We investigate the collisionless kinetic structure of the upper solar atmosphere in the presence of an expanding magnetic field. We consider a stationary two-component plasma confined within an expanding magnetic flux tube and subject to gravity, self-electrostatic interactions, the Pannekoek-Rosseland electric field, and magnetic moment conservation. Starting from the Vlasov equation, we derive fully analytical expressions for the particle distribution functions, density profiles, and the parallel, perpendicular, and total temperature profiles. We show that the combined conservation of energy and magnetic moment generates a loss-cone distribution, reducing the density with respect to the corresponding unmagnetized atmosphere and producing a pronounced temperature anisotropy. For a single-temperature boundary condition, the competition between magnetic moment conservation and gravity causes the parallel temperature to develop a maximum. We derive analytical scaling laws for its location and amplitude and validate them against numerical calculations. We further show that the anisotropy persists independently of the temperature distribution at the lower boundary. In the regime of rare but intense heating events, gravitational filtering enhances the contribution of the hottest particle populations at coronal heights, while magnetic moment conservation further amplifies the resulting velocity-space anisotropy. This work provides a fully analytical kinetic description of the combined effects of gravitational filtering and magnetic moment conservation in an expanding coronal magnetic flux tube undergoing stochastic heating at its base. These results establish a theoretical framework for investigating the role of magnetic-field expansion in shaping the density and temperature structure of weakly collisional stellar coronae.

The genesis of solar wind remains elusive due to limited multi-instrument observations of its source regions. Here, we introduce a novel “see and touch” technique, integrating remote-sensing observations with in situ measurements from Parker Solar Probe (PSP). This approach allows us to obtain 3D trajectories of flow structures such as streamer blobs and explore their in situ properties. With this approach, we link blobs observed by remote sensing and high-density jets (HDJs) measured in situ. The blobs are embedded in streamer rays, while the HDJs are found when PSP crosses the heliospheric current sheet (HCS). Our findings suggest that large-scale blobs/HDJs originate from primary reconnection in the near-Sun HCS, while secondary reconnection in smaller-scale current sheets forms multiple flux ropes, which merge to trigger further small-scale reconnection. Detailed in situ analysis reveals that turbulent magnetic reconnection is a key mechanism for dissipating filamentary HCS and energizing plasmas in blobs/HDJs. The multiscale magnetic reconnection accelerates the proton core population and mixes it with the beam population, driving bulk acceleration and heating of the nascent slow solar wind.

The solar wind cools more slowly than adiabatic expansion predicts, implying the presence of an additional heat source. A key unknown is the physical carriers that transport energy into the solar wind. Here we track the same fast solar wind stream from its source to Earth’s orbit. Using a rare quasi-radial alignment of Solar Orbiter near 0.38 au and wind near 1 au, together with remote sensing from Chinese Hα Solar Explorer and Solar Dynamics Observatory, we show that intermittent velocity spikes are the dominant energy carriers that drive the solar wind’s non-adiabatic evolution. These spikes dissipate during propagation, reducing the cooling rate and increasing plasma entropy. The resulting temperature and entropy at 1 au are in good agreement with predictions from Alfvénic turbulence theory. The solar wind source region shows abundant magnetic reconnection activity, probably providing the initial fluctuations that seed the formation of velocity spikes. Our results provide observational constraints on how intermittent velocity spikes and Alfvénic turbulence shape the thermodynamic evolution of collisionless plasmas in the heliosphere. By tracking the same fast solar wind stream from Solar Orbiter to Earth, this study shows that intermittent velocity spikes transport energy and contribute to entropy increase, helping explain why the solar wind cools more slowly than adiabatic expansion predicts.

The coronal magnetic field plays a fundamental role in governing coronal activities, driving space-weather events, and shaping the heliosphere. Due to a lack of direct observations, extrapolation models such as the Potential Field Source Surface (PFSS) model become the primary method to obtain the three-dimensional magnetic field distribution in the corona. However, the PFSS model cannot solve the long-standing open-flux problem, in which the extrapolated open magnetic flux is significantly lower than that inferred from in-situ measurements. To address this issue, we develop a Non-Spherical Potential Field (NSPF) model. The model introduces a Non-Spherical Source Surface (NSSS) defined as an isosurface of the total magnetic field. The NSSS naturally forms concave structures beneath external current sheets, enabling the model to generate substantially more open magnetic flux while yielding a physically plausible distribution of open field regions. As a result, the NSPF model successfully reproduces complex coronal magnetic topologies, interplanetary magnetic field properties, and solar wind source mappings. Our refined coronal magnetic model provides a useful framework for future research on solar and heliospheric magnetic coupling.

