Space Plasmas Research Articles

Relativistic $R$-matrix studies on direct and resonant photoionization of magnesium-like Si$^{2+}$ ions

Abstract The direct and resonant photoionization of magnesium-like Si$^{2+}$ ions from the ground state and three low-lying excited states [Ne]$3s3p~^3P_{0, \, 1, \,2}$ is studied using the relativistic R-matrix theory. Special attention is paid to the direct and resonance ionization limits and the resonance structure. To this aim, we compute the energy levels of Si$^{2+}$ and neighboring Si$^{3+}$ ions as well as the direct and resonant photoionization cross sections, using which the direct and resonance ionization limits are determined, being consistent with the NIST results. Moreover, for the ground-state photoionization 25 relatively isolated resonance peaks of different Rydberg series are resolved at the level of configuration or even fine structure by pairing the energy levels with the peak positions, which are determined to be associated with the resonance channels [Ne]$3lnl'$ ($l=p,d$). In contrast, for the excited-state scenarios just several resonance peaks are resolved due to the complexities of resonance structure, though the cross sections are calculated with a step size of $10^{-6}$ Ryd. This work fills the gap in the photoionization study of Si$^{2+}$ ions and, thus, is expected to contribute to the diagnosis and simulation of relevant astrophysical and laboratory silicon plasmas.

Flux equipartition in astrophysical plasma turbulence.

Expanding plasmas are ubiquitous in the Universe, from supernovae to stellar atmospheres and winds, carrying various forms of energy. Crucial for understanding their behavior, the characterization of the scale-to-scale energy transfer resulting from the interplay of turbulent motions, propagating waves, and instabilities is a key scope of major space missions. Here, we show how simultaneous upscale and downscale energy transfers occur in solar wind, leading statistically to equipartition of the turbulent energy flux. Our study sheds light on the paradigm of the existence of dual energy cascades in astrophysical plasmas, identifying the scales at which energy sources act in the magnetohydrodynamic regime driving turbulent dynamics in solar wind. These findings suggest that a significant fraction of the energy injected into stellar winds at scales much smaller than those of galaxies could be transferred to larger scales through turbulence, potentially influencing star formation processes.

On a Propagation of Gendrin‐Mode Waves in the Magnetosphere

Abstract This paper presents results from the analytical and numerical investigations of the propagation of Gendrin‐mode whistler waves in the magnetosphere. These waves are important for the interpretation of satellite observations and planning experiments in laboratory and space plasma because the perpendicular component of their group velocity is equal to zero, and hence, they can supposedly propagate over a significant distance along an ambient magnetic field without attenuation. Simulations of the electron MHD equations demonstrate that these waves generated by the source with a finite transverse size experience a substantial diffraction, leading to the diminishing of wave amplitude with distance from the source. The effect of the diffraction is inversely proportional to the transverse size of the wave packet. Simulations also reveal that these waves can be trapped and guided along the ambient magnetic field by the field‐aligned irregularities (aka ducts) in the plasma density or the magnetic field. Specifically, the Gendrin‐mode waves can be efficiently trapped by the low‐density or high‐magnetic ducts with a very small (1%) difference in magnitudes of the parameters inside and outside the ducts. High‐density and low‐magnetic ducts do not trap these waves.

How does the limited resolution of space plasma analyzers affect the accuracy of space plasma measurements?

We investigate the systematic errors in measured plasma velocity distribution functions and their corresponding velocity moments, arising from the limited energy and angular resolution of top-hat electrostatic analyzers. For this purpose, we develop a forward model of a concept analyzer that simulates observations of typical solar wind proton plasma particles with their velocities following a Maxwell distribution function. We then review the standard conversion of the observations to physical parameters and evaluate the errors arising from the limited resolution of the modeled instrument. We show that the limited resolution of the instrument results in velocity distributions that underestimate the core and overestimate the tails of the actual Maxwellian plasma velocity distribution functions. As a consequence, the velocity moments of the observed plasma underestimate the proton density and overestimate the proton temperature. Moreover, we show that the examined errors become significant for cold and fast plasma protons. We finally determine a mathematical formula that predicts these systematic inaccuracies based on specific plasma inputs and instrument features. Our results inform and contextualize future evaluations of observations by analyzers in various plasma regimes.

