arXiv subjects

Gregory G. Howes

Publications and source records attributed to Gregory G. Howes.

At least 19 recordsLinked to original sources

arXiv2026

Evidence for Solar-Cycle Modulation of the Alpha-to-Proton Temperature Ratio in Solar Wind

The influence of collisional age $(A_c)$ on the alpha-to-proton temperature ratio $(T_\alpha/T_p)$ has been explored in the past. However, the modulation of this ratio with respect to the solar cycle has remained unexplored so far. We show solar-cycle modulation of $T_\alpha/T_p$ and $A_c$ using nearly three decades of in-situ observations from Wind spacecraft across distinct solar wind speed regimes and solar activity phases. Our results reveal that in the slow solar wind with velocity $<400$ km s$^{-1}$, where $A_c$ happens to be typically $>1$, the ratio $T_\alpha/T_p$ stays close to unity. This suggests frequent Coulomb collisions efficiently iron out temperature differences. In contrast, the fast wind with velocity $>500$ km s$^{-1}$, where $A_c$ happens to be typically $<1$, mass-proportional heating is most pronounced, with $T_\alpha/T_p$ often exceeding 4. The intermediate speed regime ($400$-$500$ km s$^{-1}$) represents a gradual transition between the slow and fast wind populations in terms of their solar-cycle dependence. This behavior reflects the changing dominance of high-speed streams from polar coronal holes during minima to denser slow wind during maxima. These results suggest that mass-proportional ion heating at 1 AU is not solely governed by local collisional physics but is significantly modulated by the solar cycle dependent variations in the solar wind sources.

astro-ph.SR↗

arXiv2026

Statistical insights on the decorrelation lengths of solar wind parameters at L1 point under varying conditions

Understanding the spatial coherence of solar wind plasma and magnetic field properties is essential for interpreting multi-spacecraft observations and for characterizing the large-scale structure of heliospheric transients. In this study, we quantify the spatial correlation of six key solar wind parameters - interplanetary magnetic field components, bulk flow speed, proton number density, and the alpha-to-proton abundance ratio - using simultaneous measurements from the ACE and Wind spacecraft as a function of their instantaneous separation distance. The analysis is performed separately for intervals of background solar wind, Interplanetary Coronal Mass Ejections (ICMEs), and Stream Interaction Regions (SIRs). The decay of the Pearson correlation coefficient with distance is modeled using an exponential function to infer characteristic de-correlation length scales. We find that the bulk solar wind speed is the most spatially coherent parameter in all regimes, while plasma composition exhibits the weakest coherence. Magnetic field coherence shows strong dependence on solar wind structure: ICMEs display near-unity correlations and the largest magnetic coherence scales, consistent with organized, flux-rope-like configurations, whereas SIRs exhibit reduced coherence - particularly in the north - south magnetic field component - reflecting compressed and turbulent plasma. The background solar wind exhibits intermediate behavior, with large-scale coherence in bulk plasma properties but shorter coherence lengths in magnetic fluctuations. These results provide a quantitative framework for distinguishing solar wind structures based on their spatial coherence properties and have important implications for multi-point solar wind studies and space weather applications.

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

Finding Novel Precursors for Solar Wind Stream Interaction Regions with Interpretable Deep Learning

Solar wind stream interaction regions (SIRs) drive recurrent geomagnetic storms, yet most existing catalogs rely on expert inspection and simple thresholds that are subjective and can miss events with complex morphologies. We present SIREN (SIR Encoder Network), a lightweight Transformer based model for per timestep SIR detection from in situ solar wind observations. The model ingests sequences of 11 solar wind parameters, spanning magnetic field, velocity, and thermodynamic properties. With approximately 100,000 trainable parameters in a two layer encoder architecture, SIREN is trained using weighted binary cross entropy loss and a cosine annealing learning rate. Platt scaling is applied to produce well-calibrated detection probabilities. On a held-out test set of 102 events, the calibrated model achieves a ROC-AUC of 0.93, F1 score of 0.78, and true skill statistic of 0.67. Analysis of the self-attention weights confirms that the model concentrates on the SIR, grounding its decisions in the physically relevant portion of each sequence. Integrated Gradients attribution reveals a quantifiable feature hierarchy: proton density (24.3%) and magnetic field magnitude (21.6%) dominate, followed by temperature (13.9%) and bulk speed (12.1%). Notably, the transverse velocity component Vy and east-west flow angle together contribute 13-17%, identifying flow deflection as a consistent but previously under-quantified SIR signature. By producing continuous probabilities rather than binary labels, SIREN enables flexible threshold tuning for operational use and provides a template for compact, interpretable deep-learning systems in space weather.