Space plasmas are weakly collisional since characteristic timescales related to Coulomb collisions are much larger than those of Larmor gyration or wave-particle interactions. Thus, wave activity is likely to drive some of the nonthermal features that are observed in space plasma velocity distributions, such as temperature anisotropy, beams, and skewness. Therefore, we study how wave-particle interactions shape the velocity distribution functions of minor ions, and how these ions and their statistical properties modify the dispersion relation of electromagnetic waves. To achieve this, we derive the motion of heavy ions in electromagnetic waves using the Boris algorithm. We take the waves to be solutions of the fully kinetic dispersion relation of electromagnetic waves in two-ion component plasmas with parameters representative of solar wind. We use the Arbitrary Linear Plasma Solver code to derive the linear Vlasov-Maxwell dispersion relation based on the actual distribution of the ions. The test particles are initially in thermal equilibrium, and their distribution evolves due to interactions with the waves. By solving the dispersion relation using the evolved distributions, we show that the system evolves into a steady wave-particle equilibrium, which is characterized by a minimization of the interaction and energy transfer between waves and particles.

We present a framework for decomposing solar-wind velocity distribution functions (VDFs) using orthogonal polynomial bases. We aim to establish a practical procedure for applying polynomial decompositions to in-situ spacecraft VDFs and to clarify how the resulting spectra of expansion-coefficient power can be used for noise reduction, VDF reconstruction, and diagnostics of velocity-space structure. The method represents measured VDF structure with Hermite-Hermite and Hermite-Laguerre expansions, providing a nonparametric description of departures from Maxwellians, such as anisotropy, skewness, beams, and suprathermal tails. Expansion coefficients are estimated by Gaussian-weighted quadrature after interpolation of measured distributions onto polynomial nodes. We demonstrate several applications of polynomial decomposition to Solar Orbiter, Parker Solar Probe, and Magnetospheric Multiscale 1 measurements, including noise identification through high-order spectral flattening, noise-reduced VDF reconstruction, and characterization of VDF-structure variations under different plasma conditions, e.g., turbulent solar-wind streams and shocks. For instance, noise-reduced reconstructed VDFs can provide smoother estimates of distinct ion populations and VDF gradients. Examples from solar-wind streams and collisionless-shock crossings further show that the resulting spectra respond to changes in parallel and perpendicular VDF structure, illustrating their potential for comparing kinetic modifications under different plasma conditions. Overall, orthogonal-polynomial decomposition provides a bridge between measured particle distributions and kinetic plasma physics by converting complex VDF morphology into quantitative velocity-space spectra.

Cosmic magnetic fields are typically inhomogeneous and often highly tangled due to large-scale plasma flows, turbulence, and instabilities. If the variations in the magnetic field occur on scales that are large compared to the gyro-radius of the plasma electrons, the electrons are primarily confined to gyro-centre trajectories along the field lines. Therefore, in-situ electron measurements help us map out the connectivity of the magnetic field in space plasmas. Gyro-centre drifts, wave-particle interactions, trapping, and cross-field diffusion are processes related to field inhomogeneities and fluctuations; they have the potential to modify or even disrupt the transport of electrons along field lines. We introduce the basic principles of electron transport in tangled magnetic fields and review the creation of tangled fields through turbulence and instabilities as well as the modulation of parallel electron transport through kinetic instabilities. We then describe trapping and de-trapping effects in inhomogeneous magnetic fields, as well as electron diffusion and energisation across the magnetic field. The transport of electrons in tangled fields results from a complex interplay of plasma processes that occur on a broad range of scales. A combination of in-situ plasma measurements, remote-sensing plasma observations, and plasma theory and simulations is required to resolve this contemporary challenge to the fields of heliophysics and astrophysics.

We investigate stationary states of a collisionless, gravitationally stratified plasma atmosphere composed of electrons, protons, and alpha particles by extending Pannekoek–Rosseland theory to multispecies and multi-temperature plasmas. Starting from Liouville's theorem, we derive the self-consistent ambipolar electric field from kinetic equilibrium and charge neutrality. For a single-temperature atmosphere, we obtain analytical expressions for the ambipolar field, show its dependence on alpha-particle abundance, and determine the relative stratification of the three species. A first-order analytical approximation to the electrostatic potential accurately reproduces the numerical solution. We then generalize the formalism to multi-temperature plasmas generated by stochastic boundary heating, representing the stationary distribution as a superposition of Maxwellian populations. Gravitational filtering produces non-exponential density profiles and increasing temperatures with altitude, while preserving the relative species stratification, with alpha particles most strongly stratified and protons least. The ambipolar electric field contains a dominant gravitational contribution, corresponding to the generalized Pannekoek–Rosseland field, and a thermoelectric contribution arising from species-dependent temperature gradients, which accounts for its non-monotonic structure. These results provide a framework for studying the combined effects of plasma composition and stochastic heating in gravitationally stratified astrophysical plasmas.

