3-D Rpic Simulations of Relativistic Jets: Particle Acceleration, Magnetic Field Generation, and Emission

We have applied numerical simulations and modeling to the particle acceleration, magnetic field generation, and emission from relativistic shocks. We investigate the nonlinear stage of theWeibel instability and compare our simulations with the observed gamma-ray burst emission. In collisionless shocks, plasma waves and their associated instabilities (e.g., the Weibel, Buneman and other two-stream instabilities) are responsible for particle (electron, positron, and ion) acceleration and magnetic field generation. 3-D relativistic electromagnetic particle (REMP) simulations with three different electron-positron jet velocity distributions and also with an electron-ion plasma have been performed and show shock processes including spatial and temporal evolution of shocks in unmagnetized ambient plasmas. The growth time and nonlinear saturation levels depend on the initial jet parallel velocity distributions. Simulations show that the Weibel instability created in the collisionless shocks accelerates jet and ambient particles both perpendicular and parallel to the jet propagation direction. The nonlinear fluctuation amplitude of densities, currents, electric, and magnetic fields in the electron-positron shocks are larger for smaller jet Lorentz factor. This comes from the fact that the growth time of the Weibel instability is proportional to the square of the jet Lorentz factor. We have performed simulations with broad Lorentz factor distribution of jet electrons and positrons, which is assumed to be created by photon annihilation. Simulation results with this broad distribution show that the Weibel instability is excited continuously by the wide-range of jet Lorentz factor from lower to higher values. In all simulations the Weibel instability is responsible for generating and amplifying magnetic fields perpendicular to the jet propagation direction, and contributes to the electron’s (positron’s) transverse deflection behind the jet head. This small scale magnetic field structure contributes to the generation of “jitter” radiation from deflected electrons (positrons), which is different from synchrotron radiation in uniform magnetic fields. The jitter radiation resulting from small scale magnetic field structures may be important for understanding the complex time structure and spectral evolution observed in gamma-ray bursts or other astrophysical sources containing relativistic jets and relativistic collisionless shocks. The detailed studies of shock microscopic process evolution may provide some insights into early and later GRB afterglows.     

Entrainment and Deceleration of Relativistic Jets

Extragalactic radio sources are separated in two classes according to their specific luminosity: Fanaroff-Riley I and II. The origin of this dichotomy can be due either to intrinsec different properties of the AGN or to interaction of the jet with different enviroments. We assume that jets are always relativistic and supersonic close to their source, as recent observations suggest, and we explore the conditions in which the jets decelerate assuming FR I morphology. We have carried out high resolution 3D simulations for a set of parameters and in this paper we concentrate our discussion on two extreme cases.     

Accretion and Ejections in Magnetized Disks

In this contribution I review the main challenges for theory and numerical simulation of accretion turbulence in disks. I then present briefly a solution we have elaborated in recent years to a part of these questions: we have found an MHD instability which occurs in the inner region of a disk in the configuration (poloidal field of the order of equipartition with the gas pressure) used for MHD jet models. This instability has the unique characteristic that it re-emits toward the corona a fraction of the energy and angular momentum it extracts from the disk. It is a good candidate to explain the low-frequency Quasi-Periodic Oscillation observed in X-ray binaries, and we believe that it might occur also in YSO.     

On the local magnetorotational instability of astrophysical jets

We present the local linear stability analysis of rotating jets confined by a toroidal magnetic field. Under the thin flux tube approximation, we derive the equation of motion for slender magnetic flux tubes. In addition to the terms responsible for the conventional instability of the toroidal magnetic field, a term related to the magnetic buoyancy and a term corresponding to the differential rotation become relevant for the stability properties. We find that the rigid rotation stabilizes while the differential rotational destabilizes the jet in a way similar to the Balbus–Hawley instability. Within the frame of our local analysis, we find that if the azimuthal velocity is of the order of or higher than the Alfvén azimuthal speed, the rigidly rotating part of the jet interior can be completely stabilized, while the strong shearing instability operates in the transition layer between the rotating jet interior and the external medium. This can explain the limb-brightening effect observed in several jets. However, it is still possible to find jet equilibria that are stable all across the jet, even in the presence of differential rotation. We discuss observational consequences of these results.     