astro-ph.SR↗

arXiv2026

Velocity space origins of pressure-strain interaction in multi-population distributions and its application to magnetic reconnection

A forefront research question is how energy evolves in weakly collisional plasmas for which departures from local thermodynamic equilibrium (LTE) are significant. The standard approach is studying the terms in the non-LTE energy evolution equation derived by taking the second moment of the Boltzmann equation, but the resultant fluid metrics do not retain information about which particles at which velocities drive energy evolution. A widely studied channel for internal energy density evolution is the pressure-strain interaction. Here we employ the kinetic pressure-strain [S. A. Conley et al., ${\it Phys. Plasmas,} {\bf 31}$, 122117 (2024)], a phase space diagnostic whose velocity-space integral recovers the pressure-strain interaction to disambiguate the contributions to pressure-strain interaction from disparate particle populations in composite phase-space densities. We develop phase-space analogs of the pressure-strain interaction decompositions to provide the phase-space origins of normal vs. sheared flow. We introduce the "kinetic strain-rate" tensor, the phase-space analog of strain-rate tensor, which we argue is needed to interpret phase-space origins of pressure-strain interaction. To demonstrate the utility of these quantities, we investigate them for composite electron distributions near the electron diffusion region in two-dimensional particle-in-cell simulations of antiparallel symmetric magnetic reconnection. We find that the phase space-based diagnostics isolate the roles of distinct populations. These results contribute to a growing body of work providing new methods for quantifying phase space energy evolution for a broad array of processes, from magnetic reconnection to collisionless shocks and turbulence, opening new pathways for answering longstanding problems of particle energization in weakly collisional plasmas.

physics.plasm-ph↗

arXiv2026

Compressive Structures in the Foreshock of Collisionless Shocks

Collisionless shocks are fundamental accelerators of energetic particles; yet, the observations of nonlinear foreshock structures, which are essential in acceleration processes, differ significantly between Interplanetary (IP) shocks and planetary bow shocks. We present a direct comparison of two high-Mach-number, quasi-parallel shocks: an IP shock observed by Solar Orbiter and the Earth's bow shock measured by the Magnetospheric Multiscale (MMS) mission during the 2024-2025 ``string-of-pearls'' campaign. We show that Foreshock Compressive Structures (FCSs) initiate upstream of both shocks at similar normalized distances ($\lesssim$50 ion inertial lengths, $d_i$) when the suprathermal ($>10$ keV) ion density exceeds $\sim$1\% of the background. However, the IP shock lacks the fully evolved, high-amplitude Short Large Amplitude Magnetic Structures (SLAMS) characteristic of the terrestrial foreshock. We demonstrate that the ``growth zone'' capable of sustaining these structures is spatially limited ($\sim$135 $d_i$), which, due to the high speed of the propagating IP shock, corresponds to a brief observational window of $<10$ s. Beyond this observational constraint, we suggest an additional physical mechanism that can inhibit foreshock maturity at IP shocks. The lack of global curvature prevents the lateral supply (``cross-talk'') of energetic ions from different shock regions. These findings suggest that while the fundamental physics of FCS initiation is unified across collisionless shocks, the achievement of full nonlinearity can be regulated by the unique shock geometry and upstream properties, while ultimately remaining observationally challenging to identify.

physics.space-ph↗

arXiv2026

Solar Wind Heating Near the Sun: A Radial Evolution Approach

Characterizing the plasma state in the near-Sun environment is essential to constrain the mechanisms that heat and accelerate the solar wind. In this study, we use Parker Solar Probe (PSP) observations from Encounters 1 through 24 to investigate the radial evolution of solar wind plasma and magnetic field properties in this region. Using intervals with high field-of-view ($>85\%$) coverage, we derive the radial profiles of magnetic field strength ($|B|$), proton density ($N$), bulk speed ($V$), total proton temperature ($T$), parallel ($T_\parallel$) and perpendicular ($T_\perp$) temperatures, temperature anisotropy ($T_\perp/T_\parallel$), plasma beta ($\beta$), Alfv\'{e}n Mach number ($M_A$), and magnetic field fluctuations ($\delta B/B$) for sub and super-Alfv\'{e}nic regions. In super-Alfv\'{e}nic regions, power-law of $|B|$, $N$, $V$, and $T$ as a function of heliocentric distance are broadly consistent with previous \textit{Helios} results at $>0.3$ AU. The radial evolution of the components of the temperature tensor reveals distinct behavior: $T_\perp$ decreases monotonically with distance, whereas $T_\parallel$ exhibits a non-monotonic trend -- decreasing in the sub-Alfv\'{e}nic region, increasing just beyond the Alfv\'{e}n surface. We interpret the increase in $T_\parallel$ as a proxy for proton beam occurrence. We further examine the evolution of magnetic field fluctuations, finding decreasing radial/parallel fluctuations but enhanced tangential/normal/perpendicular fluctuations in sunward direction. These fluctuations may provide free energy for beam generation and particle heating via wave-particle interactions.

