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1. Preferential Energization of Solar Wind Ions Below the Alfvénic Surface

   https://doi.org/10.3847/2041-8213/ae0732  

   Mostafavi, Parisa; Jagarlamudi, V K; Raouafi, N E; Palacios, J C; Allen, R C; Hill, M E; Ofman, L; Ho, G C (September 2025, The Astrophysical Journal Letters)

   Abstract Understanding how solar wind ions are heated and accelerated remains a central question in heliophysics. Observations consistently show that alpha particles are hotter and faster than protons, particularly close to the Sun. This suggests that kinetic processes play a dominant role in shaping ion distributions. Theoretical models have proposed that much of this preferential energization occurs near the Alfvén critical surface, a spatially varying boundary where the solar wind transitions from magnetically dominated to thermally dominated flow. With in situ measurements from Parker Solar Probe (PSP), it is possible to explore this question near the Sun directly. In this study, we analyze PSP observations from near-perihelion encounters to statistically compare the thermodynamic properties of protons and alpha particles across the Alfvén surface. We find that ion temperature anisotropies, differential flows, and heating signatures are coherently organized by the local Alfvén Mach number (M_A). In the sub-Alfvénic regime, alpha particles exhibit strong perpendicular temperature heating and elevated alpha-to-proton temperature ratio, which decline across the Alfvénic transition. Meanwhile, proton heating peaks nearM_A ∼ 1, consistent with anisotropic wave–particle energization. These findings highlight the Alfvén surface as a key boundary for ion energization and momentum exchange, and establishM_Aas a fundamental organizing parameter in the kinetic evolution of solar wind ions in the inner heliosphere. 

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2. Energy Budget in the Solar Corona

   https://doi.org/10.3847/1538-4357/aceb64  

   Telloni, Daniele; Romoli, Marco; Velli, Marco; Zank, Gary P.; Adhikari, Laxman; Zhao, Lingling; Downs, Cooper; Halekas, Jasper S.; Verniero, Jaye L.; McManus, Michael D.; et al (August 2023, The Astrophysical Journal)

   Abstract This paper addresses the first direct investigation of the energy budget in the solar corona. Exploiting joint observations of the same coronal plasma by Parker Solar Probe and the Metis coronagraph aboard Solar Orbiter and the conserved equations for mass, magnetic flux, and wave action, we estimate the values of all terms comprising the total energy flux of the proton component of the slow solar wind from 6.3 to 13.3 R ⊙ . For distances from the Sun to less than 7 R ⊙ , we find that the primary source of solar wind energy is magnetic fluctuations including Alfvén waves. As the plasma flows away from the low corona, magnetic energy is gradually converted into kinetic energy, which dominates the total energy flux at heights above 7 R ⊙ . It is found too that the electric potential energy flux plays an important role in accelerating the solar wind only at altitudes below 6 R ⊙ , while enthalpy and heat fluxes only become important at even lower heights. The results finally show that energy equipartition does not exist in the solar corona. 

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3. Transonic Turbulence and Density Fluctuations in the Near-Sun Solar Wind

   https://doi.org/10.3847/2041-8213/ada3d8  

   Zhao, L-L; Silwal, A; Zhu, X; Li, H; Zank, G P (January 2025, The Astrophysical Journal Letters)

   Abstract We use in situ measurements from the first 19 encounters of Parker Solar Probe and the most recent five encounters of Solar Orbiter to study the evolution of the turbulent sonic Mach numberM_t(the ratio of the amplitude of velocity fluctuations to the sound speed) with radial distance and its relationship to density fluctuations. We focus on the near-Sun region with radial distances ranging from about 11 to 80R_⊙. Our results show that (1) the turbulent sonic Mach numberM_tgradually moves toward larger values as it approaches the Sun, until at least 11R_⊙, whereM_tis much larger than the previously observed value of 0.1 at and above 0.3 au; (2) transonic turbulence withM_t ∼ 1 is observed in situ for the first time and is found mostly near the Alfvén critical surface; (3) Alfvén Mach number of the bulk flowM_Ashows a strong correlation with the plasma beta, indicating that most of the observed sub-Alfvénic intervals correspond to a low-beta plasma; (4) the scaling relation between density fluctuations andM_tgradually changes from a linear scaling at larger radial distances to a quadratic scaling at smaller radial distances; and (5) transonic turbulence is more compressible than subsonic turbulence, with enhanced density fluctuations and slightly flatter spectra than subsonic turbulence. A systematic understanding of compressible turbulence near the Sun is necessary for future solar wind modeling efforts. 

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4. Solar Wind Heating near the Sun: A Radial Evolution Approach

   https://doi.org/10.3847/1538-4357/ae4582  

   Yogesh; Ofman, Leon; Klein, Kristopher G; Niranjana; Martinović, Mihailo; Howes, Gregory G; Mostafavi, Parisa; Boardsen, Scott A; Sadykov, Viacheslav M; Pal, Sanchita; et al (March 2026, The Astrophysical Journal)

   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. 

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5. Scaling of Magnetic Reconnection Electron Bulk Heating in the High-Alfvén-speed and Low-β Regime of Earth’s Magnetotail

   https://doi.org/10.3847/1538-4357/acdf44  

   Øieroset, M; Phan, T D; Oka, M; Drake, J F; Fuselier, S A; Gershman, D J; Maheshwari, K; Giles, B L; Zhang, Q; Guo, F; et al (August 2023, The Astrophysical Journal)

   Abstract We have surveyed 21 reconnection exhaust events observed by Magnetospheric MultiScale in the low-plasma-βand high-Alfvén-speed regime of the Earth’s magnetotail to investigate the scaling of electron bulk heating produced by reconnection. The ranges of inflow Alfvén speed and inflow electronβ_ecovered by this study are 800–4000 km s⁻¹and 0.001–0.1, respectively, and the observed heating ranges from a few hundred electronvolts to several kiloelectronvolts. We find that the temperature change in the reconnection exhaust relative to the inflow, ΔT_e, is correlated with the inflow Alfvén speed,V_Ax,in, based on the reconnecting magnetic field and the inflow plasma density. Furthermore, ΔT_eis linearly proportional to the inflowing magnetic energy per particle, ${m_{\mathrm{i}}{V}_{\mathrm{Ax},\mathrm{in}}}^2$ , and the best fit to the data produces the empirical relation ΔT_e= 0.020 ${m_{\mathrm{i}}{V}_{\mathrm{Ax},\mathrm{in}}}^2$ , i.e., the electron temperature increase is on average ∼2% of the inflowing magnetic energy per particle. This magnetotail study extends a previous magnetopause reconnection study by two orders of magnitude in both magnetic energy and electronβ, to a regime that is comparable to the solar corona. The validity of the empirical relation over such a large combined magnetopause–magnetotail plasma parameter range ofV_A∼ 10–4000 km s⁻¹andβ_e∼ 0.001–10 suggests that one can predict the magnitude of the bulk electron heating by reconnection in a variety of contexts from the simple knowledge of a single parameter: the Alfvén speed of the ambient plasma. 

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