Hadley circulations and Kelvin wave-driven equatorial jets in the atmospheres of Jupiter and Saturn

We propose a dynamical mechanism that can plausibly explain the origin of the broad prograde equatorial winds observed on Jupiter and Saturn, and examine the feasibility of this mechanism using two- (2D) and three-dimensional (3D) numerical simulation models. The idea is based on combining a narrow Gaussian jet peaking at the equator, which is induced by the momentum transfer from an upward propagating equatorial Kelvin-wave, and a pair of off-equatorial jets due to a meridional-vertical circulation similar to the tropical Hadley circulation on Earth. We employ for this feasibility study a 2D mechanistic mean-flow model which incorporates the influence of prescribed waves, and a 3D general circulation model, based on the generalised primitive equations of atmospheric motion. We then confirm that the dynamical models of both kinds can successfully reproduce theoretically expected flows of a reasonable magnitude, and that when two mechanisms are combined, a broad super-rotating jet is produced with off-equatorial maxima in zonal velocity for both Jupiter and Saturn, approximately in accordance with observations.     

Traveling waves in the martian atmosphere from MGS TES Nadir data

We have characterized the annual behavior of martian atmospheric traveling waves in the MGS TES data set from the first two martian years of mapping. There is a high degree of repeatability between the two years. They are dominated by strong low zonal wavenumber waves with high amplitudes near the polar jets, strongest in late northern fall and early northern winter. The m=1 waves have amplitudes up to about 20 K, are vertically extended, and occasionally extend even into the tropics. Periods for m=1 range from 2.5 to 30 sols. Much weaker waves were identified in the south, with amplitudes less than about 3.5 K. Traveling waves with m=2 and m=3 are also seen, but their amplitudes are typically limited to less than 4 K, and are generally more confined near the surface. In the north, they are more evident in fall and spring rather than winter solstice, which is clearly dominated by m=1 waves. Some evidence of storm tracks has been identified in the data, with accentuated weather-related temperature perturbations near longitudes 200° to 320° E for both the southern and northern hemispheres near latitude ±65° at the surface. Some evidence was also found for a sharpening of longitudinal gradients into what may be frontal systems. EP flux divergences show the waves extracting energy from the zonal mean winds. When the m=1 waves were strongest, decelerations of the zonal jet of order 30 m/(s sol) were measured. Above 1 scale height, the waves extract energy from the jet predominately through barotropic processes, but their character is overall mixed barotropic/baroclinic. Inertial instabilities may exist at altitude on the equatorward flanks of the polar jets, and marginal stability extends through to the tropics. This may explain the coordination of the tropical behavior of the waves with that centered along the polar jet, consistent with the ideas expressed in Wilson et al. (2002, Geophys. Res. Lett. 29, #1684) and similar to those in Barnes et al. (1993, J. Geophys. Res. 98, 3125-3148). Throughout the year, there exist large regions with the meridional gradient of PV less than zero, but they are strongest near winter solstice. Poleward of the winter jet, the regions of instability reach the surface, equatorward they do not. These regions, satisfying a necessary criterion for instability, likely explain the genesis of the waves, and perhaps also their bimodal character between surface (faster waves) and altitude (slow m=1 waves).     

Stratospheric superrotation in the TitanWRF model

TitanWRF general circulation model simulations performed without sub-grid-scale horizontal diffusion of momentum produce roughly the observed amount of superrotation in Titan’s stratosphere. We compare these results to Cassini-Huygens measurements of Titan’s winds and temperatures, and predict temperature and winds at future seasons. We use angular momentum and transformed Eulerian mean diagnostics to show that equatorial superrotation is generated during episodic angular momentum ‘transfer events’ during model spin-up, and maintained by similar (yet shorter) events once the model has reached steady state. We then use wave and barotropic instability analysis to suggest that these transfer events are produced by barotropic waves, generated at low latitudes then propagating poleward through a critical layer, thus accelerating low latitudes while decelerating the mid-to-high latitude jet in the late fall through early spring hemisphere. Finally, we identify the dominant waves responsible for the transfers of angular momentum close to northern winter solstice during spin-up and at steady state. Problems with our simulations include peak latitudinal temperature gradients and zonal winds occurring ∼60 km lower than observed by Cassini CIRS, and no reduction in zonal wind speed around 80 km, as was observed by Huygens. While the latter may have been due to transient effects (e.g. gravity waves), the former suggests that our low (∼420 km) model top is adversely affecting the circulation near the jet peak, and/or that we require active haze transport in order to correctly model heating rates and thus the circulation. Future work will include running the model with a higher top, and including advection of a haze particle size distribution.     

