Space Plasma Processes Research Articles

Plasma Measurements in the Space Plasma Operation Chamber (SPOC)

Plasma with a temperature and density similar to the ionospheric plasma is produced in the Space Plasma Operation Chamber (SPOC). SPOC, constructed at NCKU in 2009, is a research plasma device designed for the calibration and testing of satellite/rocket-borne instruments and for studying space plasma processes. It is a cylindrical chamber 2 meters in diameter and 3 meters in length. The vacuum pressure inside the SPOC can reach 10^(-8) Torr. The plasma in this large vacuum space is produced by two back-diffusion type plasma sources installed at the center of the chamber side doors. The sources produce ions of controllable drifting energy from a few ten eV to several hundred eV, these ions, together with the thermal electrons emitted from nickel cathodes, collide with neutral molecules at a pressure of 10^(-4) Torr in the chamber and form the plasma environment. The plasma density varies from 10^3 to 10^6 cm^(-3), and the neutral density is ~1012 cm^(-3). The electrons have a Maxwellian distribution with a temperature in the range 1000-3000 K. The ions have two component distributionsP: one is the drift-Maxwellian component with temperature close to the neutral particle temperature (room temperature), and the other is a small ion beam component. We have developed several space instruments for testing in the SPOC. This paper describes the SPOC machine and its capabilities for plasma experiments and space environmental tests. The plasma environment in the SPOC is measured by the Langmuir probe, the retarding potential analyzer, and the electron temperature and density probe. Results measured by different instruments are compared and discussed. The thermal and beam components of the ion energy distributions at different positions and the electron temperature and density spatial distributions are shown and discussed. The SPOC can be used to develop space-borne instruments and plasma thrusters, check the performance of instruments before flight, check whole payload systems in the plasma and vacuum environment, and conduct basic plasma experiments related to space sciences.

Plasma Response to a Varying Degree of Stress

We report experimental evidence of a seamless transition between three distinct modes in a magnetized plasma with a transverse sheared flow as the ratio of the ion gyroradius to the shear scale length (a measure of shear magnitude) is varied. This was achieved using a dual plasma configuration in a laboratory experiment, where a sheared flow oriented perpendicular to a background magnetic field is localized at the boundary of the plasmas. This confirms the basic theory that plasma is unstable to transverse velocity shear in a broad frequency and wavelength range. The experiment characterizes the compression or relaxation of boundary layers often generated in a variety of laboratory and space plasma processes.

Separator reconnection with antiparallel/component features observed in magnetotail plasmas

AbstractMagnetic reconnection (MR), a fundamental process in space plasmas that changes magnetic topology and converts magnetic energy into kinetic and thermal energies, is an ultimate driver of space weather. There exist two models of MR in the literatures, antiparallel and component, associated with intensive studies of their generation and applications. In this paper, we report an MR event observed by Cluster constellation in the geomagnetotail where both types of MR were detected. By reconstructing the three‐dimensional (3‐D) MR configuration, we find that a pair of A‐B nulls existed in both types of MR cases with two fan surfaces intersecting each other to form a separator line connecting the nulls. A weak or sizable magnetic field exists along the separator in the antiparallel or component case, respectively. In the latter case, field strength is finite away from the two nulls and vanishes close to the nulls. Therefore, at least in the two cases observed, both antiparallel and component MR geometries are local presentation of the separator MR configuration. This result supports the expectation that 3‐D nulls often occur as a crucial element of MR at least in the magnetotail and separator MR may play an important role in dynamics and reconfiguration of magnetic field in 3‐D MR processes.

Global Vlasov simulation on magnetospheres of astronomical objects

Space plasma is a collisionless, multi-scale, and highly nonlinear medium. There are various types of self-consistent computer simulations that treat space plasma according to various approximations. We develop numerical schemes for solving the Vlasov (collisionless Boltzmann) equation, which is the first-principle kinetic equation for collisionless plasma. The weak-scaling benchmark test shows that our parallel Vlasov code achieves a high performance and a high scalability. Currently, we use more than 1000 cores for parallel computations and apply the present parallel Vlasov code to various cross-scale processes in space plasma, such as a global simulation on the interaction between solar/stellar wind and magnetospheres of astronomical objects.

