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1. 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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2. Density Fluctuation–Mach Number Scaling in Compressible, High Plasma Beta Turbulence: In Situ Space Observations and High-Reynolds Number Simulations

   https://doi.org/10.3847/2041-8213/adbe3b  

   Bandyopadhyay, Riddhi; Beattie, James R; Bhattacharjee, Amitava (March 2025, The Astrophysical Journal Letters)

   Abstract Understanding the nature of compressible fluctuations in a broad range of turbulent plasmas, from the intracluster medium to the solar wind, has been an active field of research in the past decades. Theoretical frameworks for weakly compressible magnetohydrodynamical turbulence in an inhomogeneous background magnetic field predict a linear scaling of the normalized mass density fluctuation (δρ/ρ₀), as a function of the turbulent Mach number ( ${\mathcal{M}}_t$ ), $\delta \rho /{\rho }_0\propto {\mathcal{M}}_t$ . However, so far, the scaling relation has been tested only using moderate to low plasma beta (β ≲ 1) solar wind observational data, where the compressibility is weakδρ/ρ₀ ∼ 0.1. Here, we combine NASA’s Magnetospheric Multiscale Mission data in Earth’s magnetosheath, whereβ ∼ 10 is high, andβ ∼ 1/8 highly compressible magnetohydrodynamic turbulence simulations at unprecedented resolutions. Both show that $\delta \rho /{\rho }_0\propto {\mathcal{M}}_t$ holds across a broad range ofδρ/ρ₀, ${\mathcal{M}}_t$ , andβ, demonstrating that $\delta \rho /{\rho }_0\propto {\mathcal{M}}_t$ is a robust compressible turbulence relation, going beyond the asymptotics of the weakly compressible theory. We discuss the findings in the context of understanding the nature of strongly compressible turbulent fluctuations and the driving parameter in astrophysical and space plasmas. 

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3. Parker Solar Probe analysis across the Alfvénic transition: velocity shear, magnetic deflection, and switchback formation

   https://doi.org/10.1093/mnras/stag242  

   Goodwill, Joshua; Adhikari, Subash; Payne, Dominic; Bandyopadhyay, Riddhi; Badman, Samuel; Pecora, Francesco; Pongkitiwanichakul, Peera; Pradata, Rayta; Romeo, Orlando; Roy, Sohom; et al (March 2026, Monthly Notices of the Royal Astronomical Society)

   ABSTRACT Close to the Sun, Parker Solar Probe (PSP) traverses the sub-Alfvénic solar wind, a magnetically controlled plasma environment. Farther from the Sun, the magnetic field amplitude and plasma density weaken enough to establish a super-Alfvénic environment where kinetic energy and turbulence become more prominent. Switchbacks (i.e. large directional deviations from the mean field) are shown to occur almost exclusively in the super-Alfvénic regime. In this study, we analyse magnetic fluctuations from PSP encounters 8 through 19 to study their relationship to Alfvén Mach number ($$M_\mathrm{ A}$$) and switchback parameter (Z). We find that the fluctuation of the magnetic field magnitude normalized to the mean magnetic field and of the radial velocity normalized to local Alfvén speed increase with both Alfvén Mach number and switchback parameter. However, there is distinct saturation in the increments of the normalized radial velocity fluctuations in the Alfvén Mach number range of $$\sim$$4–6. Results are interpreted in terms of the Chandrasekhar criterion for Kelvin–Helmholtz activity. Overall, these findings are in agreement with earlier studies suggesting switchback generation through non-linear shear flow dynamics. 

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4. 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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5. A study of particle acceleration, heating, power deposition, and the damping length of kinetic Alfvén waves in non-Maxwellian coronal plasma

   https://doi.org/10.1051/0004-6361/202452376  

   Ayaz, S; Zank, G P; Khan, I A; Li, G; Rivera, Y J (February 2025, Astronomy & Astrophysics)

   Context.The heating of the solar corona and solar wind, particularly through suprathermal particles and kinetic Alfvén waves (KAWs) within the 0–10 R_Sunrange, has been a subject of great interest for many decades. This study investigates and explores the acceleration and heating of charged particles and the role of KAWs in the solar corona. Aims.We investigate how KAWs transport energy and accelerate and heat the charged particles, focusing on the behavior of perturbed electromagnetic (EM) fields, the Poynting flux vectors, net power transfer through the solar flux loop tubes, resonant particles’ speed, group speed, and the damping length of KAWs. The study examines how these elements are influenced by suprathermal particles (κ) and the electron-to-ion temperature ratios (T_e/T_i). Methods.We used kinetic plasma theory coupled with the Vlasov-Maxwell model to investigate the dynamics of KAWs and particles. We assumed a collisionless, homogeneous, and low-beta electron-ion plasma in which Alfvén waves travel in the kinetic limits; that is,m_e/m_i ≪ β ≪ 1. Furthermore, the plasma incorporates suprathermal high-energy particles, necessitating an appropriate distribution function to accurately describe the system. We adopted the Kappa distribution function as the most suitable choice for our analysis. Results.The results show that the perturbed EM fields are significantly influenced byκand the effect of T_e/T_i. We evaluate both the parallel and perpendicular Poynting fluxes and find that the parallel Poynting flux (S_z) dissipates gradually for lowerκvalues. In contrast, the perpendicular flux (S_x) dissipates quickly over shorter distances. Power deposition in solar flux tubes is significantly influenced byκand T_e/T_i. We find that particles can heat the solar corona over long distances (R_Sun) in the parallel direction and short distances in the perpendicular direction. The group velocity of KAWs increases for lowerκvalues, and the damping length, L_G, is enhanced under lowerκ, suggesting longer energy transport distances (R_Sun). These findings offer a comprehensive understanding of particle-wave interactions in the solar corona and wind, with potential applications for missions such as the Parker Solar Probe, (PSP), and can also apply to other environments where non-Maxwellian particle distributions are frequently observed. 

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