arXiv subjects

Kristopher G. Klein

Publications and source records attributed to Kristopher G. Klein.

At least 19 recordsLinked to original sources

arXiv2026

Direct Measurement of Polar Coronal Hole-like Solar Wind in its Acceleration Phase

The early evolution of fast polar coronal hole (PCH) solar wind remains largely unconstrained by in situ measurements. In March 2025, Parker Solar Probe (Parker) at its closest approach of 9.86 Solar Radii ($R_\odot$) measured outflow from a large equatorial coronal hole (ECH) which was also measured at 1\,au and at intermediate distances by Solar Orbiter (also near its perihelion). At 1\,au the stream properties are consistent with PCH properties established by Ulysses. The stream was measured by Parker substantially below the Alfv\'en surface, with proton temperatures in excess of 2\,MK and a speed at $\sim$10\,$R_\odot$ which was only $\sim$60\% of its asymptotic value. The Solar Orbiter data indicates that the acceleration is largely complete by 60~$R_{\odot}$. Spherically-polarized fluctuations in the stream are observed to develop from near-transverse and small-angle at Parker to full reversal ``switchbacks'' at Solar Orbiter. Comparison of the implied acceleration profile to historical doppler-dimming measurements suggests that the stream's low coronal acceleration is similar to that of PCH flows. Consistent with previous work, this acceleration requires significantly more energy than can be provided by the observed thermal pressure gradients, with a significant contribution likely from the abundant Alfv\'enic fluctuation energy observed at Parker. These observations provide unique constraints on models of the radial evolution of the fastest solar wind, and indicate that these wind streams experience gradual, steady acceleration over their first few tens of solar radii of evolution.

astro-ph.SR↗

arXiv2026

Physics of Circular Polarized Ion-Scale Waves in Hybrid Simulations of Alfv\'enic Fluctuations

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 Alfv\'{e}nic 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 $|\gamma/\omega_r| \gtrsim 0.368$, 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.

physics.plasm-ph↗

arXiv2026

Direct Measurement of Diffusion Coefficients: Evidence for Diffusive Stochastic Heating in Collisionless Plasmas

Open questions in collisionless plasma dissipation can be addressed using space-based observations in different astrophysical environments, with implications for both astrophysical and laboratory plasma systems. We study a low-$\beta$, highly imbalanced, sub-Alfv\'enic stream observed by Parker Solar Probe (PSP) to identify and distinguish between signatures of stochastic heating (SH) and resonant heating (RH) by parallel ion cyclotron waves (ICWs). Prior work studying this stream (Bowen et al., 2025) showed that the SH rate, accounting for intermittency, matched the amplitude of the local energy transfer (LET) rate while the RH rate did not. This comparison relied on a number of assumptions regarding the nature of the diffusive process, and the calculation of the LET rate. We introduce a novel technique of inverting the proton guiding center equation to empirically measure velocity-space diffusion coefficients using three-dimensional proton velocity distribution functions (VDFs), from the ion electrostatic analyzer (SPANi) on PSP. Measured diffusion coefficients are used to determine phase-space heating rates, leading to a calculation of a fully kinetic heating rate independent of assumptions made in prior work. We show that scale-dependent analytic expressions for SH via non-coherent fluctuations match the empirical measurements from PSP data, provided that we account for intermittency in the heating calculation. In contrast, the derived heating rates for SH that accounts for the effects of the helicity barrier, and heating rates for RH via $\parallel$-ICWs do not peak in the same region of velocity-space as the empirical measurements, nor reach the required magnitude. Our approach provides novel methodology to uniquely identify and constrain heating processes in collisionless plasmas, and shows evidence of a Fokker-Planck like diffusive process in the near-Sun solar wind.

astro-ph.SR↗

arXiv2026

Phase-Space Energy Transfer of Wave-Particle Interactions using the Field-Particle Correlation Technique and Linear Plasma Theory with JET-PLUME

