Magnetosonic Pulse Research Articles

High Mach Number Quasi‐Perpendicular Shocks: Spatial Versus Temporal Structure

AbstractPlanetary bow shocks are a natural laboratory for investigating the physics of collisionless shocks. Given that spacecraft observations provide us with information regarding the temporal structure of the shock, often multi‐spacecraft missions and kinetic simulations are needed to discern the nature of the dissipation processes at the shock and the resulting spatial structure. Such an approach through the ISEE mission and 1.5‐D hybrid (kinetic ions and fluid electrons) simulations led to an understanding of supercritical quasi‐perpendicular shocks. In such shocks, the reflection of a fraction of incident ions and their gyration back into the shock is part of the dissipation process resulting in the formation of a foot, ramp, overshoot, and undershoot. More recently, observations of high Mach number quasi‐perpendicular shocks show a seemingly turbulent structure in time series data. In this study, we use the results of 2.5‐D global hybrid simulation of a high Mach number quasi‐perpendicular shock to demonstrate the major differences between the spatial and temporal structure of such shocks. The spatial structure of the shock is the same as that of supercritical shocks while the temporal structure consists of fast magnetosonic pulses. The reason for this difference is attributed to the generation of surface waves at the shock which results in partial encounters between the shock and spacecraft. The surface waves are shown to be generated by large velocity shears present at the shock due to ion reflection and gyration into the upstream.

Variations in the Pickup Ion Density Structure in Response to the Growth of the Kelvin–Helmholtz Instability along the Heliopause

Abstract Features of the response of pickup ions (PUIs) to the Kelvin–Helmholtz instability (KHI) on the heliopause (HP) are examined by means of two-dimensional hybrid simulations. We assume the supersonic neutral solar wind as the source of PUIs gyrating about the magnetic field in the outer heliosheath. These PUIs become energetic neutral atoms (ENAs) via charge exchange with interstellar hydrogen, and a portion of these ENAs are detected by spacecraft such as the Interstellar Boundary Explorer (IBEX). To evaluate the possibility of identifying the KHI on the HP from ENA observations, we assume that an imprint of the KHI may be displayed in spatial and temporal variations in the observed ENA profile. As an alternative to ENA, the column density of PUIs integrated across the HP is calculated. The KH-inducing vortex forces not only background protons but also PUIs to roll up deep in the inner heliosheath. The KH vortex also results in the emission of magnetosonic pulses that sweep PUIs in the outer heliosheath and lead to their local confinement. These effects elongate the PUIs’ spatial distribution in the direction normal to the HP. The appearance of the local confined structure in the PUIs’ column density is consequently confirmed, and this feature can be confirmed as the KHI evolution. Although the simulation cannot be quantitatively compared with the observations currently available because its resolution is too low, we expect that the derived properties will be useful for diagnosing the nature of HP fluctuation in future missions.

Energy and momentum conservation upon reflection of a solitary pulse in a bounded magnetized plasma

When the nature of a magnetosonic pulse propagating in a bounded magnetized plasma slab is successively transformed from compression to rarefaction and vice versa upon reflection at a plasma-vacuum interface, both the energy and the longitudinal electromagnetic (EM) momentum of the plasma-pulse system are found to oscillate between two states. Simple analytical models and particle-in-cell simulations show that these oscillations are associated with EM radiation to and from the surrounding magnetized vacuum. For partial reflection supplemental losses in total pulse energy and mechanical momentum are identified and shown to follow, respectively, Fresnel's transmission coefficients for the energy and the magnetic perturbation. This mechanical momentum loss upon partial reflection is traced to the momentarily nonzero volume-integrated Lorentz force, which in turn supports that mechanical and EM momentum transfers are, respectively, associated with the magnetic and electric parts of the momentum flux density.

