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1. Ion Heating by a Fast Magnetosonic Turbulence in the Solar Corona

   https://doi.org/10.3847/1538-4357/ad3727  

   Markovskii, S A; Vasquez, Bernard J (April 2024, The Astrophysical Journal)

   Abstract Observational data at heliocentric distances of tens of solar radii suggest that fast magnetosonic modes make up a considerable fraction of the solar wind fluctuations. Furthermore, this fraction appears to increase closer to the Sun. We carry out three-dimensional kinetic simulations with particle ions and fluid electrons to evaluate the proton and alpha-particle heating produced by the damping of the fast waves in the solar corona. Realistic parameters at 5 solar radii, including the fluctuation amplitude, are used. We show that, due to the cyclotron resonance, the alphas are heated preferentially perpendicularly to the magnetic field and much more strongly than the protons. The presence of the alpha particles alters the energy partition by reducing the heating of the protons. Nevertheless, the proton heating is sufficient to account for the solar wind acceleration. 

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2. Proton-Alpha Drift Instability of Electromagnetic Ion-Cyclotron Modes: Quasilinear Development

   https://doi.org/10.3390/ physics3040075  

   Shaaban M. Shaaban, Marian Lazar (December 2021, Physics)

   The ability of space plasmas to self-regulate through mechanisms involving self-generated fluctuations is a topic of high interest. This paper presents the results of a new advanced quasilinear (QL) approach for the instability of electromagnetic ion-cyclotron modes driven by the relative alpha-proton drift observed in solar wind. For an extended parametric analysis, the present QL approach includes also the effects of intrinsic anisotropic temperatures of these populations. The enhanced fluctuations contribute to an exchange of energy between proton and alpha particles, leading to important variations of the anisotropies, the proton-alpha drift and the temperature contrast. The results presented here can help understand the observational data, in particular, those revealing the local variations associated with the properties of protons and alpha particles as well as the spatial profiles in the expanding solar wind. 

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3. Turbulence, Waves, and Taylor’s Hypothesis for Heliosheath Observations

   https://doi.org/10.3847/1538-4357/ad64c8  

   Zhao, L-L; Zank, G P; Opher, M; Zieger, B; Li, H; Florinski, V; Adhikari, L; Zhu, X; Nakanotani, M (September 2024, The Astrophysical Journal)

   Abstract Magnetic field fluctuations measured in the heliosheath by the Voyager spacecraft are often characterized as compressible, as indicated by a strong fluctuating component parallel to the mean magnetic field. However, the interpretation of the turbulence data faces the caveat that the standard Taylor’s hypothesis is invalid because the solar wind flow velocity in the heliosheath becomes subsonic and slower than the fast magnetosonic speed, given the contributions from hot pickup ions (PUIs) in the heliosheath. We attempt to overcome this caveat by introducing a 4D frequency-wavenumber spectral modeling of turbulence, which is essentially a decomposition of different wave modes following their respective dispersion relations. Isotropic Alfvén and fast mode turbulence are considered to represent the heliosheath fluctuations. We also include two dispersive fast wave modes derived from a three-fluid theory. We find that (1) magnetic fluctuations in the inner heliosheath are less compressible than previously thought, an isotropic turbulence spectral model with about 25% in compressible fluctuation power is consistent with the observed magnetic compressibility in the heliosheath; (2) the hot PUI component and the relatively cold solar wind ions induce two dispersive fast magnetosonic wave branches in the perpendicular propagation limit, PUI fast wave may account for the spectral bump near the proton gyrofrequency in the observable spectrum; (3) it is possible that the turbulence wavenumber spectrum is not Kolmogorov-like although the observed frequency spectrum has a −5/3 power-law index, depending on the partitioning of power among the various wave modes, and this partitioning may change with wavenumber. 

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4. Modeling proton and electron heating in the fast solar wind

   https://doi.org/10.1051/0004-6361/202039297  

   Adhikari, L.; Zank, G. P.; Zhao, L.-L.; Nakanotani, M.; Tasnim, S. (June 2021, Astronomy & Astrophysics)

   null (Ed.)

   Context. The Parker Solar Probe (PSP) measures solar wind protons and electrons near the Sun. To study the thermodynamic properties of electrons and protons, we include electron effects, such as distributed turbulent heating between protons and electrons, Coulomb collisions between protons and electrons, and heat conduction of electrons. Aims. We develop a general theoretical model of nearly incompressible magnetohydrodynamic (NI MHD) turbulence coupled with a solar wind model that includes electron pressure and heat flux. Methods. It is important to note that 60% of the turbulence energy is assigned to proton heating and 40% to electron heating. We use an empirical expression for the electron heat flux. We derived a nonlinear dissipation term for the residual energy that includes both the Alfvén effect and the turbulent small-scale dynamo effect. Similarly, we obtained the NI/slab time-scale in an NI MHD phenomenology to use in the derivation of the nonlinear term that incorporates the Alfvén effect. Results. A detailed comparison between the theoretical model solutions and the fast solar wind measured by PSP and Helios 2 shows that they are consistent. The results show that the nearly incompressible NI/slab turbulence component describes observations of the fast solar wind periods when the solar wind flow is aligned or antialigned with the magnetic field. 

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5. 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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