arXiv subjects

W. H. Matthaeus

Publications and source records attributed to W. H. Matthaeus.

At least 19 recordsLinked to original sources

arXiv2025

Factors Controlling the Statistics of Magnetic Reconnection in MHD Turbulence

We study the statistics of dynamical quantities associated with magnetic reconnection events embedded in a sea of strong background magnetohydrodynamic (MHD) turbulence using direct numerical simulations. We focus on the relationship of the reconnection properties to the statistics of global turbulent fields. For the first time, we show that the distribution in turbulence of reconnection rates (determined by upstream fields) is strongly correlated with the magnitude of the global turbulent magnetic field at the correlation scale. The average reconnection rates, and associated dissipation rates, during turbulence are thus much larger than predicted by using turbulent magnetic field fluctuation amplitudes at the dissipation or kinetic scales. Magnetic reconnection may therefore be playing a major role in energy dissipation in astrophysical and heliospheric turbulence.

physics.plasm-ph↗

arXiv2024

Magnetic reconnection-driven energization of protons up to 400 keV at the near-Sun heliospheric current sheet

We report observations of direct evidence of energetic protons being accelerated above ~400 keV within the reconnection exhaust of a heliospheric current sheet (HCS) crossing by NASA's Parker Solar Probe (PSP) at a distance of ~16.25 solar radii (Rs) from the Sun. Inside the extended exhaust, both the reconnection-generated plasma jets and the accelerated protons propagated toward the Sun, unambiguously establishing their origin from HCS reconnection sites located beyond PSP. Within the core of the exhaust, PSP detected stably trapped energetic protons up to ~400 keV, which is approximately 1000 times greater than the available magnetic energy per particle. The differential energy spectrum of the accelerated protons behaved as a pure power-law with spectral index of about -5. Supporting simulations using the kglobal model suggest that the trapping and acceleration of protons up to ~400 keV in the reconnection exhaust is likely facilitated by merging magnetic islands with a guide field between ~0.2-0.3 of the reconnecting magnetic field, consistent with the observations. These new results, enabled by PSP's proximity to the Sun, demonstrate that magnetic reconnection in the HCS is a significant new source of energetic particles in the near-Sun solar wind. The discovery of in-situ particle acceleration via magnetic reconnection at the HCS provides valuable insights into this fundamental process which frequently converts the large magnetic field energy density in the near-Sun plasma environment and may be responsible for heating the sun's atmosphere, accelerating the solar wind, and energizing charged particles to extremely high energies in solar flares.

astro-ph.SR↗

arXiv2024

Parker Solar Probe Observations of Energetic Particles in the Flank of a Coronal Mass Ejection Close to the Sun

We present an event observed by Parker Solar Probe at $\sim$0.2 au on March 2, 2022 in which imaging and \emph{in situ} measurements coincide. During this event, PSP passed through structures on the flank of a streamer blowout CME including an isolated flux tube in front of the CME, a turbulent sheath, and the CME itself. Imaging observations and \emph{in situ} helicity and principal variance signatures consistently show the presence of flux ropes internal to the CME. In both the sheath, and the CME interval, the distributions are more isotropic, the spectra are softer, and the abundance ratios of Fe/O and He/H are lower than those in the isolated flux tube, and yet elevated relative to typical plasma and SEP abundances. These signatures in the sheath and the CME indicate that both flare populations and those from the plasma are accelerated to form the observed energetic particle enhancements. In contrast, the isolated flux tube shows large streaming, hard spectra and large Fe/O and He/H ratios, indicating flare sources. Energetic particle fluxes are most enhanced within the CME interval from suprathermal through energetic particle energies ($\sim$ keV to $>10$ MeV), indicating particle acceleration, and confinement local to the closed magnetic structure. The flux-rope morphology of the CME helps to enable local modulation and trapping of energetic particles, particularly along helicity channels and other plasma boundaries. Thus, the CME acts to build-up energetic particle populations, allowing them to be fed into subsequent higher energy particle acceleration throughout the inner heliosphere where a compression or shock forms on the CME front.

astro-ph.SR↗

arXiv2024

Correlation of Coronal Mass Ejection Shock Temperature with Solar Energetic Particle Intensity