The collisionless transfer of energy between fields and particles through wave-particle interactions is a fundamental process in space plasmas but remains incompletely characterized because many mechanisms operate across a wide parameter range and diverse plasma conditions. The Field-Particle Correlation (FPC) technique reveals velocity-space signatures of particle energization by correlating measured electric field fluctuations with changes in the velocity distribution. Fully mapping these signatures across plasma parameters requires an impractically large number of kinetic simulations or observations. To address this challenge, we introduce JET-PLUME (Judging Energy Transfer in a Plasma in a Linear Uniform Magnetized Environment), an extension of the PLUME Vlasov-Maxwell dispersion solver. JET-PLUME uses PLUME's ability to model parallel drifting bi-Maxwellian distributions to examine phase-space energy transfer by adding an analytic Fourier-space formulation of the FPC. This approach isolates the contribution of individual resonances, separates degenerate entropy mode components, and allows systematic analysis of unstable, growing modes. Dimensionless expressions extend the results across a broad parameter range and highlight the role of off-diagonal elements of the susceptibility tensor in coupling electric field and current response. We show that during kinetic Alfv\'en wave damping, the perpendicular field can drive parallel ion currents among particles with large perpendicular velocity, reducing the net Landau damping. The resulting velocity-space signatures, accessible through JET-PLUME, demonstrate how analytic formulations of phase-space energy transfer can reveal novel physics of wave-particle interactions across diverse plasma environments.

physics.plasm-ph↗

arXiv2026

Plasma Instabilities in Arbitrary Distributions: Comparison between ALPS and BO

Determining accurate wave dispersion relations is a central problem in plasma physics. Recent advances have enabled the numerical computation of linear dispersion relation in plasmas with arbitrary particle velocity distribution functions (VDFs), using two distinct solvers, BO and ALPS. Their reliability and mutual consistency, however, have not been systematically tested for a broad range of VDFs. Here we compare the dispersion relations obtained from BO and ALPS for several representative distributions. We find that the two solvers give consistent unstable modes for kappa distributions with large values of $\kappa$, as well as for ring-beam, shell, and proton core-beam distributions. BO, however, becomes unreliable for kappa distributions with $\kappa < 4$. For an observationally derived VDF, the two solvers give similar real frequencies for the unstable waves but substantially different growth rates. This difference is mainly caused by the imperfect fitting of the input distribution required by BO. Despite this limitation, BO has a clear computational advantage because it can obtain all roots in a single run. Considering the complementary strengths of the two solvers, their combined use can provide a more reliable and effective framework for investigating instabilities in non-Maxwellian plasma environments.

physics.plasm-ph↗

arXiv2026

Inertial-Range Energy Transfer Free from Isotropic Assumption in Turbulent Space Plasma

The idea of an energy cascade in the inertial range is often invoked in turbulent space plasmas to estimate the energy dissipation rate. Laws governing the behavior of third-order structure functions in the inertial range, so-called third-order laws, are among the few rigorous theoretical results quantifying cross-scale energy transfer. The widely used third-order-law derived rate assumes isotropy, which fundamentally conflicts with the anisotropic nature of space plasmas. Elementary questions persist regarding how such anisotropic energy cascades can be quantified using multi-spacecraft constellations. As the heliospheric community increasingly progresses towards multi-spacecraft, multi-scale constellations, such as Plasma Observatory and HelioSwarm, we revisit these crucial issues pertinent to accurately measuring the inertial-range energy transfer. Here we make a systematic comparison between two methods: direction-averaging (DA) and lag polyhedral derivative ensemble (LPDE) to determine the full three-dimensional (3D) dependence of cross-scale energy transfer. We find that DA exhibits both polar and azimuthal dependence, but is insensitive to spacecraft configuration. By contrast, LPDE is strongly affected by spacecraft separation and tetrahedral shape, while being comparatively insensitive to the sampling trajectory. Our findings have direct implications for current and future multi-spacecraft missions. Both DA and LPDE will provide crucial information on the nature of turbulence in space and astrophysics.

physics.plasm-ph↗

arXiv2026

Wave-particle equilibria with heavy ions in weakly collisional space plasmas

Space plasmas are weakly collisional since characteristic time scales 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 non-thermal 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 the solar wind. We use the Arbitrary Linear Plasma Solver (ALPS) 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 wave and particles.

physics.plasm-ph↗

arXiv2026

Strong Prevalence of Hammerhead Velocity Distributions Close to the Heliospheric Current Sheet