The coronal magnetic field plays a fundamental role in governing coronal activity, driving space weather events, and shaping the heliosphere. Due to a lack of direct observations, extrapolation models such as the potential field source surface (PFSS) model become the primary method to obtain the three-dimensional magnetic field distribution in the corona. However, the PFSS model cannot solve the long-standing open-flux problem, in which the extrapolated open magnetic flux is significantly lower than that inferred from in situ measurements. To address this issue, we develop a nonspherical potential field (NSPF) model. The model introduces a nonspherical source surface (NSSS) defined as an isosurface of the total magnetic field. The NSSS naturally forms concave structures beneath external current sheets, enabling the model to generate substantially more open magnetic flux while yielding a physically plausible distribution of open field regions. As a result, the NSPF model successfully reproduces complex coronal magnetic topologies, interplanetary magnetic field properties, and solar wind source mappings. Our refined coronal magnetic model provides a useful framework for future research on solar and heliospheric magnetic coupling.

Quantification of energy transport and dissipation in weakly collisional heliospheric plasmas that are far from local thermodynamic equilibrium is an outstanding scientific problem. A central challenge is determining how non-Maxwellian velocity-space structure affects damping and emission of coherent ion-scale waves, especially compared to simplified analytical models for background plasma velocity distributions. In this work, we study the damping and emission of parallel-propagating proton cyclotron waves for two models of proton velocity distributions measured by the SPAN-I instrument on board Parker Solar Probe during an extended storm of waves with left-hand polarization in the solar wind at a heliocentric distance of 30.1 solar radii. Using the measured velocity distribution rather than a two-component bi-Maxwellian model predicts instabilities consistent with the observed coherent waves. For intervals in which both models predict net damping, the observed VDF model yields weaker damping in 90% of cases, with a reduction in the integrated heating rate of 0.44 relative to the bi-Maxwellian model. These results suggest that simplified analytical velocity distribution models may overestimate cyclotron damping and underestimate wave emission in the near-Sun solar wind.

Observations of solar-wind velocity distribution functions (VDFs) commonly reveal fine-scale structures. These features strongly influence kinetic processes such as wave damping and instability, yet their role remains poorly understood. We use a Gaussian Mixture Model to separate proton and alpha-particle (fully ionized helium) VDFs from Solar Orbiter Proton and Alpha-particle Sensor (PAS) measurements, and assess with the Arbitrary Linear Plasma Solver (ALPS) how measured VDFs affect the damping of compressive fluctuations. We analyze the dispersion relation and polarization properties of ion-acoustic (IA) waves in the solar wind. Protons and alpha-particles are represented by the measured VDFs derived from PAS observations. For comparison, we also perform calculations using the bi-Maxwellian assumption for the VDFs. Fine-scale structures of the measured proton VDFs reduce the damping rate of IA waves, even when Te similar or equal to Tp. In some cases, we find that the measured VDFs drive the IA mode unstable, while the corresponding bi-Maxwellian representations predict strong damping. These results demonstrate that resolving the fine-scale structures of VDFs is essential for accurately capturing the kinetic physics of the solar wind.

Space plasmas like the solar wind or the Earth's space environment offer unique opportunities to observe fundamental plasma processes and their impact in situ. With modern space instrumentation, we measure the velocity distribution function of the plasma particles as well as the electromagnetic fields at high resolution and with minimal perturbation of the observed plasma systems. Plasma measurements like this are often not possible in laboratory settings on Earth. This review article focuses on modern diagnostic methods for the in-situ detection of plasma particles in space. It presents the detection principle of top-hat electrostatic analysers and highlights recent examples of scientific discoveries based on data from the heliospheric space missions Parker Solar Probe and Solar Orbiter. These examples demonstrate the capabilities of modern space plasma instrumentation. The article then discusses future directions in space plasma physics as well as the involved challenges in terms of the required plasma diagnostics. These new developments include, for example, upcoming and proposed space missions such as the operational space-weather mission Vigil, the multi-spacecraft mission HelioSwarm, the Mars mission M-MATISSE, and the electron-astrophysics mission Debye.