Effect of expansion and magnetic field configuration on mass entrainment of jets

We investigate the growth of jet plus entrained mass in simulations of supermagnetosonic cylindrical and expanding jets. The entrained mass spatially grows in three stages: from an initially slow spatial rate to a faster rate and finally at a flatter rate. These stages roughly coincide with the similar rates of expansion in simulated radio intensity maps, and also appear related to the growth of the Kelvin–Helmholtz instability through linear, nonlinear, and saturated regimes. In the supermagnetosonic cylindrical jets, we found that a jet with an embedded primarily toroidal magnetic field is more stable than a jet with a primarily axial magnetic field. Also, pressure-matched expanding jets are more stable and entrain less mass than cylindrical jets with equivalent inlet conditions. We investigate the growth of jet plus entrained mass in simulations of supermagnetosonic cylindrical and expanding jets. The entrained mass spatially grows in three stages: from an initially slow spatial rate to a faster rate and finally at a flatter rate. These stages roughly coincide with the similar rates of expansion in simulated radio intensity maps, and also appear related to the growth of the Kelvin–Helmholtz instability through linear, nonlinear, and saturated regimes. In the supermagnetosonic cylindrical jets, we found that a jet with an embedded primarily toroidal magnetic field is more stable than a jet with a primarily axial magnetic field. Also, pressure-matched expanding jets are more stable and entrain less mass than cylindrical jets with equivalent inlet conditions.     

Shock instabilities

It is well known that adiabatic shocks in ordinary gases are stable to both tranverse and longitudinal perturbations, but this need not be true if there are significant thermal effects due to chemical reactions or cooling processes. For example, detonation waves in gases are observed to form cellular structures if the chemical reaction is sufficiently temperature sensitive and a similar instability occurs in radiative shocks in the ISM if their speed exceeds 150 km s^–1. This means that interstellar shocks will be subject to this radiative instability in many cases. The temperature sensitivity of the nuclear reactions in Type I supernovae is also such that we would expect detonation waves in these objects to have a cellular structure.     

Alfvén wave heating and acceleration of plasmas in the solar transition region producing jet-like eruptive activity

We present an analysis of observations and theory of selected transition-region phenomena, concentrating on small scale jet-like structures known as spicules and macrospicules. We examine a number of mechanisms that may be responsible for their formation and conclude that Alfvén waves could provide the necessary acceleration through the ponderomotive force and dissipation for heating forming a beam or jet like structure. In applying the Alfvén wave model we make no fundamental distinction between spicules and macrospicules. In this respect we consider them to be manifestations of the same phenomenon on different scales. We predict that the most effective Alfvén waves have frequencies around 1 Hz and amplitudes of 1 V m^–1. The resulting plasma jet sets up plasma conditions suitable for creating rotating structures which are also observed.     

太阳spike辐射中的MHD振荡

我们在1981年5月16日所观测到的典型的微波大爆发的spike辐射中,发现存有～1.4—1.6秒的准周期振荡特征。本文依据MHD波理论,对观测进行了分析讨论,本文认为在日冕圈内外传播着的快磁声波(“腊肠”模)调制了源区的磁场以及电子束的投射角分布,从而影响了ECM不稳定性的增长率,因此而产生了spike辐射中的准周期振荡。另外,本文还对一些有关的物理参数作了定量的估算。     

Accretion of jet streams and formation of asteroids

Our basic view on the formation of asteroids, stated in [1], is that the initial physical and chemical conditions in the asteroid region led to a slow growth of planetesimals in the region and a transfer of accretable matter to the Jupitor region, resulting in the planetesimals stopping at the “half-finished” stage, eventually forming only asteroids and not major planets. In this paper, using the conditions of the nebular disk obtained in that paper and the formula for gravitational instability and regarding the rings resulting from gravitational instability as “jet streams”, we apply the theory of accretion of jet streams to calculate the growth of the planetesimals and discuss the question of the transfer of accretable material, providing further confirmation of our basic view.     

