Chijie Xiao

Interactions between magnetosonic waves and radiation belt electrons: Comparisons of quasi‐linear calculations with test particle simulations

Quasi-linear theory (QLT) has been commonly used to study the Landau resonant interaction between radiation belt electrons and magnetosonic (MS) waves. However, the long-parallel wavelengths of MS waves can exceed their narrow spatial confinement and cause a transit-time effect during interactions with electrons. We perform a careful investigation to validate the applicability of QLT to interactions between MS waves, which have a distribution in frequency and wave normal angle, and radiation belt electrons using test particle simulations. We show agreement between these two methods for scattering rate of intense MS waves at L = 4.5 inside the plasmapause, but find a significant inconsistency for MS waves outside the plasmapause, due to the broad transit-time region in (Ek,α) space. Consequently, we introduce a particle-independent criterion to justify the validity of QLT for MS waves: the wave spatial confinement should be longer than two parallel wavelengths.

EMHD theory and observations of electron solitary waves in magnetotail plasmas

A new approach of electron magnetohydrodynamics (EMHD) is developed by including the anisotropy of the electron pressure tensor to take Biermann battery effect into account. Based on the model, the dispersion relation of slow and fast electron magnetosonic modes are derived. A Korteweg-de Vries equation is then obtained from the wave equation to get a solution of one-dimensional slow-mode soliton. Furthermore, according to measurements of Cluster and Time History of Events and Macroscale Interactions during Substorms, we find a good agreement between the theory and observations of magnetic field depression and perpendicular pressure increase.

SYNTHETIC SOLAR CORONAL HEATING ON CURRENT SHEETS

Synthetic heating of solar coronal loops combining Joule and wave-heating mechanisms on current sheets is proposed. The formation of singular current structures such as current sheets can be caused not only by magnetic reconnection and footpoint convection of coronal loops, but also Alfv?n resonance induced by shear flows. On the other hand, with the two-fluid effect, the kinetic Alfv?n wave can also be excited on such current structures. Therefore, a synthetic energy dispassion process of both collisional and wave-heating mechanisms on the current structures may lead to a better understanding of the coronal-heating problem.

Plasma waves around separatrix in collisionless magnetic reconnection with weak guide field

Electrostatic and electromagnetic waves excited by electron beam around the separatrix region are analyzed in detail during the collisionless magnetic reconnection with a weak guide field by using 2-D particle-in-cell simulation with the adaptive mesh refinement. Broadband electrostatic waves are excited both in the inflow and outflow regions around the separatrices due to the electron bump-on-tail, two-stream, and Buneman instabilities. In contrast, the quasi-monochromatic electromagnetic waves are excited only in the inflow side of the separatrices due to a beam-driven whistler instability. The localization of the whistler waves is attributed to the nonuniformity of the out-of-plane magnetic field By. The whistler instability is suppressed in the outflow side where By is too small for the oblique propagation. The electrostatic waves with distinct speeds can explain the in situ spacecraft observations. From the causality point of view, the waves are generated as the consequence of the electron bulk acceleration to thermalize the particles through wave-particle interactions. These simulation results provide guidance to analyze high-resolution wave observations during reconnection in the ongoing and upcoming satellite missions, as well as in dedicated laboratory experiments.

Effects of out-of-plane shear flows on fast reconnection in a two-dimensional Hall magnetohydrodynamics model

Effects of out-of-plane shear flows on fast magnetic reconnection in Hall magnetohydrodynamics approximation are studied in a slab model with an initial Harris sheet equilibrium. It is found that the out-of-plane shear flows can also generate a series of out-of-plane magnetic perturbations that significantly modify the out-of-plane quadrupolar magnetic perturbation induced by the Hall effect in magnetic reconnection. The modification to out-of-plane magnetic perturbations then considerably affects the magnetic reconnection process. Particularly, flows along the X-line can either enhance or reduce the reconnection rate, even altering the X-point to an O-point and resulting in a secondary island. The impact on satellite observations is also discussed.

