arXiv subjects

Terry Z. Liu

Publications and source records attributed to Terry Z. Liu.

At least 19 recordsLinked to original sources

arXiv2026

Evolution of lunar wake potentials: structure, energy conversion, and their imprints on velocity distributions

We study the evolution of electric potentials in the lunar wake. The wake potential exhibits two distinct spatial scales. The macroscopic scale arises from solar wind expansion into the vacuum, with a potential length-scale growing with distance from the Moon; the microscopic scales arises from ion acoustic shocks near the wake center, with transition layers spanning tens of local Debye lengths. This two-scale potential mediates energy conversion between ions and electrons during wake refilling. The macroscale potential retards electrons and accelerates ions to supersonic velocities, converting electron thermal energy to ion kinetic energy. The microscale potential then decelerates ions to subsonic velocities and heats both species, converting ion kinetic energy back to thermal energy. Together, the two-scale potential imprints distinct signatures on velocity distributions, including ion beams and electron flat-top distributions, consistent with ARTEMIS observations.

physics.space-ph↗

arXiv2026

Spatiotemporal Properties of Compressible Magnetohydrodynamic Turbulence from Space Plasma

Previous studies have established that a weak-to-strong transition occurs in Alfvenic magnetohydrodynamic (MHD) turbulence as energy cascades from large to small scales. However, the spatiotemporal (frequency-wavenumber) properties of compressible MHD turbulence involving all eigenmodes, which encode the strength of nonlinear interactions, remain difficult to characterize observationally. Consequently, whether a similar weak-to-strong transition occurs in compressible turbulence remains elusive. Using a novel multi-spacecraft, polarization-based mode-decomposition technique with measurements from the Cluster spacecraft in Earth's magnetosheath, we obtain spatiotemporal power spectra of all MHD eigenmodes and present the first quantitative assessment of nonlinear frequency broadening. Our results show that slow modes exhibit a weak-to-strong transition, evolving from wave-like peaks to frequency-broadened spectra as nonlinearity increases, whereas fast modes remain weakly turbulent with narrow peaks near their eigenfrequencies. Both Alfvenic and compressible fluctuations contribute significantly to low-frequency, large-scale quasi-two-dimensional structures. These findings provide a comprehensive observational characterization of compressible turbulence across mode composition, spatiotemporal scales, and weak-strong turbulence regimes, with implications for energetic particle transport, turbulent dynamos, plasma heating, and solar wind-magnetosphere coupling.

physics.plasm-ph↗

arXiv2026

Inferring lunar wake potentials from electron phase space densities

Inferring electric potentials from electron phase space density measurements in the lunar wake is complicated by two challenges: the asymmetry between the sunward and anti-sunward sides of the wake driven by the solar wind strahl, and the presence of ion acoustic shocks in the central wake. We develop the Hamiltonian inversion method, which infers the full spatial electric potential profile by exploiting the quasi-static Vlasov equilibrium condition $f = f(H)$, where $H$ is the electron Hamiltonian. The method addresses both challenges through a domain-decomposition strategy: on the two sides of the wake the potential is inferred independently by minimizing the misfit between the observed phase space density and a self-consistently reconstructed $f_\mathrm{interp}(\tilde{H})$, while in the central wake where flat-top trapped electron distributions are present the potential is inferred directly from the flat-top width. We validate the method against particle-in-cell simulation data at two evolutionary stages of the lunar wake: an early stage where strahl asymmetry is strong but no shocks have formed, and a later stage where ion acoustic shocks and flat-top distributions are present. We then apply the method to two ARTEMIS lunar wake crossings at the same evolutionary stages, inferring normalized potential drops of $eΔφ/T_e \sim 15$ and $\sim 5$ respectively and capturing shock-associated potential enhancements in the central wake. The method is broadly applicable to plasma environments where electrons are in quasi-static equilibrium with a field-aligned electric potential.