Solar energetic particle (SEP) events have been observed by the Parker Solar Probe (PSP) spacecraft since its launch in 2018. These events include sources from solar flares and coronal mass ejections (CMEs). Onboard PSP is the IS$\odot$IS instrument suite measuring ions over energies from ~ 20 keV/nucleon to 200 MeV/nucleon and electrons from ~ 20 keV to 6 MeV. Previous studies sought to group CME characteristics based on their plasma conditions and arrived at general descriptions with large statistical errors, leaving open questions on how to properly group CMEs based solely on their plasma conditions. To help resolve these open questions, plasma properties of CMEs have been examined in relation to SEPs. Here we reexamine one plasma property, the solar wind proton temperature, and compare it to the proton SEP intensity in a region immediately downstream of a CME-driven shock for seven CMEs observed at radial distances within 1 au. We find a statistically strong correlation between proton SEP intensity and bulk proton temperature, indicating a clear relationship between SEPs and the conditions in the solar wind. Furthermore, we propose that an indirect coupling of SEP intensity to the level of turbulence and the amount of energy dissipation that results is mainly responsible for the observed correlation between SEP intensity and proton temperature. These results are key to understanding the interaction of SEPs with the bulk solar wind in CME-driven shocks and will improve our ability to model the interplay of shock evolution and particle acceleration.

physics.space-ph↗

arXiv2023

Estimates of Proton and Electron Heating Rates Extended to the Near-Sun Environment

A central problem of space plasma physics is how protons and electrons are heated in a turbulent, magnetized plasma. The differential heating of charged species due to dissipation of turbulent fluctuations plays a key role in solar wind evolution. Measurements from previous heliophysics missions have provided estimates of proton and electron heating rates beyond 0.27 au. Using Parker Solar Probe (PSP) data accumulated during the first ten encounters, we extend the evaluation of the individual rates of heat deposition for protons and electrons in to a distance of 0.063 au (13.5 solar radii), in the newly formed solar wind. The PSP data in the near-Sun environment show different behavior of the electron heat conduction flux from what was predicted from previous fits to Helios and Ulysses data. Consequently, the empirically derived proton and electron heating rates exhibit significantly different behavior than previous reports, with the proton heating becoming increasingly dominant over electron heating at decreasing heliocentric distances. We find that the protons receive about 80% of the total plasma heating at ~ 13 solar radii, slightly higher than the near-Earth values. This empirically derived heating partition between protons and electrons will help to constrain theoretical models of solar wind heating.

physics.space-ph↗

arXiv2023

Parker Solar Probe: Four Years of Discoveries at Solar Cycle Minimum

Launched on 12 Aug. 2018, NASA's Parker Solar Probe had completed 13 of its scheduled 24 orbits around the Sun by Nov. 2022. The mission's primary science goal is to determine the structure and dynamics of the Sun's coronal magnetic field, understand how the solar corona and wind are heated and accelerated, and determine what processes accelerate energetic particles. Parker Solar Probe returned a treasure trove of science data that far exceeded quality, significance, and quantity expectations, leading to a significant number of discoveries reported in nearly 700 peer-reviewed publications. The first four years of the 7-year primary mission duration have been mostly during solar minimum conditions with few major solar events. Starting with orbit 8 (i.e., 28 Apr. 2021), Parker flew through the magnetically dominated corona, i.e., sub-Alfvénic solar wind, which is one of the mission's primary objectives. In this paper, we present an overview of the scientific advances made mainly during the first four years of the Parker Solar Probe mission, which go well beyond the three science objectives that are: (1) Trace the flow of energy that heats and accelerates the solar corona and solar wind; (2) Determine the structure and dynamics of the plasma and magnetic fields at the sources of the solar wind; and (3) Explore mechanisms that accelerate and transport energetic particles.

astro-ph.SR↗

arXiv2022

The essential role of multi-point measurements in investigations of turbulence, three-dimensional structure, and dynamics: the solar wind beyond single scale and the Taylor Hypothesis

Space plasmas are three-dimensional dynamic entities. Except under very special circumstances, their structure in space and their behavior in time are not related in any simple way. Therefore, single spacecraft in situ measurements cannot unambiguously unravel the full space-time structure of the heliospheric plasmas of interest in the inner heliosphere, in the Geospace environment, or the outer heliosphere. This shortcoming leaves numerous central questions incompletely answered. Deficiencies remain in at least two important subjects, Space Weather and fundamental plasma turbulence theory, due to a lack of a more complete understanding of the space-time structure of dynamic plasmas. Only with multispacecraft measurements over suitable spans of spatial separation and temporal duration can these ambiguities be resolved. We note that these characterizations apply to turbulence across a wide range of scales, and also equally well to shocks, flux ropes, magnetic clouds, current sheets, stream interactions, etc. In the following, we will describe the basic requirements for resolving space-time structure in general, using turbulence' as both an example and a principal target or study. Several types of missions are suggested to resolve space-time structure throughout the Heliosphere.