Two-dimensional particle-in-cell simulation of magnetic reconnection in Space Plasma Environment Research Facility (SPERF)

Space Plasma Environment Research Facility (SPERF) is a new platform to study fundamental plasma processes in space environments using a controlled manner in laboratory. One scientific object of this device is to study Earth's magnetopause reconnection. During the experiment, a dipole coil is used to generate a magnetic field of the magnetosphere, and four sheath coils are used to generate the magnetic field in the solar wind. A reconnection site with upstream conditions similar to magnetopause reconnection is expected to be detected between the sheath coils and dipole coil. In this paper, we perform a two-dimensional particle-in-cell simulation with the boundary and driving conditions designed to model SPERF to study magnetic reconnection in SPERF. Magnetic reconnection is observed during both the growth and decay stages of the driving current in the sheath coils, which are referred to as “push” and “pull” reconnection stages. During the push stage, the plasma conditions in the two inflow regions are asymmetric, and this stage can be used to study magnetopause reconnection; during the pull stage, there are some asymmetric features between the two reconnection outflow regions, and we propose that this stage can be used to study magnetotail reconnection. Our simulation results show the capability for SPERF to produce magnetopause-like and magnetotail-like reconnection geometries.

Trivelpiece-Gould Mode Excitation in Magnetized Plasmas: A Review of Electron, Ion, Relativistic Beam, and Streaming Ion Effects

This review explores the excitation of Trivelpiece–Gould (TG) modes in bounded magnetized plasmas under different streaming particle beams, such as electron beams and ion beams, as well as relativistic electron streams. The TG mode, an electrostatic wave trapped in cylindrical geometries with an axial magnetic field, is strongly affected by beam–plasma interactions and boundary conditions. Evidence indicates that electron beams, especially those aligned along the magnetic field direction, are capable of driving TG mode instabilities effectively with growth rates highly dependent on beam velocity, density, and anisotropy of temperature. Contrarily, ion beams—more so in low-temperature dusty plasmas—may excite TG modes by resonance with the modified plasma eigen frequencies, as demonstrated in recent analytical and simulation works. The addition of relativistic electron beams results in increased coupling and more extended instability regimes from relativistic mass effects and longitudinal electric field interactions. The presence of dust grains also changes the dispersion and increases low-frequency modes, making the instability richer. These results have been validated by experimental and theoretical research which obtains dispersion relations, calculates growth rates, and investigates mode structures by employing Bessel function eigenmodes in radially confined plasmas. Overall, particle streaming excitation of TG modes presents new opportunities for manipulating plasma wave phenomena in laboratory and space plasmas.

Electron-Helium Elastic Scattering Cross Sections: A Theoretical Study using the R-matrix Method

In this work, we theoretically study electron-helium elastic scattering over a wide impact energy range. Using the R-matrix method, which we carried out with the Quantemol-N computer code, we compute different electron-helium elastic scattering cross sections. The theory behind the R-matrix method, as described by Tennyson (2010) [1] and Tennyson et al. (2007) [2], is presented. Our results are compared with those of existing experimental and theoretical values in the literature in order to determine how accurate and useful the R-matrix method is for electron-helium systems. Special care is given to possible overestimations in cross sections obtained from calculations, and the reasons behind it due to model approximations are investigated. This research advances the basic knowledge of electron-atom interactions, which are important for a range of applications in plasma physics, astrophysics, and radiation chemistry. Key Words: Scattering, Cross Section, R-Matrix

Comprehensive Laboratory Benchmark of K-shell Dielectronic Satellites of Fe xxv–xxi Ions

Abstract We report on comprehensive laboratory studies of the K-shell dielectronic recombination resonances of Fe xxv–xxi ions that prominently contribute to the hard X-ray spectrum of hot astrophysical plasmas. By scanning a monoenergetic electron beam to resonantly excite trapped Fe ions in an electron beam ion trap and achieving a high electron–ion collision energy resolution of ∼7 eV, we resolve their respective KLn satellites up to n ′ = 11 . By normalization to known radiative recombination cross sections, we also determine their excitation cross sections and that of the continuum with uncertainties below 15% and verify our results with an independent normalization based on previous measurements. Our experimental data excellently confirm the accuracy and suitability of distorted-wave calculations obtained with the Flexible Atomic Code for modeling astrophysical and fusion plasmas.