Understanding Electron Dropout Echoes Induced by Interplanetary Shocks: Test Particle Simulations

AbstractRecently, interplanetary shocks have been reported to cause “electron dropout echoes” in the outer radiation belt, which is manifested as repeated dropout and recovery signals in electron fluxes. Both previous case and statistical studies have shown that electron dropout echoes are mostly found for high‐energy (>300 keV) electrons, and the initial dropout region is mainly located at the dusk magnetosphere, regardless of shock parameters such as shock normal. To understand these properties, we model the electron dropout echoes at geosynchronous orbit by tracing electrons in the analytic field model of the shock‐induced propagating pulse. It is shown that the characteristics of shock‐induced electron dropout echo events including energy dependence and localization are well reproduced by our model. By analyzing the trajectories of typical electrons, we find that electrons are inward transported and accelerated through “drift‐resonance‐like” interactions with the magnetosonic pulse. Two causes of the dawn‐dusk asymmetric response are presented: (1) the difference between the interaction time of electrons with the magnetosonic pulse and (2) the opposite radial ∇B drift of the electrons at dawnside and duskside. Further, we calculate the contributions to electron dynamics and phase space density variations from three terms: E×B drift, radial ∇B drift, and gyrobetatron acceleration. The details of electron flux variations could vary with the form of the shock‐induced pulse and the initial electron distribution, thus be different from our results; however, the basic ingredients of the electron interaction with the pulse could provide a general frame for understanding and evaluating electron flux responses.

Magnitude of parallel pseudo potential in a magnetosonic shock wave

The parallel pseudo potential F, which is the integral of the parallel electric field along the magnetic field, in a large-amplitude magnetosonic pulse (shock wave) is theoretically studied. Particle simulations revealed in the late 1990's that the product of the elementary charge and F can be much larger than the electron temperature in shock waves, i.e., the parallel electric field can be quite strong. However, no theory was presented for this unexpected result. This paper first revisits the small-amplitude theory for F and then investigates the parallel pseudo potential F in large-amplitude pulses based on the two-fluid model with finite thermal pressures. It is found that the magnitude of F in a shock wave is determined by the wave amplitude, the electron temperature, and the kinetic energy of an ion moving with the Alfvén speed. This theoretically obtained expression for F is nearly identical to the empirical relation for F discovered in the previous simulation work.

Particle reflection along the magnetic field in nonlinear magnetosonic pulses

Reflection of electrons and positrons in oblique, nonlinear magnetosonic pulses is theoretically analyzed. With the use of the parallel pseudo potential F, which is the integral of the parallel electric field along the magnetic field, a simple equation for reflection conditions is derived, which shows that reflection along the magnetic field is caused by two forces: one arising from the parallel pseudo potential multiplied by the particle charge and the other from the magnetic mirror effect. The two forces push electrons in the opposite directions. In compressive solitons, in which the magnetic field is intensified, electrons with large magnetic moments can be reflected by the magnetic mirror effect, whereas in rarefactive solitons, in which the magnetic field is weaker than outside, electrons with small magnetic moments can be reflected by the parallel pseudo potential. Although F is basically positive and large in shock waves, it occasionally becomes negative in some regions behind the shock front in nonstationary wave evolution. These negative spikes of F can reflect electrons. In contrast to the case of electrons, the two forces push positrons in the same direction. For this reason, compressive solitons in an electron-positron-ion plasma reflect a large fraction of positrons compared with electrons, whereas rarefactive solitons will reflect no positrons. A shock wave can reflect a majority of positrons with its large F. However, in a pure electron-positron plasma, in which F becomes zero, positron reflection will rarely occur.

Ion reflection by shock waves and pulse generation by cross-field ion beams

Comparisons are made of two different particle simulations: one for the study of plasma-based accelerators (Gueroult & Fisch, Phys. Plasmas, vol. 23, 2016, 032113) and the other for the study of shock formation in the interstellar medium (Yamauchi & Ohsawa, Phys. Plasmas, vol. 14, 2007, 053110). In the former, shock waves used for plasma density control create ion beams by reflection. In the latter, a fast and dense beam of exploding ions penetrates a surrounding plasma. In both simulations, magnetic bumps are generated from the motion of ion beams perpendicular to a magnetic field. Despite the apparent differences of their purposes, configurations and spatial scales, the two simulations show strong similarities in the generation processes and effects of the bumps, suggesting that these are not rare plasma phenomena. The bump created by the exploding ions develops into backward and forward magnetosonic pulses.