physics.space-ph↗

arXiv2025

Mode Composition Shapes Magnetic Anisotropy in Solar Wind Turbulence

Turbulence is a ubiquitous process that transfers energy across many spatial and temporal scales, thereby influencing particle transport and heating. Recent progress has improved our understanding of the anisotropy of turbulence with respect to the mean magnetic field; however, its exact form and implications for magnetic topology and energy transfer remain unclear. In this study, we investigate the nature of magnetic anisotropy in compressible magnetohydrodynamic (MHD) turbulence within low-$β$ solar wind using measurements from the Cluster spacecraft. By decomposing small-amplitude fluctuations into Alfvén and compressible modes, we reveal that magnetic anisotropy is largely mode dependent: Alfvenic fluctuations are broadly distributed in propagation angle, whereas compressible fluctuations are concentrated near the quasi-parallel (slab) direction, a feature closely linked to collisionless damping of compressible modes. For $β\rightarrow0$, compressible modes become dominant within the slab component at smaller scales. These findings advance our understanding of magnetic anisotropy in solar wind turbulence and offer a new perspective on the three-dimensional turbulence cascade, with broad implications for particle transport, acceleration, and magnetic reconnection.

astro-ph.SR↗

arXiv2025

The Role of Gyrating Ions in Reformation of a Quasi-parallel Supercritical Shock

Collisionless shocks in space and astrophysical plasmas mediate energy exchange between charged particles and fields in two or more plasma flows. In this study we analyze the evolution of ion distributions around a reformation cycle of a quasi-parallel shock. We use multi-point in-situ observations in the foreshock region of the Earths bow shock of a transient foreshock structure as it generates a shock. We find that backstreaming ions in the foreshock create a density and magnetic field depletion known as caviton which locally changes the shock geometry. Gyrating suprathermal ions that emerge within the caviton and reach the upstream edge of the core create a cross-field current imbalance that results in the nonlinear growth of a new shock layer. The new shock forms from the background foreshock fields over a distance of ~6 ion inertial lengths ($l_i$) and within 4.5 to 11.2 $l_i$ from the main bow shock. We find that plasma compression at the new thin shock layer is due to compactification of the cold upstream ion beam by high amplitude magnetic field-aligned electrostatic fields. At later stages, the plasma compression expands to form a new sheath.

physics.space-ph↗

arXiv2025

ARTEMIS observations of electrostatic shocks inside the lunar wake

When the solar wind encounters the Moon, a plasma void forms downstream of it, known as the lunar wake. In regions where the magnetic field is quasi-parallel to the plasma-vacuum boundary normal, plasma refills the wake primarily along magnetic field lines. As faster electrons outpace slower ions, an ambipolar electric field is generated, accelerating ions and decelerating electrons. Recent particle-in-cell simulations have shown that when accelerated supersonic ion beams from opposite sides of the wake meet near the wake center, electrostatic shocks may form, decelerating ions and heating electrons into flat-top velocity distributions. Using data from the Acceleration, Reconnection, Turbulence and Electrodynamics of the Moon's Interaction with the Sun (ARTEMIS) spacecraft, we present the first observational evidence of the predicted electrostatic shocks. Near the wake center of one event, we observed an electrostatic solitary structure with an amplitude of ~2 mV/m and a spatial scale of ~50 local Debye lengths. This structure generated a potential increase of ~50 V from upstream to downstream, heating incoming electrons by ~50 eV in the parallel direction while decelerating ions by ~60 km/s leading to a density enhancement. At a second event representing a more evolved stage, we observed more dissipated structures dominated by strong electrostatic waves, with persistent potential increases driving continued field-aligned electron heating and ion deceleration. These observations confirm simulation predictions of electrostatic shock formation and the associated particle dynamics within the lunar wake, with potential applications to understanding plasma interactions around other airless celestial bodies.

physics.space-ph↗

arXiv2025

Plasma refilling of the lunar wake: plasma-vacuum interactions, electrostatic shocks, and electromagnetic instabilities

