Session UO07: Plasma Astrophysics III

Understanding the Kinetic Physics of Particle Energization at Quasiperpendicular Collisionless Shocks Using the Field-Particle Correlation Technique

Collisionless shocks play an important role in the conversion of supersonic flow energy to thermal energy at important boundaries in the heliosphere, such as at planetary bow shocks, the termination shock in the outer heliosphere, and interplanetary shocks propagating through the solar wind. In addition, collisionless shocks can lead to the acceleration of a small fraction of particles to high energy.  Many of these energization mechanisms remain poorly understood, but kinetic simulations and spacecraft observations present valuable opportunities to improve our understanding of the fundamental kinetic physics.   The recently developed field-particle correlation technique was devised to identify and characterize the mechanisms that energize particles in the six-dimensional (3D-3V) phase space of kinetic plasmas---such mechanisms underlie the fundamental plasma processes of kinetic turbulence, collisionless magnetic reconnection, collisionless shocks, and kinetic instabilities.  Here we present an overview of how the field-particle correlation method can be applied to gain deeper insight into the kinetic plasma processes that govern how particles are energized at collisionless shocks.  Requiring only single-point measurements in space, the technique can be used to identify well-known acceleration mechanisms, such as shock drift acceleration and shock surfing acceleration.  In addition, it shows promise to be able to separate the energization mediated by micro-instabilities arising in the shock transition from that due to the macroscopic shock fields.   

Characterizing Solar Wind Electron Energization Signatures Using the Field-Particle Correlation Technique

In this project we seek to characterize the field-particle correlation signature of electron Landau damping using gyrokinetic simulations of the dissipation of plasma turbulence. The field-particle correlation technique uses single point measurements of the distribution function and electromagnetic fields to reveal mechanism-specific signatures of changes in phase space energy density, making it applicable to the study of energization mechanisms in both simulated and in situ data. The signature of electron Landau damping of dispersive kinetic Alfvén waves is particularly worthy of study, as this mechanism may be a key player in the dissipation of solar wind turbulence. We create 3D-2V simulations of Alfvénic solar wind turbulence with a range of plasma beta in order to characterize how the phase space energization signature of this mechanism may vary throughout the heliosphere. Additionally, we confirm the prediction that the field-particle correlation technique should be able to recover energization signatures of electron Landau damping even from undersampled spacecraft measurements, where an instrument’s Nyquist frequency is below the frequency of the wave undergoing collisionless damping.    

Excitation of whistler waves via nonlinear scattering in the ion ring instability

An ion ring velocity distribution is unstable and excites lower hybrid waves. When these waves reach sufficient amplitude, nonlinear scattering of lower hybrid waves can excite whistler and magnetosonic waves and contribute to saturation of the lower hybrid instability. Ring distributions, and subsequent instabilities and waves, are relevant to multiple areas of space plasma physics. We present results from 3D electromagnetic particle-in-cell simulations of the nonlinear evolution of the ion ring instability, including the nonlinear excitation of long-wavelength electromagnetic waves. In the past, there has been some question as to 1) the conversion efficiency from kinetic energy in the ring to electromagnetic waves, and 2) the mechanism responsible for electromagnetic waves (three wave coupling or nonlinear scattering). From these simulations, we account for energy flow from ring to electrostatic waves to electromagnetic waves, including heating of the background plasma and ring via quasilinear and nonlinear stochastic processes. We also show multiple types of evidence from these simulations that nonlinear scattering is responsible for the long-wavelength electromagnetic waves. Finally, we discuss how this analysis affects ring distributions in natural space phenomena.   

Nonlinear whistler wave generation, in low beta plasmas, by induced scattering: 2D PIC simulations.*

We present recent results on whistler wave generation by nonlinear induced scattering. Nonlinear induced scattering (NLIS) is a process that allows transfer of energy from one unstable mode ω₁ into another mode ω₂ and the particles that can satisfy the nonlinear Landau resonant condition. In our particular example, the generation of whistler waves through NLIS is achieved via lower-hybrid (LH) beat-wave coupling with thermal particles. The LH waves are first generated by a cold but energetic ring ion distribution that is unstable to these waves [1]. The nonlinear induced scattering acts as a saturation mechanism for the linearly unstable LH modes. We show for the first time, in a PIC simulation, how the damped nonlinear density perturbations associated with the beat-wave coupling, a.k.a. quasi-modes, arise in the nonlinear induced scattering of LH/whistler waves. This fundamental, and interesting, nonlinear mechanism is at the heart of the upcoming SMART experiment where LH electrostatic waves will be converted to whistler waves via NLIS [2]. Although NLIS is fastest in a realistic three-dimensional (3D) setup, a slower version of the phenomenon survives in 2D and thus it can be investigated with a careful choice of simulation parameters. We present 2D PIC simulations with parameters close to those found in the low beta plasma β<10^-3 environment of Earth’s ionosphere at  500 km of altitude --where the SMART experiment will be performed. However, we point out that nonlinear induced scattering is a universal phenomenon in turbulent plasmas.