Ion-driven plasma instability thresholds, derived from linear theory, constrain the distribution of solar observations in parameter space, defining boundaries of stable plasma parameters. Excursions beyond these thresholds result in the emission of energy, transferred from particles to coherent electromagnetic waves, acting to adjust the system toward a more stable configuration. In this Letter, we use linear Vlasov-Maxwell theory to define parametric limits for a low-β plasma that contains a drifting proton beam or helium (α-particle) population. A sufficiently fast and dense drifting population triggers an oblique drift instability (ODI). This instability decreases the velocity drift between the thermal core proton and secondary populations and prevents the ratio of core thermal to magnetic pressure β_{c} from decreasing below a minimum value by increasing the temperatures-i.e., heating-of both the core and drifting populations. Our theoretical results are of interest for Parker Solar Probe observations, as they provide an additional mechanism for perpendicular heating of ions active in the sub-Alfvénic solar wind. The ODI may explain the discrepancy between long-standing expectations of measurements of very low-β plasmas with very large ion temperature anisotropies in the near-Sun environment and in situ observations, where β is consistently measured above a few percentages and the secondary ion populations drift faster than the bulk of the proton population by no more than approximately the local Alfvén speed.

Interplanetary coronal mass ejections (ICMEs) are large-scale magnetic structures that influence heliospheric dynamics and space weather. While wave activity has been observed within their low- β (≪1) magnetic obstacles, the role of temperature anisotropy and instability remains underexplored. This study examines proton temperature anisotropies, heating, cooling, turbulence, and collisional effects within ICME magnetic obstacles, which are low- β plasmas. Using Wind spacecraft data from 382 ICME magnetic obstacles at 1 au (spanning 1995–2021), we observe that proton temperature and proton β p follow log-normal distributions. The anisotropy within these regions is primarily constrained to the stable parameter regime below the thresholds for the mirror-mode and oblique firehose instabilities. Additionally, plasmas unstable to proton cyclotron and firehose instabilities exhibit temperatures that are significantly higher – 50 to 100 times higher than those of stable plasma. Notably, enhanced magnetic fluctuations and low collisional age are observed near instability thresholds, regardless of beta. Although a clear relationship exists between temperature and collisional age, the correlation between turbulent amplitude and collisional age is weak, differing from trends observed in the solar wind. Our results suggest a causal chain whereby high turbulence amplitudes are associated with enhanced heating, linked to reduced collisions, causing increased temperature anisotropy, and ultimately favouring the development of instabilities within ICME magnetic obstacles.

Parameters of solar wind velocity distributions are well constrained by thresholds of ion-driven plasma instabilities derived from linear theory. Surpassing these thresholds results in the transfer of energy from particles to coherent electromagnetic waves as the system is altered toward a more stable configuration. We use linear Vlasov-Maxwell theory to describe an Oblique Drift Instability (ODI) that constrains the limits of stable parametric space for a low-beta plasma that contains a drifting proton beam or helium population. This instability decreases the relative drift of secondary populations and prevents beta from decreasing below a minimum value by heating both the core and drifting populations. Our predictions are of interest for Parker Solar Probe (PSP) observations, as they provide an additional mechanism for perpendicular heating of ions active in the vicinity of Alfven surface. The ODI may explain the discrepancy between long-standing expectations of measurements of very low-beta plasmas in the near-Sun environment and in situ observations, where beta is consistently measured above 1%. In parallel, it proposes an interpretation why the drift of the secondary ion populations with respect to the bulk of thermal protons is reduced to no more than approximately the local Alfven speed, as observed in earlier PSP encounters.

Solar wind alpha particles exhibit preferential heating and acceleration relative to protons; however, their behavior in the vicinity of turbulent coherent structures remains less understood. We report the first evidence of localized alpha particle and proton heating within coherent structures identified using the Partial Variance of Increments (PVI) method, based on Parker Solar Probe (PSP) observations. Our results show that high-PVI events are associated with significant, species-dependent temperature enhancements: protons undergo a relative larger temperature increase than alpha particles. This preferential proton heating produces a localized decrease in the alpha-to-proton temperature ratio, indicating that the plasma is driven toward thermal equilibration between species. The heating is also anisotropic, being dominated by enhancements in the perpendicular temperature. These temperature-signatures coincide with a pronounced reduction in the normalized alpha-proton differential flow speed and a localized minimum in the Coulomb collision age, suggesting that the relaxation is affected primarily by collisionless kinetic effects. These findings provide new insight into the intermittent energy conversion and ion thermodynamics in the solar wind.