A theoretical study for parallel electric field in nonlinear magnetosonic waves in three-component plasmas

The electric field parallel to the magnetic field in nonlinear magnetosonic waves in three component plasmas (two-ion-species plasma and electron-positron-ion plasma) is theoretically studied based on a three-fluid model. In a two-ion-species plasma, a magnetosonic mode has two branches, high-frequency mode and low-frequency mode. The parallel electric field E∥ and its integral along the magnetic field, F=−∫E∥ds, in the two modes propagating quasiperpendicular to the magnetic field are derived as functions of the wave amplitude ϵ and the density ratio and cyclotron frequency ratio of the two ion species. The theory shows that the magnitude of F in the high-frequency-mode pulse is much greater than that in the low-frequency-mode pulse. Theoretical expressions for E∥ and F in nonlinear magnetosonic pulses in an electron-positron-ion plasma are also obtained under the assumption that the wave amplitudes are in the range of (me/mi)1/2<ϵ<1, where me/mi is the electron to ion mass ratio.

Three-dimensional MHD modeling of vertical kink oscillations in an active region plasma curtain

Observations on 2011 August 9 of an X6.9-class flare in active region (AR) 11263 by the Atmospheric Imaging Assembly (AIA) on-board the Solar Dynamics Observatory (SDO), were followed by a rare detection of vertical kink oscillations in a large-scale coronal active region plasma curtain in EUV coronal lines. The damped oscillations with periods in the range 8.8-14.9 min were detected and analyzed recently. Our aim is to study the generation and propagation of the MHD oscillations in the plasma curtain taking into account realistic 3D magnetic and density structure of the curtain. We also aim at testing and improving coronal seismology for more accurate determination of the magnetic field than with standard method. We use the observed morphological and dynamical conditions, as well as plasma properties of the coronal curtain based on Differential Emission Measure (DEM) analysis to initialize a 3D MHD model of its vertical and transverse oscillations by implementing the impulsively excited velocity pulse mimicking the flare generated nonlinear fast magnetosonic propagating disturbance interacting with the curtain obliquely. The model is simplified by utilizing initial dipole magnetic field, isothermal energy equation, and gravitationally stratified density guided by observational parameters. Using the 3D MHD model, we are able to reproduce the details of the vertical oscillations and study the process of their excitation by nonlinear fast magnetosonic pulse, propagation, and damping, finding agreement with the observations. We estimate the accuracy of simplified slab-based coronal seismology by comparing the determined magnetic field strength to actual values from the 3D MHD modeling results and demonstrate the importance of taking into account more realistic magnetic geometry and density for improving coronal seismology.

Laser-driven, magnetized quasi-perpendicular collisionless shocks on the Large Plasma Device

The interaction of a laser-driven super-Alfvénic magnetic piston with a large, preformed magnetized ambient plasma has been studied by utilizing a unique experimental platform that couples the Raptor kJ-class laser system [Niemann et al., J. Instrum. 7, P03010 (2012)] to the Large Plasma Device [Gekelman et al., Rev. Sci. Instrum. 62, 2875 (1991)] at the University of California, Los Angeles. This platform provides experimental conditions of relevance to space and astrophysical magnetic collisionless shocks and, in particular, allows a detailed study of the microphysics of shock formation, including piston-ambient ion collisionless coupling. An overview of the platform and its capabilities is given, and recent experimental results on the coupling of energy between piston and ambient ions and the formation of collisionless shocks are presented and compared to theoretical and computational work. In particular, a magnetosonic pulse consistent with a low-Mach number collisionless shock is observed in a quasi-perpendicular geometry in both experiments and simulations.

Shock formation processes due to interactions of two plasmas in a magnetic field and modified two-stream instabilities

The study of interactions of exploding and surrounding plasmas in an external magnetic field [K. Yamauchi and Y. Ohsawa, Phys. Plasmas 14, 053110 (2007)] is verified with two-dimensional (2D) electromagnetic particle simulations, for a case in which the initial velocity of the exploding plasma is perpendicular to the external magnetic field. The 2D simulations show essentially the same shock-formation processes as those in the previous one-dimensional simulation, including penetration of exploding ions into surrounding plasma, formation of a strong magnetic-field pulse due to deceleration of the exploding ions, ion reflection by the pulse, and subsequent splitting of the pulse into two magnetosonic pulses which then develop into forward and reverse shock waves. Furthermore, the 2D structure of electromagnetic fields in the region, where the exploding and surrounding ions overlap, is investigated with particular attention to the linear and nonlinear evolution of modified two-stream instabilities in the magnetic field that is being gradually compressed. The effects of these instabilities on ion reflection and on 2D magnetic fluctuations in the two generated pulses are also discussed.