Simulation studies of high-latitude magnetospheric boundary dynamics

Magnetic reconnection at the high-latitude magnetopause is studied using 2.5-dimensional Hall-MHD simulation. Concentric flow vortices and magnetic islands appear when both Hall effect and sheared flow are considered. Plasma mixing across the magnetopause occurs in the presence of the flow vortices. Reconnected structure generated in the vicinity of the subsolar point changes its geometry with increasing flow shear while moving to high latitudes. In the presence of flow shear, with the Hall-MHD reconnection a higher reconnection rate than with the traditional MHD is obtained. The out-of-plane components of flow and magnetic field produced by the Hall current are redistributed under the action of the flow shear, which makes the plasma transport across the boundaries more complicated. The simulation results provide some help in understanding the dynamic processes at the high latitude magnetopause.

Long-lasting energetic particle modes in tokamak plasmas with low magnetic shear

A long-lasting (for hundreds of milliseconds) m/n = 1 energetic particle mode driven by trapped fast ions, other than conventional fishbone bursts, is studied theoretically and in comparison with HL-2A experimental results. The mode can be observed in weak shear tokamak plasmas during neutral beam injection with a mostly steady amplitude envelope of long-lasting magnetic perturbation signals. The dispersion relation and radial structure of the mode are calculated with a weak shear q-profile. Both the m/n = 1/1 component and its higher frequency m/n = 2/2 harmonics are found to be unstable, in good agreement with experimental observations on HL-2A. On the other hand, due to the feature of weak magnetic shear, the mode is also significantly different from bursty fishbones, especially the mode structure, temporal behavior, instability threshold and growth rate dependence on the fast ion gradient. The nonlinear evolution of the mode and the comparison with fishbone bursts are also further investigated.

Effects of magnetic islands on drift wave instability

Magnetic islands have been implemented in the gyrokinetic toroidal code to study the effects of the islands on microturbulence. The pressure profile flattening is verified in the simulation with the islands. Simulations of ion temperature gradient instability find that different toroidal modes are linearly coupled together and that toroidal spectra become broader when the island width increases. The real frequencies and growth rates of different toroidal modes approach each other with the averaged value independent of the island width. The linear mode structures are enhanced at the island separatrices and weakened at the island centers, consistent with the flattening of the pressure profile inside the islands.

A current sheet traced from the Sun to interplanetary space

Context. Magnetic reconnection is a central concept for understanding solar activity, including filament eruptions, flares, and coronal mass ejections (CMEs). The existence of transverse and vertical current sheets, sites where reconnection takes place in the solar atmosphere, is frequently proposed as a precondition for flare/CME models, but is rarely identified in observations. Aims. We aim at identifying a transverse current sheet that existed in the pre-CME structure and persisted from the CME solar source to interplanetary space. Methods. STEREO A/B provide us a unique opportunity to calculate the interplanetary current sheets for the magnetic cloud. We analyze such a structure related to the fast halo CME of 2006 December 13 with assembled observations. A current sheet at the front of the magnetic cloud is analyzed to its origin in a transverse current sheet in the CME solar source, which can be revealed in the magnetic field extrapolations, XRT, and LASCO observations. Results. An interplanetary current sheet is identified as coming from the CME solar source by carefully mapping and examining multiple observations from the Sun to interplanetary space, along with nonlinear force-free magnetic field extrapolations of the active region NOAA 10930. Conclusions. The structure identified in the pre-flare state is a global transverse current sheet, which plays a role in the CME initiation, and propagates from the corona to interplanetary space.

The Parametric Decay Instability of Alfvén Waves in Turbulent Plasmas and the Applications in the Solar Wind

We perform three dimensional (3D) ideal magnetohydrodynamic (MHD) simulations to study the parametric decay instability of Alfven waves in turbulent plasmas and explore its possible applications in the solar wind. We find that, over a broad range of parameters in background turbulence amplitudes, the parametric decay instability of an Alfven wave with various amplitudes can still occur, though its growth rate in turbulent plasmas tends to be lower than both the theoretical linear theory prediction and that in the non-turbulent situations. Spatial - temporal FFT analyses of density fluctuations produced by the parametric decay instability match well with the dispersion relation of the slow MHD waves. This result may provide an explanation of the generation mechanism of slow waves in the solar wind observed at 1 AU. It further highlights the need to explore the effects of density variations in modifying the turbulence properties as well as in heating the solar wind plasmas.