Structures In Space Plasmas Research Articles

Origin of the filamentary structure in space plasmas

We propose a scenario that filamentary structure appears together with propagating waves on the scale of ion gyroradius. The method is based on two-dimensional ion particle-in-cell (or hybrid) simulation in low-beta plasmas. Coherent, filamentary structures in space plasmas are found as the zero-frequency mode in the wavenumber-frequency domain. The existence of the wave modes might be the key to understand filament formation in space plasmas.

Positron acoustic solitary waves interaction in a four-component space plasma

The characteristics of the head-on collision (HOC) between two positron acoustic solitary waves (PASWs) in a four component electron-positron-ion (EPI) space plasma have been investigated theoretically, using the extended Poincare-Lighthill-Kuo (PLK) method. The analytical phase shifts after the collision of the two solitary waves occurs are derived. Numerically, the influences of the cold/hot positron parameters on the phase shifts are explicitly investigated. The present theory is applied to analyze the formation and the interaction of localized coherent PASWs structures in space plasmas (pulsar environments).

Clarification on Polarity of Bipolar Electric Field Solitary Structures in Space Plasmas with Satellite Observation

The bipolar electric field solitary (EFS) structures observed frequently in space plasmas by satellites have two different polarities, first positive electric field peak then negative (i.e., positive/negative) and first negative then positive peak (i.e., negative/positive). We provide the physical explanation on the polarity of observed bipolar EFS structures with an electrostatic ion fluid model. The results show that if initial electric field E0 > 0, the polarity of the bipolar EFS structure will be positive/negative; and if E0 < 0, the polarity of the bipolar EFS structure will be negative/positive. However, for a fixed polarity of the EFS, either positive/negative or negative/positive, if the satellite is located at the positive side of the EFS, the observed polarity should be positive/negative, if the satellite is located at the negative side of the EFS, the observed polarity should be negative/positive. Therefore, we provide a method to clarify the natural polarity of the EFS with observed polarity by satellites. Our results are significant to understand the physical process in space plasma with the satellite observation.

MHD Waves and Instabilities in Space Plasma Flows

Shear flow instabilities are an important aspect of hydrodynamic studies. The present review article discusses the role of an ambient magnetic field which both modifies the Kelvin-Helmholtz instability and may introduce new types of magnetohydrodynamic waves and instabilities. A brief overview of magnetospheric pulsations is presented with an emphasis on the long-period resonant Alfven waves associated with the high speed solar wind. The spatio-temporal evolution of magnetically modified shear flow instabilities in various space plasma structures is addressed. A distinction between convective and absolute instabilities is necessary for proper understanding of theory and correct interpretation of the observations. Finally, it is shown how incompressible Alfvenic disturbances may become unstable in a compressible flow in the absence of any shear. An application to coronal loops is presented.

Theoretical properties of offset bipolar electric field solitary structures in space plasmas

The bipolar electric field solitary (EFS) structures have been frequently observed in the near Earth plasma regions, such as auroral zone, magnetopause, cusp regions, and magneto-tail. Sometimes these structures are observed as offset bipolar structures. In this paper, the properties of the offset bipolar EFS structures parallel to the magnetic field are studied with an ion fluid model in a cylindrical symmetry by considering electrostatic condition. The model results show that the offset bipolar EFS structures can develop from both ion-acoustic waves and ion cyclotron waves, and propagate along the magnetic field line in the space plasmas if plasma satisfies some conditions. The offset bipolar EFS structures can have both polarities. It will be first negative pulse and then positive pulse if the initial electric field E0 0. The amplitude of the offset bipolar EFS structures first decreases and then increases with the wave propagation velocity. Some results from our model are consistent with the observations.

“Reduced” multidimensional Vlasov simulations, with applications to electrostatic structures in space plasmas

A class of two-dimensional open-boundary kinetic plasma simulations characterized as reduced Vlasov simulations have been developed and applied to the study of double layers and electron holes in Earth’s auroral downward current region. In these simulations, full Vlasov phase-space dynamics is followed only in the direction of the magnetic field or imposed current, with a limited representation of phase space employed in the perpendicular spatial dimension. These reduced methods include a dissipative moment closure for unmagnetized species and limitation of perpendicular velocity space to a ring of modes at a fixed v⊥ for weakly magnetized species. Simulations with weakly magnetized ions exhibit ion-Bernstein waves in association with electron holes and simulations with weakly magnetized or unmagnetized electrons result in electron holes elongated perpendicular to B. These results are in qualitative agreement with satellite observations and previous particle simulations.