A ROLE OF MAGNETOSONIC PULSES ON VARIATIONS OF VOYAGER-1 MeV ELECTRON INTENSITY IN THE HELIOSHEATH

Voyager 1 (V1) spacecraft observed electrons of 6-14 MeV in the heliosheath which showed several enhancements of significant flux variation. We compare these temporal electron flux variations, from the time when V1 crossed the termination shock (TS) up to mid-2008, with dynamical phenomena in the heliosheath that are obtained from our MHD simulations which are based on Voyager 2 (V2) observed solar-wind data. Our simulations indicate that all electron flux enhancements, except for one, correspond fairly well to the times when a magnetosonic (MS) pulse was driven downstream of the TS due to collision of interplanetary shock (IPS) or shock-driven MS pulse and its reflection in the heliosheath that either passed V1, or collided with the TS or with the plasma sheet near the heliopause (HP). This result suggests that these enhancements in the electron flux should correspond to either direct or indirect effects of MS pulses in the heliosheath driven by IPSs. The scale of the heliosphere is estimated by comparing V1-observed magnetic field intensity with the simulated intensity which suggests that V1 is possibly located near the HP within 4-8 AU at the present time.

THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC MODELS OF TWISTED MULTITHREADED CORONAL LOOP OSCILLATIONS

The multithreaded structure of active region coronal loops was deduced from past spectroscopic observations. Recent high-resolution observations by Transition Region and Coronal Explorer and Hinode satellites provided direct evidence that active region loops consist of multiple magnetic threads filled with plasma with higher density than neighboring loop material. High-resolution observations of loops near a flare site suggest that the threads are twisted or tangled, the magnetic field is not force free, and flows are present. To better understand these observations, we developed for the first time a three-dimensional magnetohydrodynamic model of twisted multithreaded loops that oscillate as a result of an impulsive event. The twist is induced by applying a rotating velocity field at the footpoint of the initially parallel set of threads, and parallel flow is included. The oscillations of the twisted loops are excited by a fast magnetosonic pulse. The evolution and the damping of the fast magnetosonic wave excited in the twisted multithreaded loop are compared to oscillations of a four-parallel-threaded loop. It was found that twisted loop oscillations result in filamented current and velocity structure that cannot be described by the fundamental kink mode. When parallel flow is present, the oscillation induces nonlinear compressive modulation of the flow and density in the threads. The twisted loop oscillates and damps faster than the parallel-threaded loop. The results of the study demonstrate the effects of the twist, internal loop structure, and flow on the evolution of the waves in coronal active region loops.

Current sheets formed by Alfvén and magnetosonic pulses in the vicinity of the null line of the magnetic field

A study is made of the propagation of an Alfven pulse and the superposition of an Alfven and a magnetosonic pulse in the vicinity of the magnetic null line (the X point). It is shown that, on long time scales, the poloidal components of the velocity and magnetic field relax to steady-state distributions similar to those in the case of a magnetosonic pulse. In the essentially nonlinear problem under investigation, the steady-state distributions of the toroidal components of the velocity and magnetic field are found to be close to those in the corresponding linear problem. It is established that two-dimensional effects play an important role in the evolution of the forming current sheets.

Damping of perpendicular magnetosonic pulses in a two-ion-species plasma

The damping of finite-amplitude magnetosonic pulses propagating perpendicular to a magnetic field in a collisionless plasma containing two ion species (majority light ions and minority heavy ions) is studied through theory and simulation. A magnetosonic pulse accelerates heavy ions in the direction parallel to the wave front; this cross-filed motion of heavy ions then generates a long-wavelength perturbation behind the original pulse. Because of these processes the original pulse is damped. The damping rate of a solitary pulse is theoretically obtained. It decreases with increasing amplitude. The damping is further investigated by means of a one-dimensional, fully electromagnetic code based on a three-fluid model. The theoretically obtained damping rate is in good agreement with the simulation result. Also, it is confirmed that small-amplitude periodic waves are not damped.