α Effect and Magnetic Diffusivity β in Helical Plasma Under Turbulence Growth

We investigate the transport coefficients α and β in plasma systems with varying Reynolds numbers while maintaining a unit magnetic Prandtl number (PrM). The α and β tensors parameterize the turbulent electromotive force (EMF) in terms of the large-scale magnetic field B¯ and current density as follows: ⟨u×b⟩=αB¯−β∇×B¯. In astrophysical plasmas, high fluid Reynolds numbers (Re) and magnetic Reynolds numbers (ReM) drive turbulence, where Re governs flow dynamics and ReM controls magnetic field evolution. The coefficients αsemi and βsemi are obtained from large-scale magnetic field data as estimates of the α and β tensors, while βtheo is derived from turbulent kinetic energy data. The reconstructed large-scale field B¯ agrees with simulations, confirming consistency among α, β, and B¯ in weakly nonlinear regimes. This highlights the need to incorporate magnetic effects under strong nonlinearity. To clarify α and β, we introduce a field structure model, identifying α as the electrodynamic induction effect and β as the fluid-like diffusion effect. The agreement between our method and direct simulations suggests that plasma turbulence and magnetic interactions can be analyzed using fundamental physical quantities. Moreover, αsemi and βsemi, which successfully reproduce the numerically obtained magnetic field, provide a benchmark for future theoretical studies.

Temporal Properties of Compressible Magnetohydrodynamic Turbulence

Abstract Describing the temporal properties of compressible magnetohydrodynamic (MHD) turbulence is a fundamental problem that has important implications for particle acceleration and transport in astrophysical plasmas. Here, by carefully analyzing the spatial and temporal properties of compressible MHD turbulence, we derive a new spectral power density function that is supported by simulations. This new function reveals that the low-frequency fluctuations are dominated by modes with small parallel wavenumbers with respect to the mean background magnetic field. Furthermore, for fluctuations with dynamically significant parallel wavenumbers, broadening around their eigenfrequencies is described by this function, which is in close agreement with simulations. We use this formalism to present the scaling properties of individual MHD modes. Such broadening is a direct consequence of nonlinear processes and is different for the three fundamental MHD modes. Our results provide a new window to investigate the temporal properties of turbulence and will enable further studies on the interaction between compressible MHD turbulence and energetic plasmas.

Laboratory Magnetoplasmas as Stellar-like Environment for 7Be β-Decay Investigations Within the PANDORA Project

Laboratory magnetoplasmas can become an intriguing experimental environment for fundamental studies relevant to nuclear astrophysics processes. Theoretical predictions indicate that the ionization state of isotopes within the plasma can significantly alter their lifetimes, potentially due to nuclear and atomic mechanisms such as bound-state β-decay. However, only limited experimental evidence on this phenomenon has been collected. PANDORA (Plasmas for Astrophysics, Nuclear Decay Observations, and Radiation for Archaeometry) is a novel facility which proposes to investigate nuclear decays in high-energy-density plasmas mimicking some properties of stellar nucleosynthesis sites (Big Bang Nucleosynthesis, s-process nucleosynthesis, role of CosmoChronometers, etc.). This paper focuses on the case of 7Be electron capture (EC) decay into 7Li, since its in-plasma decay rate has garnered considerable attention, particularly concerning the unresolved Cosmological Lithium Problem and solar neutrino physics. Numerical simulations were conducted to assess the feasibility of this possible lifetime measurement in the plasma of PANDORA. Both the ionization and atomic excitation of the 7Be isotopes in a He buffer Electron Cyclotron Resonance (ECR) plasma within PANDORA were explored via numerical modelling in a kind of “virtual experiment” providing the expected in-plasma EC decay rate. Since the decay of 7Be provides γ-rays at 477.6 keV from the 7Li excited state, Monte-Carlo GEANT4 simulations were performed to determine the γ-detection efficiency by the HPGe detectors array of the PANDORA setup. Finally, the sensitivity of the measurement was evaluated through a virtual experimental run, starting from the simulated plasma-dependent γ-rate maps. These results indicate that laboratory ECR plasmas in compact traps provide suitable environments for β-decay studies of 7Be, with the estimated duration of experimental runs required to reach 3σ significance level being few hours, which prospectively makes PANDORA a powerful tool to investigate the decay rate under different thermodynamic conditions and related charge state distributions.

