the generation of solar magnetic fields, the deciphering of the solar cycle, the conditions leading to the explosive activity, and the structure and dynamics of the corona as it drives the heliosphere.