Collisionless Damping of Perpendicular Magnetosonic Waves in a Two-Ion-Species Plasma

Propagation of finite-amplitude magnetosonic waves in a collisionless plasma containing two ion species is studied with a one-dimensional, fully electromagnetic code based on a three-fluid model. It is found that perpendicular magnetosonic waves are damped in a two-ion-species plasma; a magnetosonic pulse accelerates heavy ions in the direction parallel to the wave front, which results in the excitation of a longer wavelength perturbation behind the pulse. The damping due to the energy transfer from the original pulse to the longer wavelength perturbation occurs even if the plasma is collisionless and the pulse amplitude is small. The theoretically obtained damping rate is in agreement with the simulation result.

A kinetic study of solar wind mass loading and cometary bow shocks

A detailed numerical study is conducted to understand the kinetic processes associated with solar wind mass loading due to pickup of cometary ions and the formation of cometary bow shocks. Because in this study we are most interested in phenomena which take place over time and spatial scales associated with ion dynamics, electrons are treated as a massless fluid. On the other hand, solar wind protons and heavy cometary ions are treated kinetically. It is found that solar wind deceleration and pickup of cometary ions take place through both the macroscopic electromagnetic fields embedded in the solar wind, as well as the microscopic fields associated with low‐frequency electromagnetic waves that are generated by the unstable velocity distribution function of the cometary ions. These electromagnetic waves can grow to very large amplitudes and their nonlinear evolution is controlled by their wave normal angle. At parallel propagation where the waves are noncompressional, parametric instabilities seem to be operative, while the oblique, compressional waves are found to steepen and form shocklets. As for the nature of cometary bow shocks, three different regimes are found, based on the shock normal angle. For shock normal angles at or near 90° (quasi‐perpendicular), a typically thin transition region (shock) is formed through steepening of fast magnetosonic pulses. This shock is a pure proton shock in that no sharp change in density or velocity of the cometary ions is observed across the shock. At intermediate shock normal angles (oblique), a much wider transition region is seen where the solar wind is gradually heated and decelerated. This region, whose width is much larger than the gyroradius of cometary ions, is associated with a high level of turbulence due to ion pickup instabilities and is unlike a typical fast magnetosonic shock. Finally, at small shock normal angles (quasi‐parallel), a narrow transition region is found which is similar to a parallel terrestrial shock. The formation of this shock is not due to wave steepening but rather is the result of scattering and heating of the solar wind protons by the large‐amplitude electromagnetic waves that are generated through the relative drift between the protons and the cometary ions. These results are compared and shown to be in qualitative agreement with the recent observations at comets.

Strong ion acceleration by a collisionless magnetosonic shock wave propagating perpendicularly to a magnetic field

A 2 1/2 -dimension, fully relativistic, fully electromagnetic particle code is used to study the time evolution of a nonlinear magnetosonic pulse propagating in a direction perpendicular to a magnetic field. The pulse is excited by an instantaneous piston acceleration, and evolves in a totally self-consistent manner. A large amplitude pulse traps some ions and accelerates them parallel to the wave front. They are detrapped when their velocities become of the order of the sum of the E×B drift velocity and the wave phase velocity, where E is the electric field in the direction of the wave propagation. The pulse develops into a quasishock wave in a collisionless plasma because of dissipation caused by the resonant ion acceleration. A simple nonlinear wave theory for a cold plasma describes the shock properties observed in the simulation except for the effects of resonant ions. In particular, the magnitude of an electric potential across the shock region is derived analytically and is found to be in good agreement with our simulations. The potential jump is proportional to B2, and hence the E×B drift velocity of the trapped ions is proportional to B.

Resonant Ion Acceleration by Collisionless Magnetosonic Shock Wave Propagating Obliquely to a Magnetic Field

A 2–1/2 dimensional relativistic electromagnetic particle code is used to study a time evolution of a selfconsistent nonlinear magnetosonic pulse propagating obliquely to a magnetic field. It is shown that the shock wave traps some ions and resonantly accelerates them parallel to the wave front. This acceleration is most efficient when the propagation angle is in the region \(\theta_{c}{\lesssim}\theta{\leqq}\pi/2\), where θ c = arctan (( m i / m e ) 1/2 -( m e / m i ) 1/2 ).