astro-ph.SR↗

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

Unveiling the Velocity-Space Signature of Ion Cyclotron Damping Using Liouville Mapping

Ion cyclotron damping is a key mechanism for the dissipation of electromagnetic wave energy in weakly collisional plasmas. This study presents a combined approach using Liouville mapping and the field-particle correlation technique to investigate qualitatively and quantitatively the velocity-space signature of ion cyclotron damping. Liouville mapping offers a computationally efficient way to predict perturbations to the particle velocity distribution function using single-particle trajectories in prescribed electromagnetic fields. One may apply the field-particle correlation technique to these perturbed velocity distributions to reveal the unique velocity-space signatures of the secular energy transfer rate associated with specific wave-particle interactions. We validate this method by reproducing known Landau damping signatures for kinetic Alfv\'en waves, and then we apply this method to ion cyclotron waves where ion cyclotron damping dominates. The resulting velocity-space signature reveals distinct energization features of ion cyclotron damping : (i) a quadrupolar pattern in the perpendicular $(v_x, v_y)$ plane; and (ii) a localized energization near the $n = 1$ resonant velocity in gyrotropic $(v_\parallel, v_\perp)$ velocity-space. The quantitative patterns remain unchanged as the ion plasma beta $\beta_i$ is varied, ultimately showing minimal $v_\perp$ dependence on $\beta_i$ of the velocity-space signature at the $n = 1$ resonant velocity. This work provides a systematic study of how the ion cyclotron damping signature varies with $\beta_i$, offering a practical foundation to identify ion cyclotron damping using kinetic simulation data or spacecraft data.

physics.plasm-ph↗

arXiv2024

Plasma Seismology: Fully Exploiting the Information Contained in Velocity Space of Kinetic Plasmas using the Morrison G Transform

Weakly collisional plasmas contain a wealth of information about the dynamics of the plasma in the particle velocity distribution functions, yet our ability to exploit fully that information remains relatively primitive. Here we aim to present the fundamentals of a new technique denoted Plasma Seismology that aims to invert the information from measurements of the particle velocity distribution functions at a single point in space over time to enable the determination of the electric field variation over an extended spatial region. The fundamental mathematical tool at the heart of this technique is the Morrison $G$ Transform. Using kinetic numerical simulations of Langmuir waves in a Vlasov-Poisson plasma, we demonstrate the application of the standard Morrison $G$ Transform, which uses measurements of the particle velocity distribution function over all space at one time to predict the evolution of the electric field in time. Next, we introduce a modified Morrison $G$ Transform which uses measurements of the particle velocity distribution function at one point in space over time to determine the spatial variation of the electric field over an extended spatial region. We discuss the limitations of this approach, particularly for the numerically challenging case of Langmuir waves. The application of this technique to Alfven waves in a magnetized plasma holds the promise to apply the technique to existing spacecraft particle measurement instrumentation to determine the electric fields over an extended spatial region away from the spacecraft.

physics.plasm-ph↗

arXiv2024

The Kinetic Analogue of the Pressure-Strain Interaction

Energy transport in weakly collisional plasma systems is often studied with fluid models and diagnostics. However, the applicability of fluid models is necessarily limited when collisions are weak or absent, and using a fluid approach can obscure kinetic processes that provide key insights into the physics of energy transport. A kinetic technique that retains all of the information in 3D-3V phase-space for the study of energy transfer between electromagnetic fields and particle kinetic energy, which is quantified by the rate of electromagnetic work per unit volume $\mathbf{j} \cdot \mathbf{E}$ in fluid models, is the Field- Particle Correlation (FPC) technique. This technique has demonstrated that leveraging the full information contained in phase-space can elucidate the physical mechanisms of energy transfer. This provides a significant advantage over fluid diagnostics that quantify the rate at which energy is exchanged but do not distinguish between different physical processes. A different channel of energy transport, between fluid flow energy and particle internal energy, is quantified in fluid models via the pressure-strain interaction $-(\mathbf{P} \cdot \nabla ) \cdot \mathbf{u}$. Using a similar approach to that of the field-particle correlation technique, in this work we derive a kinetic analog of the pressure-strain interaction and use it alongside the field-particle correlation to analyze the flow of energy from electromagnetic fields into particle internal energy in two case studies of electron Landau damping.