Theoretical model and experimental diagnostics of nonlinear electrostatic structures in space plasma

The formation of three-dimensional ion-acoustic electrostatic structures in the magnetized magnetospheric plasma is investigated. Basing on the MHD system of equations for three-component plasmas, evolutionary equations, which are the modified Korteweg-de Vries–Zakharov–Kuznetzov equations, are derived. The mathematical methods for the solution of these nonlinear equations with different degree of nonlinearity are developed. In the specific case of the spherical symmetry, numerical solutions of the evolutionary equations with different degree of nonlinearity are found. It is shown that in dependence on the distribution function of the hot electrons solitary structures with positive potential (compressive structures) or with negative potential (rarefactive structures) can be formed. The characteristics of these solitary structures as the ratio of the dimension parallel to the magnetic field to the dimension perpendicular to the field are estimated. The basic results of the numerical models are compared with experimental data obtained by satellites FAST, POLAR and GEOTAIL.

Phase space structures generated by absorbing obstacles in streaming plasmas

Abstract. The dynamic behavior of a collisionless plasma flowing around an obstacle is investigated by numerical methods. In the present studies, the obstacle is formed by an absorbing cylinder, and a 2-D electrostatic particle-in-cell simulation is used to study the flow characteristics, with extensions to a fully 3-D generalization of the problem demonstrated as well. The formation of irregular filamented density depletions, oblique to the flow, is observed. The structures form behind the obstacle, in a region with a strong velocity shear, but also other instability mechanisms can be identified. The dynamics of these structures is highly dependent on the physical parameters of the plasma, and they can either be quasi-stationary or undergo a dynamic evolution. The structures are found to be associated with phase-space vortices, observed especially in the phase space spanned by the velocity direction perpendicular to the flow and the spatial coordinate in the same direction. The bias of the obstacle with respect to the plasma potential is found to be an important parameter for the dynamics of the structures, but seemingly not for their formation as such. The results can be of interest in the interpretation of structures in space plasmas as observed by instrumented spacecrafts.

Phase space structures generated by an absorbing obstacle in a streaming plasma

The dynamic behavior of an ion flow around an obstacle in a collisionless plasma is investigated. The obstacle consists here of an absorbing cylinder, and a 2 dimensional electrostatic particle‐in‐cell simulation is used to study the flow characteristics. The formation of irregular filamented density depletions, oblique to the flow, is observed. The dynamics of these structures depend on the physical parameters of the plasma. The structures form at the edges of the wake behind the obstacle, in a region with a strong velocity shear, and are found to be associated with phase‐space vortices, observed specially in the velocity direction perpendicular to the flow. The results can be of interest in the interpretation of structures in space plasmas as observed by instrumented space crafts.

Drift–Alfvén vortices in dusty plasmas with non-zero ion-temperature effects

Reduced two-fluid equations governing the nonlinear dynamics of drift–Alfvén waves in dusty plasmas with non-zero ion temperature are derived. In the linear limit, we find a dispersion relation that shows the coupling between the ion-drift–Shukla–Varma mode, and electron-drift (magnetostatic) and (inertial or kinetic) Alfvén waves due to the finite collisionless electron skin depth or Larmorradius corrections. In contrast to the case of an electron–ion plasma, when the nonlinear drift–Alfvén vortices are weakly localized, i.e. decrease at infinity as r−1, the presence of the charged dust grains makes exponential localization possible. The physical meaning of such a localization is connected with the fact that charged massive dust granules provide an additional screening that results in stronger localization of the vortex. In several intermediate-β plasmas with 1 [Gt ] β [Gt ] me/mi (me,i is the electron or ion mass), the localization length approaches a minimum value when the vortex velocity is of the order of the ion diamagnetic drift. It then reaches the value ρiδ−1/4d where ρi is the ion Larmor radius and δd is the ratio of the dust to ion densities multiplied by the dust charge number Zd. In the case of very low plasma pressure, β [Lt ] me/mi, the vortex is localized with typical scale (λeρi)1/2δ−1/4d where λe is the electron skin depth. Our investigation can thus predict the velocities of coherent nonlinear structures in space plasmas.