Characteristics of dusty plasma sheath over a spacecraft surface near Jupiter’s ring with oblique magnetic field and non-extensive electrons

Abstract Space plasma significantly affects spacecraft charging, and accurate sheath modeling is essential for predicting surface potential and mitigating related issues. This study investigates the characteristics of a dusty plasma sheath near a spacecraft surface in an oblique magnetic field using a 1D3V fluid model, where electrons follow a non-extensive distribution. A modified orbital-motion-limited model is used to determine the dust charge. The dust surface potential shows distinct behavior at the sheath edge and in the bulk region. By employing the Sagdeev potential method, a generalized Bohm criterion incorporating dust charging is derived. Results show that the effective Bohm velocity decreases with increasing dust radius, reflecting the strong influence of dust on sheath structure. The results reveal that as the dust particle radius decreases, the sheath floating potential increases, intensifying the surface charging effects of spacecraft. A stronger oblique magnetic field increases the positive charging of dust particles, thereby modifying the sheath structure. A key finding is that dust particles near the floating wall carry significantly lower charges than previously predicted, due to the coupling between the dust charging process and the floating potential condition. Additionally, a sub-extensive electron distribution leads to a higher wall potential than in the super-extensive case, further amplifying spacecraft charging effects.

Exploring Numerical Correlations: Models and Thermodynamic Kappa

McComas et al. (2025) introduced a numerical experiment, where ordinary uncorrelated collisions between collision pairs are followed by other, controlled (correlated) collisions, shedding light on the emergence of kappa distributions through particle correlations in space plasmas. We extend this experiment by introducing correlations indicating that (i) when long-range correlations are interwoven with collision pairs, the resulting thermodynamic kappa are described as that corresponding to an ‘interatomic’ potential interaction among particles; (ii) searching for a closer description of heliospheric plasmas, we found that pairwise short-range correlations are sufficient to lead to appropriate values of thermodynamic kappa, especially when forming correlated clusters; (iii) multi-particle correlations do not lead to physical stationary states; finally, (iv) an optimal model arises when combining all previous findings. In an excellent match with space plasmas observations, the thermodynamic kappa that describes the stationary state at which the system is stabilized behaves as follows: (a) When correlations are turned off, kappa is turning toward infinity, indicating the state of classical thermal equilibrium (Maxwell-Boltzmann distribution), (b) When collisions are turned off, kappa is turning toward the anti-equilibrium state, the furthest state from the classical thermal equilibrium (−5 power-law phase-space distribution), and (c) the finite kappa values are generally determined by the competing factor of collisions and correlations.

Solar coronal heating: role of kinetic and inertial Alfvén waves in heating and charged particle acceleration

ABSTRACT A comprehensive understanding of solar coronal heating and charged particle acceleration remains one of the most critical challenges in space and astrophysical plasma physics. In this study, we explore the contribution of Alfvén waves – both in their kinetic (KAWs) and inertial (IAWs) regimes – to particle acceleration and solar coronal heating. Employing a kinetic plasma framework in the generalized Vlasov–Maxwell model, we analyse the dynamics of these waves with a focus on the perpendicular (i.e. across the magnetic field lines) Poynting flux vectors and the net resonant speed of the particles. We found the Poynting flux of KAWs decays rapidly, indicating short-range energy transport, while IAWs exhibit slower decay, enabling energy transfer over larger distances (R$_{\text{Sun}}$) in the solar corona. We also evaluate the electric potentials associated with KAWs and IAWs and find the KAW’s potentials are significantly enhanced at larger wavenumbers ($k_x \rho _i>0.1$) regimes, while IAWs exhibit reduced parallel and enhanced perpendicular electric potentials, governed by the perturbed electric fields (${E_x}$ and ${E_z}$) values. Additionally, we determine the net resonant speed of particles in the perpendicular direction and demonstrate that these wave–particle interactions can efficiently heat the solar corona over extended distances R$_{\text{Sun}}$. Finally, we quantify the power transported by KAWs and IAWs through solar flux loop tubes, finding that both wave types deliver greater energy with increasing ${T_e/T_i}$ and $c k_x/\omega _{pe}$ values. These insights not only deepen our theoretical understanding of wave-driven heating mechanisms but also provide valuable implications for interpreting solar wind, corona, heliospheric, and magnetospheric dynamics.