A plasma void forms downstream of the Moon when the solar wind impacts the lunar surface. This void gradually refills as the solar wind passes by, forming the lunar wake. We investigate this refilling process using a fully kinetic particle-in-cell (PIC) simulation. The early stage of refilling follows plasma-vacuum interaction theory, characterized by exponential decay of plasma density into the wake, along with ion acceleration and cooling in the expansion direction. Our PIC simulation confirms these theoretical predictions. In the next stage of the refilling process, the counter-streaming supersonic ion beams collide, generating Debye-scale electrostatic shocks at the wake's center. These shocks decelerate and thermalize the ion beams while heating electrons into flat-top velocity distributions along magnetic field lines. Additionally, fast magnetosonic waves undergo convective growth via anomalous cyclotron resonance as they co-propagate with temperature-anisotropic ion beams toward the wake's center. Electromagnetic ion cyclotron waves may also be excited through normal cyclotron resonance, counter-propagating with these anisotropic ion beams. Our findings provide new insights into the kinetic aspects of lunar wake refilling and may enhance interpretation of spacecraft observations.

physics.space-ph↗

arXiv2025

Observations of Turbulence and Particle Transport at Interplanetary Shocks: Transition of Transport Regimes

The transport of energetic particles is intimately related to the properties of plasma turbulence, a ubiquitous dynamic process that transfers energy across a broad range of spatial and temporal scales. However, the mechanisms governing the interactions between plasma turbulence and energetic particles remain incompletely understood. Here we present comprehensive observations from the upstream region of a quasi-perpendicular interplanetary (IP) shock on 2004 January 22, using data from four Cluster spacecraft to investigate the interplay between turbulence dynamics and energetic particle transport. Our observations reveal a transition in energetic proton fluxes from exponential to power-law decay with increasing distance from the IP shock. This result provides possible observational evidence of a shift in transport behavior from normal diffusion to superdiffusion. This transition correlates with an increase in the time ratio from $τ_s/τ_{c}<1$ to $τ_s/τ_{c}\gg1$, where $τ_s$ is the proton isotropization time, and $τ_{c}$ is the turbulence correlation time. Additionally, the frequency-wavenumber distributions of magnetic energy in the power-law decay zone indicate that energetic particles excite linear Alfvén-like harmonic waves through gyroresonance, thereby modulating the original turbulence structure. These findings provide valuable insights for future studies on the propagation and acceleration of energetic particles in turbulent astrophysical and space plasma systems.

physics.plasm-ph↗

arXiv2024

Cross-scale energy transfer from fluid-scale Alfvén waves to kinetic-scale ion acoustic waves in the Earth's magnetopause boundary layer

In space plasmas, large-amplitude Alfvén waves can drive compressive perturbations, accelerate ion beams, and lead to plasma heating and the excitation of ion acoustic waves at kinetic scales. This energy channelling from fluid to kinetic scales represents a complementary path to the classical turbulent cascade. Here, we present observational and computational evidence to validate this hypothesis by simultaneously resolving the fluid-scale Alfvén waves, kinetic-scale ion acoustic waves, and their imprints on ion velocity distributions in the Earth's magnetopause boundary layer. We show that two coexisting compressive modes, driven by the magnetic pressure gradients of Alfvén waves, not only accelerate the ion tail population to the Alfvén velocity, but also heat the ion core population near the ion acoustic velocity and generate Debye-scale ion acoustic waves. Thus, Alfvén-acoustic energy channeling emerges as a viable mechanism for plasma heating near plasma boundaries where large-amplitude Alfvén waves are present.

physics.space-ph↗

arXiv2024

Multi-Mission Observations of Relativistic Electrons and High-Speed Jets Linked to Shock Generated Transients