Ion cyclotron waves (ICW) and fast magnetosonic/whistler waves (FMW) are fundamental electromagnetic modes at ion kinetic scales, yet their generation mechanisms and roles in plasma evolution remain poorly understood. We analyze a 2.5D hybrid simulation of broadband Alfvénic fluctuations, where the proton velocity distribution is modeled as a sum of two bi-Maxwellian components: a thermal core and a drifting beam. Using wavelet-based wave identification, bi-Maxwellian VDF fitting, and the PLUME linear dispersion solver, we find that ICW behave as linear modes. Growth is intermittent, occurring when core temperature anisotropy builds up, and is driven mainly by the core (the beam contributes negligibly). Poynting flux analysis shows that ICW are predominantly forward-propagating, with a net energy flux ratio of +1 across all frequencies, consistent with the initial condition. FMW present a stark contrast: PLUME solutions often yield very small (near-zero) linear growth/damping rates. The species decomposition breaks down when |γ/ω_r| ≳ 0.368, indicating that linear theory predicts these waves to be strongly damped and not describable by linear eigenmodes. Nevertheless, FMW are clearly observed in the wavelet helicity spectrogram, indicating that they are generated by nonlinear processes (e.g., parametric decay or phase steepening) and persist despite linear damping. The net energy flux ratio for FMW is close to +1 at low frequencies but decreases at higher frequencies, yet never reaches zero (net energy flow remains forward). These results demonstrate that ICW are linear, core-driven waves that transfer energy to the plasma, while FMW are heavily damped, nonlinearly generated waves.

Whether solar wind electrons expanding into the heliosphere can preserve information about their origin in the solar corona remains an open debate. The suprathermal strahl temperature has often been postulated as an indicator of source coronal electron temperature, while the core electron temperature has also been found to correlate with the solar wind velocity in the inner heliosphere. Here we investigate how well solar wind electron populations retain imprints of coronal electron temperature. Using Solar Orbiter measurements at similar to 0.5 au, we fit three components (i.e., the core, halo, and strahl) of the electron velocity distribution function and compare the resulting strahl and core temperatures with heavy-ion charge-state ratios, which serve as proxies for the coronal electron temperature. We present the first clear evidence from inner heliosphere observations that, in several individual streams, a proxy for the strahl parallel temperature, Tstrahl,parallel to, correlates significantly and positively with the charge-state ratios O7+/O6+ and C6+/C5+. However, this correlation is not universally present, implying that many electron streams are significantly affected by transport processes, such as scattering, that erase the signature. We find that, notably, the core perpendicular temperature ( Tcore,perpendicular to ) also strongly correlates with the charge-state ratios. We interpret this result within the framework of the exospheric solar wind model. Our results suggest that both thermal and suprathermal electrons can at times retain coronal information, but that aggregating multiple streams can obscure the underlying relationships.

Magnetic holes (MHs) are structures commonly observed in various space plasma environments throughout the solar system, including the solar wind. These structures are characterized by a localized decrease in magnetic field strength, coincident with an increase in plasma density. Previous observational studies in the solar wind link the presence of Langmuir waves to MHs, suggesting a strong correlation between these phenomena. We develop a model based on magnetic-moment conservation and its violation to explain the excitation of Langmuir waves in MHs. Our model illustrates that MHs induce changes in the electron velocity distribution function that emit electrostatic Langmuir waves due to the bump-on-tail instability. Using data from the Solar Orbiter spacecraft, we provide a comprehensive analysis of this process and test our predictions with observations. The consistency between our model and observations indicates that the proposed process is a viable mechanism for producing Langmuir waves in MHs in the solar wind.

The Earth's magnetosheath plasma frequently exhibits unequal temperatures parallel and perpendicular to the background magnetic field. This temperature anisotropy is crucial in exciting and dissipating electromagnetic fluctuations in various plasma environments. Leveraging a comprehensive dataset comprising sub‐ion scale measurements from NASA's Magnetospheric Multiscale Mission, our study unveils insights into the spatial evolution and parametric dependence of different ion kinetic instabilities within the Earth's magnetosheath. We observe a remarkable spatial dependence on the occurrence of kinetic instabilities. The condition for the mirror‐mode instability exhibits a significantly higher prevalence near the subsolar magnetopause compared to the flank regions. The condition for the oblique firehose instability is more prominent near the bow shock region than in the vicinity of the magnetopause. Furthermore, the occurrence of these instabilities and their spatial distribution are linked to the plasma's upstream conditions. Our study offers new insights into the nature of ion kinetic instabilities in the Earth's magnetosheath and similar plasma environments.