    

Neutral-charged-particle Collisions as the Mechanism for Accretion Disk Angular Momentum Transport

The matter in an accretion disk must lose angular momentum when moving radially inwards but how this works has long been a mystery. By calculating the trajectories of individual colliding neutrals, ions, and electrons in a weakly ionized 2D plasma containing gravitational and magnetic fields, we numerically simulate accretion disk dynamics at the particle level [1]. As predicted by Lagrangian mechanics, the fundamental conserved global quantity is the total canonical angular momentum, not the ordinary angular momentum. When the Kepler angular velocity and the magnetic field have opposite polarity, collisions between neutrals and charged particles cause: (i) ions to move radially inwards, (ii) electrons to move radially outwards, (iii) neutrals to lose ordinary angular momentum, and (iv) charged particles to gain canonical angular momentum. Neutrals thus spiral inward due to their decrease of ordinary angular momentum while the accumulation of ions at small radius and accumulation of electrons at large radius produces a radially outward electric field. In 3D, this radial electric field would drive an out-of-plane poloidal current that produces the magnetic forces that drive bidirectional astrophysical jets. Because this neutral angular momentum loss depends only on neutrals colliding with charged particles, it should be ubiquitous. Quantitative scaling of the model using plausible disk density, temperature, and magnetic field strength gives an accretion rate of 3 × 10⁻⁸ solar mass per year, which is in good agreement with observed accretion rates.

[1] Y. Zhang and P. M. Bellan 2022 ApJ 930 167

    

Origin of realistic magnetized cold neutral media in multiphase interstellar media : A simulation perspective

The cold neutral media (CNM) is perhaps the most important astrophysical observable in the last decade. Observations like HI4PI, GALFA and FAST have advanced our understanding of CNM, including its spatial distribution, alignment to B-field and its connection to underlying molecular phase. However, why fundamentally HI emission map is highly filamentary along B-field is still a mystery. Different proposals such as the density perturbations, MHD turbulence and reconnection are proposed recently. However, simulations suggest that CNM are significantly shorter in length and less aligned to B-field. In this talk, we demonstrate our recent high-resolution simulations on how CNM forms and its physical properties both spatially and dynamically. In particular, we showed that (1) CNM filaments are subjected to the bounding forces from the warmer, unstable phases, which restricts its aspect ratio to half of the GS95 estimate. (2) Despite the CNM does not follow the GS95 cascade, statistically CNM still follows the Kolmogorov cascade. (3) The high density CNM features that are perpendicular to the magnetic field lines are the progenitors of the star formation sites. This work will update the lack of understanding of UNM since the proposal of two-phase model by Mckee & Ostriker (1977).

    

A hydrodynamic mechanism for hot spot formation in the circumstellar ring of SN1987A

Since the light from supernova 1987A (SN1987A) first reached Earth, the evolution of the dying star has been the subject of intense study. In particular, several theories have been proposed to explain the formation of hot spots, or accumulations of mass, along the ring of gas surrounding the supernova origin. In this study, we assess the viability of a hydrodynamic mechanism related to the stability of interacting vortex cores (Crow instability) in explaining the formation of the mass accumulations. Perturbations along the circular cores, which would have formed when the progenitor star emitted the ring of gas approximately twenty-thousand years prior to the supernova, grow under the influence of their self- and mutually induced velocity fields. Our analysis predicts a dominant instability wavenumber consistent with the number of observed hot spots surrounding SN1987A, with important implications for nebula formation following supernovae.   