Shock-generated transients, such as hot flow anomalies (HFAs), upstream of planetary bow shocks, play a critical role in electron acceleration. Using multi-mission data from NASA's Magnetospheric Multiscale (MMS) and ESA's Cluster missions, we demonstrate the transmission of HFAs through Earth's quasi-parallel bow shock, associated with acceleration of electrons up to relativistic energies. Energetic electrons, initially accelerated upstream, are shown to remain broadly confined within the transmitted transient structures downstream, where betatron acceleration further boosts their energy due to elevated compression levels. Additionally, high-speed jets form at the compressive edges of HFAs, exhibiting a significant increase in dynamic pressure and potentially contributing to driving further localized compression. Our findings emphasize the efficiency of quasi-parallel shocks in driving particle acceleration far beyond the immediate shock transition region, expanding the acceleration region to a larger spatial domain. Finally, this study underscores the importance of multi-scale observational approach in understanding the convoluted processes behind collisionless shock physics and their broader implications.

astro-ph.HE↗

arXiv2024

Transient Upstream Mesoscale Structures: Drivers of Solar-Quiet Space Weather

In recent years, it has become increasingly clear that space weather disturbances can be triggered by transient upstream mesoscale structures (TUMS), independently of the occurrence of large-scale solar wind (SW) structures, such as interplanetary coronal mass ejections and stream interaction regions. Different types of magnetospheric pulsations, transient perturbations of the geomagnetic field and auroral structures are often observed during times when SW monitors indicate quiet conditions, and have been found to be associated to TUMS. In this mini-review we describe the space weather phenomena that have been associated with four of the largest-scale and the most energetic TUMS, namely hot flow anomalies, foreshock bubbles, travelling foreshocks and foreshock compressional boundaries. The space weather phenomena associated with TUMS tend to be more localized and less intense compared to geomagnetic storms. However, the quiet time space weather may occur more often since, especially during solar minima, quiet SW periods prevail over the perturbed times.

astro-ph.SR↗

arXiv2024

Interaction of the Prominence Plasma within the Magnetic Cloud of an ICME with the Earth's Bow Shock

The magnetic cloud within an interplanetary coronal mass ejection (ICME) is characterized by high magnetic field intensities. In this study, we investigate the interaction of a magnetic cloud carrying a density structure with the Earth's bow shock during the ICME event on 24 April 2023. Elevated abundances of cold protons and heavier ions, namely alpha particles and singly charged helium ions, associated with the prominence plasma are observed within this structure. The plasma downstream of the bow shock exhibits an irregular compression pattern which could be due to the presence of heavy ions. Heavy ions carry a significant fraction of the upstream flow energy; however, due to their different charge per mass ratio and rigidity, they are less scattered by the electromagnetic and electrostatic waves at the shock. We find that downstream of the shock, while the thermal ion energy is only a small fraction of the background magnetic energy density, nevertheless increased ion fluxes reduce the characteristic wave speeds in the that region. As such, we observe a transition state of an unstable bow shock layer across which the plasma flow is super Alfvénic in both upstream and downstream regions. Our findings help with understanding the intense space weather impacts of such events.

physics.space-ph↗

arXiv2024

Identification of the weak-to-strong transition in Alfvénic turbulence from space plasma

Plasma turbulence is a ubiquitous dynamical process that transfers energy across many spatial and temporal scales in astrophysical and space plasma systems. Although the theory of anisotropic magnetohydrodynamic (MHD) turbulence has successfully described phenomena in nature, its core prediction of an Alfvenic transition from weak to strong MHD turbulence when energy cascades from large to small scales has not been observationally confirmed. Here we report the first observational evidence for the Alfvenic weak-to-strong transition in MHD turbulence in the terrestrial magnetosheath using the four Cluster spacecraft. The observed transition indicates the universal existence of strong turbulence regardless of the initial level of MHD fluctuations. Moreover, the observations demonstrate that the nonlinear interactions of MHD turbulence play a crucial role in the energy cascade, widening the directions of the energy cascade and broadening the fluctuating frequencies. Our work takes a critical step toward understanding the complete picture of turbulence cascade, connecting the weak and strong MHD turbulence systems. It will have broad implications in star formation, energetic particle transport, turbulent dynamo, and solar corona or solar wind heating.