Cross-scale coupling at a perpendicular collisionless shock

A full particle simulation study is carried out on a perpendicular collisionless shock with a relatively low Alfven Mach number (M_A = 5). Recent self-consistent hybrid and full particle simulations have demonstrated ion kinetics are essential for the non-stationarity of perpendicular collisionless shocks, which means that physical processes due to ion kinetics modify the shock jump condition for fluid plasmas. This is a cross-scale coupling between fluid dynamics and ion kinetics. On the other hand, it is not easy to study cross-scale coupling of electron kinetics with ion kinetics or fluid dynamics, because it is a heavy task to conduct large-scale full particle simulations of collisionless shocks. In the present study, we have performed a two-dimensional (2D) electromagnetic full particle simulation with a “shock-rest-frame model”. The simulation domain is taken to be larger than the ion inertial length in order to include full kinetics of both electrons and ions. The present simulation result has confirmed the transition of shock structures from the cyclic self-reformation to the quasi-stationary shock front. During the transition, electrons and ions are thermalized in the direction parallel to the shock magnetic field. Ions are thermalized by low-frequency electromagnetic waves (or rippled structures) excited by strong ion temperature anisotropy at the shock foot, while electrons are thermalized by high-frequency electromagnetic waves (or whistler mode waves) excited by electron temperature anisotropy at the shock overshoot. Ion acoustic waves are also excited at the shock overshoot where the electron parallel temperature becomes higher than the ion parallel temperature. We expect that ion acoustic waves are responsible for parallel diffusion of both electrons and ions, and that a cross-scale coupling between an ion-scale mesoscopic instability and an electron-scale microscopic instability is important for structures and dynamics of a collisionless perpendicular shock.     

Investigating the Astrophysical Applicability of Radiative and Non-Radiative Blast wave Structure in Cluster Media

We describe experiments that investigate the capability of an experimental platform, based on laser-driven blast waves created in a medium of atomic clusters, to produce results that can be scaled to astrophysical situations. Quantitative electron density profiles were obtained for blast waves produced in hydrogen, argon, krypton and xenon through the interaction of a high intensity (I ≈ 10¹⁷ Wcm⁻²), sub-ps laser pulse. From this we estimate the local post-shock temperature, compressibility, shock strength and adiabatic index for each gas. Direct comparisons between blast wave structures for consistent relative gas densities were achieved through careful gas jet parameter control. From these we investigate the applicability of different radiative and Sedov-Taylor self-similar solutions, and therefore the (ρ,T) phase space that we can currently access.     

MHD oscillations in the solar spike radiation

In this paper the observed 1.4–1.6 s quasi-periodic oscillations in the spike radiation of the microwave outburst of 1981 May 16 are analysed in teras of MHD waves. We point out that the fast magnetoacoustic waves (“sausage” mode) propagating inside and outside a loop can modulate the magnetic field and the pitch angle distribution of the electron beams in the source region. The growth rate of electron-cyclotron-maser instability is then affected to give rise to the quasi-periodic oscillations. Quantitative estimates of relevant physical parameters are given.     

Hypothesis on the origin of the spiral structure of the galaxies

The spiral waves in a model galaxy consisting of differentially rotating and non-rotating subsystems are considered on the basis of participating phenomena. The subsystems involved represent the Populations I and II of normal spirals, respectively. The spiral waves in such a systems are unstable in the Landau sense. Due to this instability they grow up to attain finite amplitude. This growth is stopped by a non-linear effect (the quasilinear effect), the steady state with waves of finite amplitude being established. The hypothesis is proposed that these waves should be identified with the spiral structure of the galaxies.     

On dynamical models for radio galaxies

The tailed radio galaxies that have been called ‘Type I’ are not a uniform set. To study their dynamics, we have used the Ledlow–Owen data set, which provides a new sample of 250 radio galaxies in nearby Abell clusters. These sources divide into two clear categories based on their radio morphology. Type A sources (‘straight’) contain nearly straight jets which are embedded in outer radio lobe. Type B sources (‘tailed’) have a well-collimated jet flow which undergoes a sudden transition, at an inner hot spot, to a less collimated flow which continues on and forms a radio tail. We have not found any separation of these classes in terms of radio power, radio flux size, galaxy power or external gas density. We propose the difference is due to the development, or not, of a disruptive flow instability, such as Kelvin–Helmholtz, and the saturation of the instability when it develops.     