Formation and Ejection of Double-Helix Plasma Structures from Gravitational Wave Emitters

Double-helix plasma structures have been identified and shown to form in and propagate away from the time dependent plasma configurations in which Black Hole binaries can be imbedded [1]. These structures are envisioned to extend up to plasma regions where they can be disrupted. By now experimental observations on the termination of jets have found that they can involve double-helix magnetic topologies [2]. Theoretically, these structures are found to emerge as non­linearly coupled torsional ion sound waves which, in the presence of a background magnetic field, in both the formation and terminal plasmas generate helical magnetic field configurations while remaining nearly ``electrostatic'' [1] in regions where no significant background magnetic field is present. These (double-helix) structures corotate with the binary and can propagate independently in either of the two vertical directions. The coupling involves Intrinsic Gravitational Modes [3] originating in the circumbinary disk and Inner Gravitational Fluctuations emerging from the Swept (Toroidal) Regions [1] traced by one or both Black Holes.

[1] B. Coppi, Invited Papers: XVI M. Grossmann Conf., July 2021, and Asia Pacific Phys. Soc. Conf., Oct. 2021.

[2] A. Pagetto et al., Ap J. Letters, 923: L5 (2021).

[3] B. Coppi, Pl. Phys. Rep. 47, 9 (2021).

    

Particle and Radiation Emission Processes Associated with Plasma Geometry and Gravitational Waves Sources

The problem of understanding the coexistence of gravitational waves emission with that of plasma waves was raised originally without foreseeing [1,2] the presence of low frequency modes, involving the geometry of the plasma structures surrounding gravitational wave emitters. In fact, a class of these (modes) may transfer energy from high energy electron populations to low energy particle populations and prevent the emission of detectable radiation. Two sources of fluctuations are identified: one related to the time dependent gravitational potential produced by a BH binary with a frequency equal or twice the binary rotation frequency and the other one, generating high harmonics, related to the Pulsed Accretion associated with the Swept Torus, carved within the circumbinary disk, by one or both Black Holes. Introducing a magnetic field, to be consistent with the features of observed radiation emission, leads to find torsional modes involving helical magnetic field structures that can propagate away from the circumbinary disk as nonlinearly coupled modes to form a double-helix configuration [3]. Two representative binary cases have been considered: one with two equal masses BH’s and the other with a large BH and a planet BH.

 

[1] Y.B. Zeldovich, Zh. Eksp. Tear. Fiz. 65, 1311 (1973).

[2] B. Coppi and Y.B. Zeldovich, Enciclopedia del Novecento (1980),

[3] B. Coppi, Invited Papers: XVI M. Grossmann Conf., July 2021, and Asia Pacific Phys. Soc. Conf., Oct. 2021.

    

Collective effects for dispersive gravitational waves in plasma

Interaction of gravitational waves (GWs) with matter could be important, for example, in the early Universe, and in the vicinity of compact GW sources where the enormous amount of energy stored in the GWs can make even a weak coupling significant. The standard approach to studying these effects is to solve Einstein--Vlasov equations, but it has proven to be prohibitively cumbersome and typically involves oversimplifications. We use an alternative, variational formulation [PRD 102, 064012 (2020); arxiv:2106.05062] to derive the gauge-invariant (GI) wave equations for collective oscillations of the self-consistent metric. We also show how to find the GI part of the metric perturbation in an arbitrary background metric (arXiv:2105.04680), and propose their GI adiabatic quasilinear theory and geometrical optics (arXiv:2106.05062; arxiv:2201.08562, to appear in JPP). This forms a foundation for describing GW--plasma interactions rigorously. For example, we show that the kinetic Jeans instability can be subsumed as a collective GW mode with a peculiar polarization, which is derived from the dispersion matrix rather than assumed a priori as usual (arXiv:2204.09095, to appear in JCAP). We also briefly discuss possible future directions and applications of this general formalism.   

``Alternator'' Involving Reconnected Magnetic Field Structures in the Presence of Electron Temperature and Density Gradients

The generation of magnetic field structures over macroscopic scale distances is an issue of well recognized importance in astrophysics and for laboratory experiments. A relevant process is identified [1] involving a plasma, imbedded in a pre-existing sheared magnetic structure, with a significant gradient of the longitudinal (to the stationary component of the magnetic field) electron temperature. A periodic emergence of a reconnected magnetic structure is the basic feature of this process that is likened to an ``alternator''. The oscillations amplitude can be amplified in the presence of a density gradient (aligned with the electron temperature gradient) and a non-negligible particle diffusion. Regimes where the longitudinal electron thermal conductivities are large relative to the associated transverse conductivities are considered, a two-fluid description being adopted as a start. The introduced process differs substantially from the stationary Biermann Battery concept that depends on misaligned.

[1] B. Coppi and B. Basu, Phys. Lett. A 397, 127265 (2021).