astro-ph.SR↗

arXiv2024

Small-amplitude Compressible Magnetohydrodynamic Turbulence Modulated by Collisionless Damping in Earth's Magnetosheath: Observation Matches Theory

Plasma turbulence is a ubiquitous dynamical process that transfers energy across many spatial and temporal scales and affects energetic particle transport. Recent advances in the understanding of compressible magnetohydrodynamic (MHD) turbulence demonstrate the important role of damping in shaping energy distributions on small scales, yet its observational evidence is still lacking. This study provides the first observational evidence of substantial collisionless damping (CD) modulation on small-amplitude compressible MHD turbulence cascade in Earth's magnetosheath using four Cluster spacecraft. Based on an improved compressible MHD decomposition algorithm, turbulence is decomposed into three eigenmodes: incompressible Alfvén modes, and compressible slow and fast (magnetosonic) modes. Our observations demonstrate that CD enhances the anisotropy of compressible MHD modes because CD has a strong dependence on wave propagation angle. The wavenumber distributions of slow modes are mainly stretched perpendicular to the background magnetic field ($\mathbf{B_0}$) and weakly modulated by CD. In contrast, fast modes are subjected to a more significant CD modulation. Fast modes exhibit a weak, scale-independent anisotropy above the CD truncation scale. Below the CD truncation scale, the anisotropy of fast modes enhances as wavenumbers increase. As a result, fast mode fractions in the total energy of compressible modes decrease with the increase of perpendicular wavenumber (to $\mathbf{B_0}$) or wave propagation angle. Our findings reveal how the turbulence cascade is shaped by CD and its consequences to anisotropies in the space environment.

astro-ph.SR↗

arXiv2022

Multi-spacecraft Analysis of the Properties of Magnetohydrodynamic Fluctuations in Sub-Alfvénic Solar Wind Turbulence at 1 AU

We present three-dimensional magnetic power spectra in wavevector space to investigate anisotropy and scalings of sub-Alfvénic solar wind turbulence at magnetohydrodynamic (MHD) scale using the Magnetospheric Multiscale spacecraft. The magnetic power distributions are organized in a new coordinate determined by wavevectors (k) and background magnetic field ($b_0$) in Fourier space. This study utilizes two approaches to determine wavevectors: singular value decomposition method and timing analysis. The combination of the two methods allows an examination of magnetic field properties in terms of mode compositions without any spatiotemporal hypothesis. Observations show that fluctuations ($δB_{\perp1}$) in the direction perpendicular to k and $b_0$ prominently cascade perpendicular to $b_0$, and such anisotropy increases with wavenumber. The reduced power spectra of $δB_{\perp1}$ follow Goldreich-Sridhar scalings: $P(k_\perp)\sim k_\perp^{-5/3}$ and $P(k_{||}) \sim k_{||}^{-2}$. In contrast, fluctuations within $kb_0$ plane show isotropic behaviors: perpendicular power distributions are approximately the same as parallel distributions. The reduced power spectra of fluctuations within $kb_0$ plane follow the scalings: $P(k_\perp)\sim k_\perp^{-3/2}$ and $P(k_{||})\sim k_{||}^{-3/2}$. Comparing frequency-wavevector spectra with theoretical dispersion relations of MHD modes, we find that $δB_{\perp1}$ are probably associated with Alfven modes. Moreover, for the Alfvénic component, the ratio of cascading time to the wave period is found to be a factor of a few, consistent with critical balance in the strong turbulence regime. The magnetic field fluctuations within $kb_0$ plane more likely originate from fast modes based on isotropic behaviors.

astro-ph.SR↗

arXiv2021

Analysis of Magnetohydrodynamic Perturbations in Radial-field Solar Wind from Parker Solar Probe Observations