Radio wave active Doppler imaging of space plasma structures: Arrival angle, wave polarization, and Faraday rotation measurements with the radio plasma imager

Radio sounding in the magnetosphere by the radio plasma imager on the IMAGE spacecraft will determine the dimensions and shape of the cavity between the magnetopause and the plasmapause. Omnidirectional transmission of pulsed radio signals results in echoes arriving from many directions. Quadrature sampling and Doppler analysis of the signals received on three orthogonal antennas will make it possible to determine the angles of arrival of the echoes, their polarization ellipses, and the Faraday rotation. Decomposition of the echo signals into the two characteristic waves is used to identify the O‐ and X‐wave components.

Drift‐Alfvén vortices in dusty plasmas

A set of two‐fluid equations that governs the nonlinear dynamics of drift‐Alfvén waves in a multicomponent dusty plasma is derived. It consists of the electron continuity equation, the electron parallel equation of motion, and the divergence of the total plasma current density. In the linear limit, we obtain a dispersion relation that shows the coupling between drift‐Alfvén and drift convective cells. On the other hand, the stationary solutions of the nonlinear equations can be represented as dipolar vortices. The relevance of our investigation to coherent nonlinear structures in space plasmas is pointed out.

Analysis of thick, non-planar boundaries using the discontinuity analyser

Abstract. The advent of missions comprised of phased arrays of spacecraft, with separation distances ranging down to at least mesoscales, provides the scientific community with an opportunity to accurately analyse the spatial and temporal dependencies of structures in space plasmas. Exploitation of the multi-point data sets, giving vastly more information than in previous missions, thereby allows unique study of their small-scale physics. It remains an outstanding problem, however, to understand in what way comparative information across spacecraft is best built into any analysis of the combined data. Different investigations appear to demand different methods of data co-ordination. Of the various multi-spacecraft data analysis techniques developed to affect this exploitation, the discontinuity analyser has been designed to investigate the macroscopic properties (topology and motion) of boundaries, revealed by multi-spacecraft magnetometer data, where the possibility of at least mesoscale structure is considered. It has been found that the analysis of planar structures is more straightforward than the analysis of non-planar boundaries, where the effects of topology and motion become interwoven in the data, and we argue here that it becomes necessary to customise the analysis for non-planar events to the type of structure at hand. One issue central to the discontinuity analyser, for instance, is the calculation of normal vectors to the structure. In the case of planar and `thin' non-planar structures, the method of normal determination is well-defined, although subject to uncertainties arising from unwanted signatures. In the case of `thick', non-planar structures, however, the method of determination becomes particularly sensitive to the type of physical sampling that is present. It is the purpose of this article to firstly review the discontinuity analyser technique and secondly, to discuss the analysis of the normals to thick non-planar structures detected in magnetometer data. Key words. Space plasma physics (discontinuities; instruments and techniques)

Analysis of thick, non-planar boundaries using the discontinuity analyser

The advent of missions comprised of phased arrays of spacecraft, with separation distances ranging down to at least mesoscales, provides the scientific community with an opportunity to accurately analyse the spatial and temporal dependencies of structures in space plasmas. Exploitation of the multi-point data sets, giving vastly more information than in previous missions, thereby allows unique study of their small-scale physics. It remains an outstanding problem, however, to understand in what way comparative information across spacecraft is best built into any analysis of the combined data. Different investigations appear to demand different methods of data co-ordination. Of the various multi-spacecraft data analysis techniques developed to affect this exploitation, the discontinuity analyser has been designed to investigate the macroscopic properties (topology and motion) of boundaries, revealed by multi-spacecraft magnetometer data, where the possibility of at least mesoscale structure is considered. It has been found that the analysis of planar structures is more straightforward than the analysis of non-planar boundaries, where the effects of topology and motion become interwoven in the data, and we argue here that it becomes necessary to customise the analysis for non-planar events to the type of structure at hand. One issue central to the discontinuity analyser, for instance, is the calculation of normal vectors to the structure. In the case of planar and `thin' non-planar structures, the method of normal determination is well-defined, although subject to uncertainties arising from unwanted signatures. In the case of `thick', non-planar structures, however, the method of determination becomes particularly sensitive to the type of physical sampling that is present. It is the purpose of this article to firstly review the discontinuity analyser technique and secondly, to discuss the analysis of the normals to thick non-planar structures detected in magnetometer data. Key words. Space plasma physics (discontinuities; instruments and techniques)