physics.plasm-ph↗

arXiv2024

The Fundamental Parameters of Astrophysical Plasma Turbulence and its Dissipation: Nonrelativistic Limit

A specific set of dimensionless plasma and turbulence parameters is introduced to characterize the nature of turbulence and its dissipation in weakly collisional space and astrophysical plasmas. Key considerations are discussed for the development of predictive models of the turbulent plasma heating that characterize the partitioning of dissipated turbulent energy between the ion and electron species and between the perpendicular and parallel degrees of freedom for each species. Identifying the kinetic physical mechanisms that govern the damping of the turbulent fluctuations is a critical first step in constructing such turbulent heating models. A set of ten general plasma and turbulence parameters are defined, and reasonable approximations along with the exploitation of existing scaling theories for magnetohydrodynamic turbulence are used to reduce this general set of ten parameters to just three parameters in the isotropic temperature case. A critical step forward in this study is to identify the dependence of all of the proposed kinetic mechanisms for turbulent damping in terms of the same set of fundamental plasma and turbulence parameters. Analytical estimations of the scaling of each damping mechanism on these fundamental parameters are presented, and this information is synthesized to produce the first phase diagram for the turbulent damping mechanisms as a function of driving scale and ion plasma beta.

astro-ph.SR↗

arXiv2024

The Velocity-Space Signature of Transit-Time Damping

Transit-time damping (TTD) is a process in which the magnetic mirror force -- induced by the parallel gradient of magnetic field strength -- interacts with resonant plasma particles in a time-varying magnetic field, leading to the collisionless damping of electromagnetic waves and the resulting energization of those particles through the perpendicular component of the electric field, $E_\perp$. In this study, we utilize the recently developed field-particle correlation technique to analyze gyrokinetic simulation data. This method enables the identification of the velocity-space structure of the TTD energy transfer rate between waves and particles during the damping of plasma turbulence. Our analysis reveals a unique bipolar pattern of energy transfer in velocity space characteristic of TTD. By identifying this pattern, we provide clear evidence of TTD's significant role in the damping of strong plasma turbulence. Additionally, we compare the TTD signature with that of Landau damping (LD). Although they both produce a bipolar pattern of phase-space energy density loss and gain about the parallel resonant velocity of the Alfv\'enic waves, they are mediated by different forces and exhibit different behaviors as $v_\perp \to 0$. We also explore how the dominant damping mechanism varies with ion plasma beta $\beta_i$, showing that TTD dominates over LD for $\beta_i > 1$. This work deepens our understanding of the role of TTD in the damping of weakly collisional plasma turbulence and paves the way to seek the signature of TTD using \emph{in situ} spacecraft observations of turbulence in space plasmas.

physics.plasm-ph↗

arXiv2023

Electron Energization in Reconnection: Eulerian versus Lagrangian Perspectives

Particle energization due to magnetic reconnection is an important unsolved problem for myriad space and astrophysical plasmas. Electron energization in magnetic reconnection has traditionally been examined from a particle, or Lagrangian, perspective using particle-in-cell (PIC) simulations. Guiding-center analyses of ensembles of PIC particles have suggested that Fermi (curvature drift) acceleration and direct acceleration via the reconnection electric field are the primary electron energization mechanisms. However, both PIC guiding-center ensemble analyses and spacecraft observations are performed in an Eulerian perspective. For this work, we employ the continuum Vlasov-Maxwell solver within the Gkeyll simulation framework to re-examine electron energization from a kinetic continuum, Eulerian, perspective. We separately examine the contribution of each drift energization component to determine the dominant electron energization mechanisms in a moderate guide-field Gkeyll reconnection simulation. In the Eulerian perspective, we find that the diamagnetic and agyrotropic drifts are the primary electron energization mechanisms away from the reconnection x-point, where direct acceleration dominates. We compare the Eulerian (Vlasov Gkeyll) results with the wisdom gained from Lagrangian (PIC) analyses.