The solar wind undergoes non-adiabatic heating as it travels away from the Sun. The velocity phase space distribution of non-equilibrium ions in the solar wind indicate a source of free energy that could contribute significantly to this heating. Parker Solar Probe (PSP) has observed velocity distributions containing highly anisotropic, perpendicularly diffused proton beams with a distinctly constricted gap between the core and beam populations. These distributions resemble a ``hammerhead" shape and were first reported in the fourth PSP encounter. Numerical simulations have reproduced the qualitative nature of hammerheads under certain initial conditions, but have not convincingly captured the prevalence or extreme attributes of the observed beam. This necessitates a broad study of the occurrence conditions and the associated plasma processes to better guide simulations. We statistically investigate the occurrence of these structures from 20 recent PSP encounters, and find that hammerheads dominantly occur around the Heliospheric Current Sheet (HCS). As the inclination of the HCS at PSP crossing points increases over the rising phase of the solar cycle, the occurrence of hammerheads is increasingly concentrated in narrow time periods around the HCS crossings. For comparison with previous work, we present statistical trends in the anisotropy of the proton beam and its connection to the density of proton beams as well as the drift speed of the beam to the core. Our study establishes a consistent occurrence pattern of hammerhead distributions around the HCS indicating hammerheads are diagnostics of energization processes associated with the HCS and its escaping wind.

astro-ph.SR↗

arXiv2026

Velocity-Space Signatures of Energy Transfer for Ion-Acoustic Instabilities

Context. Observations by Parker Solar Probe (PSP) of electrostatic waves suggest that electrostatic instabilities, including the ion-ion-acoustic instability (IIAI) frequently observed in the inner heliosphere, play an important role in plasma heating and particle acceleration. Aims. Our aim is to explore the application of single spacecraft diagnostics to the IIAI, in anticipation of application to the current missions operating in the inner heliosphere, e.g. PSP and Solar Orbiter. Methods. We apply the field-particle correlation (FPC) technique to fully kinetic simulations of IIAI. We characterize the conversion of energy between the electric field and particle species, allowing the differentiation between oscillatory and secular energy transfer to and from the particles and highlighting the role of resonant energy exchange. We then identify the characteristic IIAI signatures for the proton and electron distributions, and relate them to our previous knowledge of IIAI onset and energy exchange mechanisms. Results. Applying the FPC technique to our simulations run in parameters regime compatible with solar wind conditions, we have identified IIAI signatures that would enable efficient recognition of IIAI in observations. This task is left for future missions, since the time scale over which IIAI signatures develop is too fast for the sampling rates of current missions.

physics.plasm-ph↗

arXiv2025

How the Oblique Drift Instability Alters Solar Wind Heating and Constrains the Distribution of Solar Wind Observations

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 work, we use linear Vlasov--Maxwell theory to define parametric limits for a low-$\beta$ plasma that contains a drifting proton beam or helium ($\alpha$-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 $\beta_c$ 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-\Alfvenic solar wind. The ODI may explain the discrepancy between long-standing expectations of measurements of very low-$\beta$ plasmas with very large ion temperature anisotropies in the near-Sun environment and in situ observations, where $\beta$ is consistently measured above a few percent and the secondary ion populations drift faster than the bulk of proton population by no more than approximately the local Alfven speed.

astro-ph.SR↗

arXiv2025

Free Energy Sources of Ion-scale Waves Observed by Parker Solar Probe

Parker Solar Probe (PSP) observes abundant circularly polarized ion-scale waves throughout the inner heliosphere. These waves are a signature of the interplay between plasma microinstabilities and turbulent dissipation. We perform a mission-wide statistical survey of ion-scale waves observed by PSP, investigating if the waves correspond to specific free energy sources in the measured proton velocity distributions. We find that left-handed waves (LHWs) are frequently observed, with the fraction of time they are observed increasing closer to the Sun, reaching $\sim$30\%. Right-handed waves (RHWs) are less frequently observed, with the associated time fraction decreasing closer to the Sun. The observed LHWs are generally consistent with parallel propagating ion cyclotron wave (ICW) storms that occur continuously for extended periods of time. Turbulent energy spectra are consistently steeper when LHW storms are observed; these wave storms mediate the spatial transport of the free energy associated with temperature anisotropy. The observed RHWs are generally consistent with oblique and parallel fast magnetosonic waves (FMWs), and their observation is well correlated with enhanced proton parallel heat flux, which quantifies the presence of secondary proton populations. Using observations and the SAVIC machine learning instability identification algorithm, we identify a threshold on the proton heat flux beyond which FMWs are likely to be driven unstable by the proton beams. We are thus able to associate trends in the observed ion-scale waves with known sources of free energy for Encounters 3 through 24 of the PSP's prime science phase.