First Observation of Mini Harmonic Structure in Magnetosonic Waves

Abstract We recently reported the finding of elementary rising‐tone emissions embedded within each harmonic of magnetosonic waves, by investigating wave electric field waveforms measured by Van Allen Probes. The present study further uncovers a new set of fine structures of magnetosonic waves, namely, each elementary rising‐tone may consist of a series of mini harmonics spaced around the O+ gyrofrequency. The measured ion distributions suggest that the proton ring distribution provides free energy to excite the waves, whilst the O+ ions suppress the wave growth around multiples of O+ gyrofrequency, resulting in the formation of mini harmonics. Further investigation suggests that the warm plasma dispersion relation, that is, the ion Bernstein mode instabilities, may contribute to the formation of mini harmonics. The mini harmonic structure implies a new mechanism of energy redistribution among ion species in space plasmas, potentially providing a new acceleration mechanism for O+ ions in the magnetosphere.

Chorus: Optimizing Synchrotron Transfer Coefficients with Weighted Sums

Abstract Accurate synchrotron transfer coefficients are essential for modeling radiation processes in astrophysics. However, their current calculation methods face significant challenges. Analytical approximations of the synchrotron emissivity, absorptivity, and rotativity are limited to a few simple electron distribution functions that inadequately capture the complexity of cosmic plasmas. Numerical integrations of the transfer coefficients, on the other hand, are accurate but computationally prohibitive for large-scale simulations. In this paper, we present a new numerical method, Chorus, which evaluates the transfer coefficients by expressing any electron distribution function as a weighted sum of functions with known analytical formulas. Specifically, the Maxwell-Jüttner distribution function is employed as the basic component in the weighted sum. The Chorus leverages the additivity of transfer coefficients, drawing inspiration from an analogous approach that uses stochastic averaging to approximate the κ distribution function. The key findings demonstrate median errors below $5{{\ \rm per\ cent}}$ for emissivity and absorptivity, with run times reduced from hours to milliseconds compared to first-principles numerical integrations. Validation against a single κ distribution, as well as its extension to more complicated distributions, confirms the robustness and versatility of the method. However, limitations are found, including increased errors at higher energies due to numerical precision constraints and challenges with rotativity calculations arising from fit function inaccuracies. Addressing these issues could further enhance the method’s reliability. Our method has the potential to provide a powerful tool for radiative transfer simulations, where synchrotron emission is the main radiative process.

Thermally Induced Electromagnetic Fluctuations by Protons and Alpha Particles in the Solar Wind

Abstract We present an initial study on the contribution of alpha particles, alongside electrons and protons, to the generation of thermally induced electromagnetic fluctuations (TIEFs) in space plasmas, such as the solar wind. The inclusion of alpha particles is anticipated to influence the regulation of the quasi-stable region in the beta-anisotropy diagram for both ion species, as well as the amplitude of the fluctuations. For this analysis, we filtered 30 yr of solar wind data at 1 au, focusing on conditions close to isotropy for protons and alpha particles, given that the observational distribution peaks near this state. The filtered data are displayed and organized in the (β ∥p , β ∥α ) plane, where β ∥ denotes the ratio of parallel plasma pressure to magnetic pressure for protons (p) and alpha particles (α). In this plane, we analyze the ratio of alpha particle to proton density, n α /n p , the squared perpendicular magnetic fluctuations, and compressibility. Furthermore, we compare these magnetic fluctuations with the total fluctuating magnetic energy density derived from a generalized fluctuation–dissipation theorem, highlighting a clear similarity. We also present Fourier spectra for a subset of the data and the corresponding theoretical curve predicted by TIEFs. This study suggests that TIEFs may play a significant role in explaining the unexpected fluctuations observed in space plasmas. Additionally, it provides preliminary evidence that the inclusion of additional ion species could serve as a quantitative test for the theoretical models proposed to describe these fluctuations.