physics.plasm-ph↗

arXiv2023

Characterizing the Velocity-Space Signature of Electron Landau Damping

Plasma turbulence plays a critical role in the transport of energy from large-scale magnetic fields and plasma flows to small scales, where the dissipated turbulent energy ultimately leads to heating of the plasma species. A major goal of the broader heliophysics community is to identify the physical mechanisms responsible for the dissipation of the turbulence and to quantify the consequent rate of plasma heating. One of the mechanisms proposed to damp turbulent fluctuations in weakly collisional space and astrophysical plasmas is electron Landau damping. The velocity-space signature of electron energization by Landau damping can be identified using the recently developed field-particle correlation technique. Here, we perform a suite of gyrokinetic turbulence simulations with ion plasma beta values of 0.01, 0.1, 1, and 10 and use the field-particle correlation technique to characterize the features of the velocity-space signatures of electron Landau damping in turbulent plasma conditions consistent with those observed in the solar wind and planetary magnetospheres. We identify the key features of the velocity-space signatures of electron Landau damping as a function of varying plasma \beta_i to provide a critical framework for interpreting the results of field-particle correlation analysis of in situ spacecraft observations of plasma turbulence.

physics.plasm-ph↗

arXiv2022

Isolation and Phase-Space Energization Analysis of the Instabilities in Collisionless Shocks

We analyze the generation of kinetic instabilities and their effect on the energization of ions in non-relativistic, oblique collisionless shocks using a 3D-3V simulation by $\texttt{dHybridR}$, a hybrid particle-in-cell code. At sufficiently high Mach number, quasi-perpendicular and oblique shocks can experience rippling of the shock surface caused by kinetic instabilities arising from free energy in the ion velocity distribution due to the combination of the incoming ion beam and the population of ions reflected at the shock front. To understand the role of the ripple on particle energization, we devise the new instability isolation method to identify the unstable modes underlying the ripple and interpret the results in terms of the governing kinetic instability. We generate velocity-space signatures using the field-particle correlation technique to look at energy transfer in phase space from the isolated instability driving the shock ripple, providing a viewpoint on the different dynamics of distinct populations of ions in phase space. We generate velocity-space signatures of the energy transfer in phase space of the isolated instability driving the shock ripple using the field-particle correlation technique. Together, the field-particle correlation technique and our new instability isolation method provide a unique viewpoint on the different dynamics of distinct populations of ions in phase space and allow us to completely characterize the energetics of the collisionless shock under investigation.

physics.plasm-ph↗

arXiv2022

Phase Space Energization of Ions in Oblique Shocks

Examining energization of kinetic plasmas in phase space is a growing topic of interest, owing to the wealth of data in phase space compared to traditional bulk energization diagnostics. Via the field-particle correlation (FPC) technique and using multiple means of numerically integrating the plasma kinetic equation, we have studied the energization of ions in phase space within oblique collisionless shocks. The perspective afforded to us with this analysis in phase space allows us to characterize distinct populations of energized ions. In particular, we focus on ions which reflect multiple times off the shock front through shock-drift acceleration, and how to distinguish these different reflected populations in phase space using the FPC technique. We further extend our analysis to simulations of three-dimensional shocks undergoing more complicated dynamics, such as shock ripple, to demonstrate the ability to recover the phase space signatures of this energization process in a more general system. This work thus extends previous applications of the FPC technique to more realistic collisionless shock environments, providing stronger evidence of the technique's utility for simulation, laboratory, and spacecraft analysis.

physics.plasm-ph↗

arXiv2022

Observing Particle Energization above the Nyquist Frequency: An Application of the Field-Particle Correlation Technique

The field-particle correlation technique utilizes single-point measurements to uncover signatures of various particle energization mechanisms in turbulent space plasmas. The signature of Landau damping by electrons has been found in both simulations and observations from Earth's magnetosheath using this technique, but instrumental limitations of spacecraft sampling rates present a challenge to discovering the full extent of the presence of Landau damping in the solar wind. Theory predicts that field-particle correlations can recover velocity-space energization signatures even from data that is undersampled with respect to the characteristic frequencies at which the wave damping occurs. To test this hypothesis, we perform a high-resoluation gyrokinetic simulation of space plasma turbulence, confirm that it contains signatures of electron Landau damping, and then systematically reduce the time resolution of the data to identify the point at which the signatures become impossible to recover. We find results in support of our theoretical prediction and look for a rule of thumb that can be compared with the measurement capabilities of spacecraft missions to inform the process of applying field-particle correlations to low time resolution data.

physics.plasm-ph↗