astro-ph.SR↗

arXiv2025

In situ Evidence of 5-minute Oscillations from Parker Solar Probe

The Sun's surface vibrates in characteristic 5-minute oscillations, known as p-modes, generated by sound waves trapped within the convection zone. Although these oscillations have long been hypothesized to reach into the solar wind, direct in situ evidence has remained elusive, even during previous close encounters by Parker Solar Probe (PSP). Here, we present the first promising in situ detection of 5-minute oscillations in the upper solar corona, based on observations from PSP's three closest perihelia. In two events at 9.9 solar radii, we identify statistically significant ($\sim$ 6 $\sigma$) 3.1-3.2 mHz peaks in the magnetic field power spectrum, each appearing as a large-amplitude, spherically polarized Alfv\'enic wave train lasting approximately 35 minutes. These results demonstrate that global solar oscillations can reach and potentially influence the solar wind.

astro-ph.SR↗

arXiv2025

Stochastic Heating in the Sub-Alfv\'enic Solar Wind

Collisionless dissipation of turbulence is important for heating plasmas in astrophysical, space physics, and laboratory environments, controlling energy, momentum and particle transport. We analyze Parker Solar Probe observations to understand the collisionless heating of the sub-Alfv\'enic solar wind, which is connected to the solar corona. Our results show that linear resonant heating through parallel-propagating cyclotron waves cannot account for turbulent dissipation in sub-Alfv\'enic region, which observations suggest may dissipate turbulence at distances further from the Sun. Instead, we find that stochastic heating can account for the observed ion energization; however, because the dominant contributions arise from infrequent, large-amplitude events, turbulent intermittency must be explicitly incorporated. These observations directly connect stochastic heating via breaking of the proton magnetic moment with the intermittent and inhomogeneous heating of turbulence reported in many previous studies. Our identification of stochastic heating as a dynamic mechanism responsible for intermittent heating of the solar wind has significant implications for turbulent dissipation in the lower corona, other astrophysical environments, and laboratory plasma.

astro-ph.SR↗

arXiv2025

Multi-spacecraft Measurements of the Evolving Geometry of the Solar Alfv\'en Surface Over Half a Solar Cycle

The geometry of a star's Alfv\'en surface determines stellar angular momentum loss, separates a causally distinct 'corona' and stellar wind, and potentially affects exoplanetary habitability. The solar Alfv\'en surface is the only such structure that is directly measurable and since 2021, has been routinely measured in situ by NASA's Parker Solar Probe (Parker). We use these unique measurements in concert with Solar Orbiter and L1 in situ data spanning the first half of the Solar Cycle 25 in time and from 0.045 - 1 au in heliocentric distance to develop a radial scaling technique to estimate the morphology of the Alfv\'en surface from measurements of the solar wind speed and local Alfv\'en speed. We show that accounting for solar wind acceleration and mass flux is necessary to achieve reasonable agreement between the scaled location of the Alfv\'en surface and the locations of direct crossings measured by Parker. We produce continuous 2D equatorial cuts of the Alfv\'en surface over half a Solar Cycle (ascending phase and maximum). Parker's earliest crossings clipped outward extrusions, many of which are likely transient related, while more recently Parker has unambiguously sampled deep sub-Alfv\'enic flows. We analyze the average altitude, departure from spherical symmetry, and surface roughness, finding that all are positively correlated to solar activity. For the current modest Solar Cycle, the height varies up to 30\% which corresponds to a near-doubling in angular momentum loss per unit mass loss.

astro-ph.SR↗

arXiv2025

Perpendicular ion heating in turbulence and reconnection: magnetic moment breaking by coherent fluctuations