Ponderomotive electron physics captured in a single-fluid extended magnetohydrodynamics model

The ponderomotive force, arising from the interaction between electromagnetic waves and plasma, plays a critical role in laser fusion, astrophysical plasmas, and laser diagnostics. Traditionally, modeling this force requires multi-fluid or particle-in-cell simulations due to its strong coupling to electron-scale dynamics. In this work, we demonstrate that a one-fluid, two-temperature extended magnetohydrodynamics (XMHD) model—augmented with a generalized Ohm's law (GOL) including electron inertia—can accurately reproduce key ponderomotive effects. We derive the ponderomotive force within this framework using a phasor-based approach and then validate its nonlinear manifestations through direct numerical simulations in the PERSEUS code, where steepening and density modulation phenomena typically associated with kinetic-scale models are reproduced. These results and prior work establish XMHD as a robust and efficient alternative for modeling nonlinear laser–plasma dynamics, bridging the gap between ideal MHD and fully kinetic approaches.

Characterization of Anodized Aluminum Alloy Al6061-T6 Under Simulated Leo Plasma Conditions

ABSTRACT The space plasma has relatively low energy but is dense in the low Earth orbit (LEO). In this study, we prepared various samples of anodic alloy surfaces with coating thicknesses of 20, 25, 35, and 45 μm to identify the most suitable characteristics for space applications. Ground-based tests were conducted at the Laboratory of Lean Satellite Enterprises and In-Orbit Experiments at the Kyushu Institute of Technology. A radio frequency (RF) plasma source was used to generate a simulated LEO plasma using argon gas in a vacuum chamber. The plasma properties were measured with a Langmuir probe under different test conditions. A negatively biased voltage of –450 V was applied to the samples to study charging/discharging phenomena. The samples were exposed to Ar-plasma for 1–2 h. The physical properties and structural morphology of various alloy samples were analyzed before and after exposure to plasma. This analysis involved ultraviolet (UV)/visible (Vis)/ Near-Infrared (NIR) absorption spectra, Energy Dispersive X-ray (EDX) analysis, and surface roughness testing. The results showed that space plasma notably impacts the physical properties and morphology of the alloys. A coated thickness of around 25 μm is considered more suitable for spacecraft surface structures due to its improved optical stability and resistance to plasma degradation, as indicated by the experimental results.

Bipolar Reversal in the Off‐Diagonal Ion Pressure Term in Collisionless Magnetic Reconnection

Abstract Magnetic reconnection is a fundamental process that converts magnetic energy into particle energy, with wide applications in space plasmas. A key manifestation of this energy conversion is the acceleration of fast ion outflows. However, ion processes and their associated signatures in energy conversion remains only partially understood in collisionless magnetic reconnection. In this study, we utilize statistical analyses of in‐situ observations and particle‐in‐cell (PIC) simulations to identify a distinct signature in the off‐diagonal component of the ion pressure tensor. This signature displays a bipolar reversal that correlates with ion outflows across the reconnection X‐line. The bipolar signal originates from distorted ion velocity distributions during acceleration. These signals are confirmed by statistical in‐situ evidence for the first time and captured by PIC simulations. PIC simulations further indicate the peak of the off‐diagonal ion pressure is near the magnetic pileup region associated with the ion outflow. Trajectories of ions in the distorted velocity distributions are traced within the PIC simulations. Ions in the distorted distributions undergo partial cyclotron motion around the reconnected magnetic field (Bz) and acceleration by the reconnection electric fields. The observation of bipolar reversal in the off‐diagonal ion pressure term indicate its spatial gradient, which could contribute to supporting ion‐scale reconnection electric fields. These findings provide new insights into the fundamental features of energy conversion in collisionless magnetic reconnection.