Effects of a large convective storm on Saturn's equatorial jet

Hubble Space Telescope observations revealed that Saturn's equatorial jet at the cloud level blows at ∼275 m s⁻¹ today, approximately half the ∼470 m s⁻¹ wind during the Voyager flybys in 1980-1981. Radiative transfer calculations estimate the clouds to be significantly higher today than in 1980. The higher clouds make it difficult to observationally isolate any true slowdown from the vertical wind shear because Voyager and Cassini observations show that the winds become slower with altitude. Here, we test the hypothesis that the large equatorial storm in 1990 called the Great White Spot (GWS) decelerated the equatorial jet. We first use order of magnitude estimates to show: (1) if the GWS triggers vertical momentum redistribution, a minor speed change in the troposphere can lead to a substantial stratospheric wind speed change; (2) storm-triggered turbulent mixing slows a prograde equatorial jet; and (3) a prograde equatorial jet inhibits turbulent mixing in latitude. To test whether a GWS-like large storm decelerates the equatorial jet, we perform numerical experiments using the Explicit Planetary Isentropic Coordinate (EPIC) atmosphere model. Our simulation results are consistent with our order of magnitude predictions. We show that the storm excites waves, and the waves transport westward momentum from the troposphere to the stratosphere and decelerate the equatorial jet by as much as ∼40 m s⁻¹ at the 10-mbar level. However, our results show that the storm's effect is too weak at the cloud levels to halve the jet's speed from ∼470 m s⁻¹. Our results suggest that a combination of higher clouds and a true slowdown is necessary to explain the apparent equatorial jet slowdown. We also analyze the effect of waves on the apparent cloud motions, and show that waves can influence cloud-tracking wind speed measurements.     

Nonlinear Dynamics of Kinetic Alfvén and Whistler Waves in the Solar Wind

In this article we investigate the nonlinear dynamics of 3D kinetic Alfvén waves (KAWs) and quasi-transverse weak whistler waves in a magnetized plasma. We have studied the problem numerically to examine the transient evolution of localized structures of 3D KAWs and whistler waves. The nonlinearity arises as a result of ponderomotive effects associated with 3D KAWs; consequently, the background density modifies. The weak whistler waves propagating in this modified density are localized and amplified. To improve our insight into the basic physics behind the formation of these localized structures, we have also solved the system semi-analytically. The power spectra show a Kolmogorov scaling (with a power of \(-5/3\)) in the inertial range that lies above the ion gyroradius. Below this scale, dispersive effects start to appear, and the power spectrum follows a steeper scaling (?2 to ?4). Our results show the important role that KAWs and whistler waves play in the energy cascading from larger to smaller scales. The results are consistent with the solar wind observations by the Cluster spacecraft.     

太阳大气中动力学阿尔文波的产生与加热研究进展

动力学阿尔文波是垂直波长接近离子回旋半径或者电子惯性长度的色散阿尔文波.由于波的尺度接近粒子的动力学尺度,动力学阿尔文波在太阳和空间等离子体加热、加速等能化现象中起重要作用.因此,动力学阿尔文波通常被认为是日冕加热的候选者.本研究工作深入、系统地调研了太阳大气中动力学阿尔文波的激发和耗散机制.基于日冕等离子体环境,介绍了几种常见的动力学阿尔文波激发机制:温度各向异性不稳定性、场向电流不稳定性、电子束流不稳定性、密度非均匀不稳定性以及共振模式转换.还介绍了太阳大气中动力学阿尔文波的耗散机制,并讨论了这些耗散机制对黑子加热、冕环加热以及冕羽加热的影响.不仅为认识太阳大气中动力学阿尔文波的驱动机制、动力学演化特征以及波粒相互作用提供合理的理论依据,而且有助于揭示日冕等离子体中能量储存和释放、粒子加热等能化现象的微观物理机制.