We study the interaction of an ion with a fluctuation in the electromagnetic fields that is localized in both space and time. We study the scale-dependence of the interaction in both space and time, deriving a generic form for the ion's energy change, which involves an exponential cutoff based on the characteristic timescale of the electromagnetic fluctuation. This leads to diffusion in energy in both $v_\perp$ and $v_\parallel$. We show how to apply our results to general plasma physics phenomena, and specifically to Alfv\'enic turbulence and to reconnection. Our theory can be viewed as a unification of previous models of stochastic ion heating, cyclotron heating, and reconnection heating in a single theoretical framework.

physics.plasm-ph↗

arXiv2025

Observational Constraints on the Radial Evolution of O$^{6+}$ Temperature and Differential Flow in the Inner Heliosphere

Over decades of solar wind observations, heavy ions have been observed to have a higher temperature and flow faster than protons in the solar corona and heliosphere. Remote observations have largely been limited to the low corona ($< 4R_{\odot}$), while in situ observations for heavy ions ($Z>2$) have only been sampled at 1 au and beyond. As a result, theories that address heavy ion heating and acceleration remain largely unconstrained. With the launch of Solar Orbiter, heavy ion kinetics can be probed closer to the Sun, as close as the orbit of Mercury ($65R_{\odot}$), to examine their radial behavior. Through a statistical analysis of O$^{6+}$, this work provides a comprehensive analysis of the velocity and temperature of O$^{6+}$ from 0.3 au to 1 au. The study finds that the O$^{6+}$ relative drift, normalized to the local Alfv\'en speed, and its temperature compared to protons, both decrease with distance from the Sun and show some speed dependence. The O$^{6+}$ temperature is well fit by a single temperature adiabatic profile across all wind speeds, suggesting there is no significant heating at these heliocentric distances. This is in contrast to what is observed for protons and He$^{2+}$. Alfv\'enic fluctuations, with full 180$^{\circ}$ field rotation, create momentary negative differential streaming where the speed of O$^{6+}$ trails the protons. The amount of negative differential streaming gradually increases at larger distances. These results provide critical constraints to the proposed mechanisms seeking to describe ion heating and acceleration in the solar wind.

astro-ph.SR↗

arXiv2025

Temperature anisotropy instabilities of solar wind electrons with regularized Kappa-halos resolved with ALPS

Space plasmas in various astrophysical setups can often be both very hot and dilute, making them highly susceptible to waves and fluctuations, which are generally self-generated and maintained by kinetic instabilities. In this sense, we have in-situ observational evidence from the solar wind and planetary environments, which reveal not only wave fluctuations at kinetic scales of electrons and protons, but also non-equilibrium distributions of particle velocities. This paper reports on the progress made in achieving a consistent modeling of the instabilities generated by temperature anisotropy, taking concrete example of those induced by anisotropic electrons, such as, electromagnetic electron-cyclotron (whistler) and firehose instabilities. The effects of the two main electron populations, the quasi-thermal core and the suprathermal halo indicated by the observations, are thus captured. The low-energy core is bi-Maxwellian, and the halo is described for the first time by a regularized (bi-)$\kappa$-distribution (RKD), which was recently introduced to fix inconsistencies of standard $\kappa$-distributions (SKD). In the absence of a analytical RKD dispersion kinetic formalism (involving tedious and laborious derivations), both the dispersion and (in)stability properties are directly solved numerically using the numerical Arbitrary Linear Plasma Solver (ALPS). The results have an increased degree of confidence, considering the successful testing of the ALPS on previous results with established distributions.

physics.plasm-ph↗

arXiv2025

Non-Maxwellianity of Ion Velocity Distributions in the Earth's Magnetosheath

We analyze the deviations from local thermodynamic equilibrium (LTE) of the ion velocity distribution function (iVDF) in collisionless plasma turbulence. Using data from the Magnetospheric Multiscale (MMS) mission, we examine the non-Maxwellianity of 439,685 iVDFs in the Earth's magnetosheath. We find that the iVDFs' anisotropies and the high-order non-bi-Maxwellian features are widespread and can be significant. Our results show that the complexity of the iVDFs is strongly influenced by the ion plasma beta and turbulence intensity, with high-order non-LTE features emerging in the presence of large-amplitude magnetic field fluctuations. Furthermore, our analysis indicates that turbulence-driven magnetic curvature contributes to the isotropization of the iVDFs by scattering the ions, emphasizing the complex interaction between turbulence and the velocity distribution of charged particles in collisionless plasmas.

physics.plasm-ph↗