physics.space-ph↗

arXiv2022

Energetic Particle Perpendicular Diffusion: Simulations and Theory in Noisy Reduced Magnetohydrodynamic Turbulence

The transport of energetic charged particles (e.g., cosmic rays) in turbulent magnetic fields is usually characterized in terms of the diffusion parallel and perpendicular to a large-scale (or mean) magnetic field. The nonlinear guiding center theory (NLGC) has been a prominent perpendicular diffusion theory. A recent version of this theory, based on random ballistic spreading of magnetic field lines and a backtracking correction (RBD/BC), has shown good agreement with test particle simulations for a two-component magnetic turbulence model. The aim of the present study is to test the generality of the improved theory by applying it to the noisy reduced magnetohydrodynamic (NRMHD) turbulence model, determining perpendicular diffusion coefficients that are compared with those from the field line random walk (FLRW) and unified nonlinear (UNLT) theories and our test particle simulations. The synthetic NRMHD turbulence model creates special conditions for energetic particle transport, with no magnetic fluctuations at higher parallel wavenumbers so there is no resonant parallel scattering if the particle Larmor radius $R_{\rm L}$ is even slightly smaller than the minimum resonant scale. This leads to non-monotonic variation in the parallel mean free path $λ_\parallel$ with $R_{\rm L}$. Among the theories considered, only RBD/BC matches simulations within a factor of two over the range of parameters considered. This accuracy is obtained even though the theory depends on $λ_\parallel$ and has no explicit dependence on $R_{\rm L}$. In addition, the UNLT theory often provides accurate results and even the FLRW limit provides a very simple and reasonable approximation in many cases.

physics.plasm-ph↗

arXiv2022

Sub-Alfvenic Solar Wind observed by PSP: Characterization of Turbulence, Anisotropy, Intermittency, and Switchback

In the lower solar coronal regions where the magnetic field is dominant, the Alfven speed is much higher than the wind speed. In contrast, the near-Earth solar wind is strongly super-Alfvenic, i.e., the wind speed greatly exceeds the Alfven speed. The transition between these regimes is classically described as the "Alfven point" but may in fact occur in a distributed Alfven critical region. NASA's Parker Solar Probe (PSP) mission has entered this region, as it follows a series of orbits that gradually approach more closely to the sun. During its 8th and 9th solar encounters, at a distance of 16 solar radii from the Sun, PSP sampled four extended periods in which the solar wind speed was measured to be smaller than the local Alfven speed. These are the first in-situ detections of sub-Alfvenic solar wind in the inner heliosphere by PSP. Here we explore properties of these samples of sub-Alfvenic solar wind, which may provide important previews of the physical processes operating at lower altitude. Specifically, we characterize the turbulence, anisotropy, intermittency, and directional switchback properties of these sub-Alfvenic winds and contrast these with the neighboring super-Alfvenic periods.

physics.space-ph↗

arXiv2021

PSP/IS$\odot$IS Observation of a Solar Energetic Particle Event Associated With a Streamer Blowout Coronal Mass Ejection During Encounter 6

In this paper we examine a low-energy SEP event observed by IS$\odot$IS's Energetic Particle Instrument-Low (EPI-Lo) inside 0.18 AU on September 30, 2020. This small SEP event has a very interesting time profile and ion composition. Our results show that the maximum energy and peak in intensity is observed mainly along the open radial magnetic field. The event shows velocity dispersion, and strong particle anisotropies are observed throughout the event showing that more particles are streaming outward from the Sun. We do not see a shock in the in-situ plasma or magnetic field data throughout the event. Heavy ions, such as O and Fe were detected in addition to protons and 4He, but without significant enhancements in 3He or energetic electrons. Our analysis shows that this event is associated with a slow streamer-blowout coronal mass ejection (SBO-CME) and the signatures of this small CME event are consistent with those typical of larger CME events. The time-intensity profile of this event shows that PSP encountered the western flank of the SBO-CME. The anisotropic and dispersive nature of this event in a shockless local plasma give indications that these particles are most likely accelerated remotely near the Sun by a weak shock or compression wave ahead of the SBO-CME. This event may represent direct observations of the source of low-energy SEP seed particle population.