We report analysis of sub-Alfvénic magnetohydrodynamic (MHD) perturbations in the low-\b{eta} radial-field solar wind using the Parker Solar Probe spacecraft data from 31 October to 12 November 2018. We calculate wave vectors using the singular value decomposition method and separate the MHD perturbations into three types of linear eigenmodes (Alfvén, fast, and slow modes) to explore the properties of the sub-Alfvénic perturbations and the role of compressible perturbations in solar wind heating. The MHD perturbations there show a high degree of Alfvénicity in the radial-field solar wind, with the energy fraction of Alfvén modes dominating (~45%-83%) over those of fast modes (~16%-43%) and slow modes (~1%-19%). We present a detailed analysis of a representative event on 10 November 2018. Observations show that fast modes dominate magnetic compressibility, whereas slow modes dominate density compressibility. The energy damping rate of compressible modes is comparable to the heating rate, suggesting the collisionless damping of compressible modes could be significant for solar wind heating. These results are valuable for further studies of the imbalanced turbulence near the Sun and possible heating effects of compressible modes at MHD scales in low-\b{eta} plasma.

astro-ph.SR↗

arXiv2021

Observations of an Electron-cold Ion Component Reconnection at the Edge of an Ion-scale Antiparallel Reconnection at the Dayside Magnetopause

Solar wind parameters play a dominant role in reconnection rate, which controls the solar wind-magnetosphere coupling efficiency at Earth's magnetopause. Besides, low-energy ions from the ionosphere, frequently detected on the magnetospheric side of the magnetopause, also affect magnetic reconnection. However, the specific role of low-energy ions in reconnection is still an open question under active discussion. In the present work, we report in situ observations of a multiscale, multi-type magnetopause reconnection in the presence of low-energy ions using NASA's Magnetospheric Multiscale data on 11 September 2015. This study divides ions into cold and hot populations. The observations can be interpreted as a secondary reconnection dominated by electrons and cold ions located at the edge of an ion-scale reconnection. This analysis demonstrates a dominant role of cold ions in the secondary reconnection without hot ions' response. Cold ions and electrons are accelerated and heated by the secondary process. The case study provides observational evidence for the simultaneous operation of antiparallel and component reconnection. Our results imply that the pre-accelerated and heated cold ions and electrons in the secondary reconnection may participate in the primary ion-scale reconnection affecting the solar wind-magnetopause coupling and the complicated magnetic field topology affect the reconnection rate.

physics.space-ph↗

arXiv2020

Magnetospheric Multiscale (MMS) observations of foreshock transients at their very early stage

Foreshock transients are ion kinetic structures in the ion foreshock. Due to their dynamic pressure perturbations, they can disturb the bow shock and magnetosphere-ionosphere system. They can also accelerate particles contributing to shock acceleration. However, it is still unclear how exactly they form. Recent particle-in-cell simulations point out the important role of electric field and Hall current in the formation process. To further examine this, we use data from the Magnetospheric Multiscale (MMS) mission to apply case studies on two small (1000-2000 km) foreshock transient events that just started to form. In event 1 where MMS were in a tetrahedral formation, we show that the current density configuration, which determined the magnetic field profile, was mainly driven by Hall currents generated by demagnetized foreshock ions. The resulting time variation of the magnetic field induced electric field that drove cold plasma moving outward with magnetic field lines. In event 2 where MMS were in a string-of-pearls formation, we analyze the evolution of field and plasma parameters. We show that the magnetic flux and mass flux were transported outward from the core resulting in the steepening of the boundary. The steepened boundary, which trapped more foreshock ions and caused stronger demagnetization of foreshock ions, nonlinearly further enhanced the Hall current. Based on our observations, we propose a physical formation process that the positive feedback of foreshock ions on the varying magnetic field caused by the foreshock ion Hall current enables an instability and the growth of the structure.

physics.space-ph↗