astro-ph.SR↗

arXiv2021

Energy Dissipation in Turbulent Reconnection

We study the nature of pressure-strain interaction at reconnection sites, detected by NASA's Magnetospheric Multiscale (MMS) Mission. We employ data from a series of published case studies, including a large-scale reconnection event at the magnetopause, three small-scale reconnection events at the magnetosheath current sheets, and one example of the recently discovered electron-only reconnection. In all instances, we find that the pressure-strain shows signature of conversion into (or from) internal energy at the reconnection site. The electron heating rate is larger than the ion heating rate and the compressive heating is dominant over the incompressive heating rate in all cases considered. The magnitude of thermal energy conversion rate is close to the electromagnetic energy conversion rate in the reconnection region. Although in most cases the pressure-strain interaction indicates that the particle internal energy is increasing, in one case the internal energy is decreasing. These observations indicate that the pressure-strain interaction can be used as an independent measure of energy conversion and dynamics in reconnection regions, in particular independent of measures based on the electromagnetic work. Finally, we explore a selected reconnection site in a turbulent Particle-in-Cell (PIC) simulation which further supports the observational results.

physics.plasm-ph↗

arXiv2021

Suprathermal Ion Energy spectra and Anisotropies near the Heliospheric Current Sheet crossing observed by the Parker Solar Probe during Encounter 7

We present observations of >10-100 keV/nucleon suprathermal (ST) H, He, O, and Fe ions associated with crossings of the heliospheric current sheet (HCS) at radial distances <0.1 au from the Sun. Our key findings are: 1) very few heavy ions are detected during the 1st full crossing, the heavy ion intensities are reduced during the 2nd partial crossing and peak just after the 2nd crossing; 2) ion arrival times exhibit no velocity dispersion; 3) He pitch-angle distributions track the magnetic field polarity reversal and show up to ~10:1 anti-sunward, field-aligned flows and beams closer to the HCS that become nearly isotropic further from the HCS; 4) the He spectrum steepens either side of the HCS and the He, O, and Fe spectra exhibit power-laws of the form ~E^4-6; and 5) maximum energies EX increase with the ion's charge-to-mass (Q/M) ratio as EX/EH proportional to [(QX/MX)]^alpha where alpha~0.65-0.76, assuming that the average Q-states are similar to those measured in gradual and impulsive solar energetic particle events at 1 au. The absence of velocity dispersion in combination with strong field-aligned anisotropies closer to the HCS appears to rule out solar flares and near-sun coronal mass ejection-driven shocks. These new observations present challenges not only for mechanisms that employ direct parallel electric fields and organize maximum energies according to E/Q, but also for local diffusive and magnetic reconnection-driven acceleration models. Re-evaluation of our current understanding of the production and transport of energetic ions is necessary to understand this near-solar, current-sheet-associated population of ST ions.

astro-ph.SR↗

arXiv2021

Turbulent magneto-genesis in a collisionless plasma

We investigate an efficient mechanism for generating magnetic fields in turbulent, collisionless plasmas. By using fully kinetic, particle-in-cell simulations of an initially non-magnetized plasma, we inspect the genesis of magnetization, in a nonlinear regime. The complex motion is initiated via a Taylor-Green vortex, and the plasma locally develops strong electron temperature anisotropy, due to the strain tensor of the turbulent flow. Subsequently, in a domino effect, the anisotropy triggers a Weibel instability, localized in space. In such active wave-particle interaction regions, the magnetic field seed grows exponentially and spreads to larger scales due to the interaction with the underlying stirring motion. Such a self-feeding process might explain magneto-genesis in a variety of astrophysical plasmas, wherever turbulence is present.

physics.plasm-ph↗

arXiv2021

Parker Solar Probe Observations of Helical Structures as Boundaries for Energetic Particles

Energetic particle transport in the interplanetary medium is known to be affected by magnetic structures. It has been demonstrated for solar energetic particles in near-Earth orbit studies, and also for the more energetic cosmic rays. In this paper, we show observational evidence that intensity variations of solar energetic particles can be correlated with the occurrence of helical magnetic flux tubes and their boundaries. The analysis is carried out using data from Parker Solar Probe orbit 5, in the period 2020 May 24 to June 2. We use FIELDS magnetic field data and energetic particle measurements from the Integrated Science Investigation of the Sun (\isois) suite on the Parker Solar Probe. We identify magnetic flux ropes by employing a real-space evaluation of magnetic helicity, and their potential boundaries using the Partial Variance of Increments method. We find that energetic particles are either confined within or localized outside of helical flux tubes, suggesting that the latter act as transport boundaries for particles, consistent with previously developed viewpoints.

astro-ph.SR↗

arXiv2021

Dissipation measures in weakly-collisional plasmas

The physical foundations of the dissipation of energy and the associated heating in weakly collisional plasmas are poorly understood. Here, we compare and contrast several measures that have been used to characterize energy dissipation and kinetic-scale conversion in plasmas by means of a suite of kinetic numerical simulations describing both magnetic reconnection and decaying plasma turbulence. We adopt three different numerical codes that can also include interparticle collisions: the fully kinetic particle-in-cell VPIC, the fully kinetic continuum Gkeyll, and the Eulerian Hybrid Vlasov-Maxwell (HVM) code. We differentiate between (i) four energy-based parameters, whose definition is related to energy transfer in a fluid description of a plasma, and (ii) four distribution function-based parameters, requiring knowledge of the particle velocity distribution function. There is an overall agreement between the dissipation measures obtained in the PIC and continuum reconnection simulations, with slight differences due to the presence/absence of secondary islands in the two simulations. There are also many qualitative similarities between the signatures in the reconnection simulations and the self-consistent current sheets that form in turbulence, although the latter exhibits significant variations compared to the reconnection results. All the parameters confirm that dissipation occurs close to regions of intense magnetic stresses, thus exhibiting local correlation. The distribution function-based measures show a broader width compared to energy-based proxies, suggesting that energy transfer is co-localized at coherent structures, but can affect the particle distribution function in wider regions. The effect of interparticle collisions on these parameters is finally discussed.

physics.plasm-ph↗

arXiv2021

Sonic and Alfvenic Critical Point and the Solar Probe Mission: presentation to the Solar Probe Science and Technology Definition Team

Are critical points important in the Solar Probe Mission? This is a brief discussion of the nature of critical points in solar wind models, what this means physically in the 'real' solar wind, and what can be expected along a nominal Solar Probe Orbit. The conclusion is that the regions where the wind becomes transonic and trans-Alfvenic, which may be irregular and varying, may reveal interesting physics, but the mathematically defined critical points themselves are of less importance.

astro-ph.SR↗

arXiv2021

Magnetic reconnection as an energy cascade process

Reconnection and turbulence are two of the most commonly observed dynamical processes in plasmas, but their relationship is still not fully understood. Using 2.5D kinetic particle-in-cell simulations of both strong turbulence and reconnection, we compare the cross-scale transfer of energy in the two systems by analyzing the generalization of the von Kármán Howarth equations for Hall magnetohydrodynamics, a formulation that subsumes the third-order law for steady cascade rates. Even though the large scale features are quite different, the finding is that the decomposition of the energy transfer is structurally very similar in the two cases. In the reconnection case, the time evolution of the energy transfer also exhibits a correlation with the reconnection rate. These results provide explicit evidence that reconnection itself is fundamentally an energy cascade process.

physics.plasm-ph↗

arXiv2021

Solar Orbiter Observations of the Kelvin-Helmholtz Instability in the Solar Wind

The Kelvin-Helmholtz instability (KHI) is a nonlinear shear-driven instability that develops at the interface between shear flows in plasmas. KHI has been inferred in various astrophysical plasmas and has been observed in situ at the magnetospheric boundaries of solar-system planets and through remote sensing at the boundaries of coronal mass ejections. While it was hypothesized to play an important role in the mixing of plasmas and in triggering solar wind fluctuations, its direct and unambiguous observation in the solar wind was still lacking. We report in-situ observations of ongoing KHI in the solar wind using Solar Orbiter during its cruise phase. The KHI is found in a shear layer in the slow solar wind in the close vicinity of the Heliospheric Current Sheet, with properties satisfying linear theory for its development. An analysis is performed to derive the local configuration of the KHI. A 2-D MHD simulation is also set up with empirical values to test the stability of the shear layer. In addition, magnetic spectra of the KHI event are analyzed. We find that the observed conditions satisfy the KHI onset criterion from the linear theory analysis, and its development is further confirmed by the simulation. The current sheet geometry analyses are found to be consistent with KHI development. Additionally, we report observations of an ion jet consistent with magnetic reconnection at a compressed current sheet within the KHI interval. The KHI is found to excite magnetic and velocity fluctuations with power-law scalings that approximately follow $k^{-5/3}$ and $k^{-2.8}$ in the inertial and dissipation ranges, respectively. These observations provide robust evidence of KHI development in the solar wind. This sheds new light on the process of shear-driven turbulence as mediated by the KHI with implications for the driving of solar wind fluctuations.

astro-ph.SR↗