Topological skeleton of the 2-D slightly non-ideal MHD system close to X-type magnetic null points – an analysis of the general solution for the generic case

Abstract. The appearance of eruptive space plasma processes, e.g. in eruptive flares as observed in the solar atmosphere, is usually assumed to be caused by magnetic reconnection, often connected with singular points of the magnetic field. We are interested in the general relation between the eigenvalues of the Jacobians of the plasma velocity and the magnetic field and their relation to the shape of a spatially varying, localized non-idealness or resistivity, i.e. we are searching for the general solution. We perform a local analysis of almost all regular, generic, structurally stable non-ideal or resistive MHD solutions. Therefore we use Taylor expansions of the magnetic field, the velocity field and all other physical quantities, including the non-idealness, and with the method of comparison of coefficients, the non-linear resistive MHD system is solved analytically, locally in a close vicinity of the null point. We get a parameterised general solution that provides us with the topological and geometrical skeleton of the resistive MHD fields. These local solutions provide us with the "roots" of field and streamlines around the null points of basically all possible 2-D reconnection solutions. We prove mathematically that necessarily, the flow close to the magnetic X-point must also be of X-point type to guarantee positive dissipation of energy and annihilation of magnetic flux. We also prove that, if the non-idealness has only a one-dimensional, sheet-like structure, only one separatrix line can be crossed by the plasma flow, similar to known reconnective annihilation solutions.

Recent progresses in theoretical studies and satellite observations for collisionless magnetic reconnection

As an essential mechanism in large scale fast magnetic energy releases and field reconfigurations processes in space, astrophysical, and laboratory plasmas, magnetic reconnection, particularly collisionless magnetic reconnection, has been studied for more than 65 years. Many progresses have been achieved in recent years and basic features of the process have been well understood, largely due to more and more satellite observation data available in the last decade. However, a few outstanding issues are still remained unresolved. We in the paper review the development of collisionless magnetic reconnection studies and major achievements in recent years, and also briefly discuss the open questions remained to be answered in studies of collisionless magnetic reconnection.

Kinetic simulations of the structures of magnetic island in multiple X line guide field reconnection

Magnetic reconnection is one of the most important processes in astrophysical, space, and laboratory plasmas, and magnetic island is an important feature in reconnection. Therefore, identifying the structures of magnetic island is crucial to improving our understanding of magnetic reconnection. Using two-dimensional (2-D) particle-in-cell (PIC) simulations, we demonstrate that the out-of-plane magnetic field has a dip in the center of magnetic island, which is formed during multiple X line guide field reconnection. Such structures are considered to be produced by the current system in the magnetic island. At the edge of the magnetic island, there exists a current anti-parallel to the in-plane magnetic field, while the current is parallel to the in-plane magnetic field inside the magnetic island. Such a dual-ring current system, which is attributed to the electron dynamics in the magnetic island, leads to the dip of the out-of-plane magnetic field in the center of the island. The relevance between our simulations and crater flux transfer events (C-FTEs) is also discussed.

Magnetopause response to variations in the solar wind: Conjunction observations between Cluster, TC-1, and SuperDARN

How the solar wind affects the location of the magnetopause has been widely studied and excellent models of the magnetopause based on in situ observations in the solar wind and at the magnetopause have been established, while the careful insight into the responses of the magnetopause to the variations in the solar wind can still provide us some new information about the processes in space plasmas. The short distance from Cluster to TC-1 on 9 March 2004, between 06: 10 and 08: 10 UT, gives us a good opportunity to precisely monitor the responses of the magnetopause to the variations in the solar wind. On the basis of the combined observations between Cluster, TC-1, and SuperDARN we analyze the magnetopause crossings associated with magnetopause motion or magnetic reconnection when the solar wind conditions have a series of variations. New results about the time delays for the propagation from the solar wind monitor to the magnetopause of the interplanetary magnetic fields (IMF) and of the solar wind dynamic pressure, respectively, and the intrinsic time for reconnection onset at the magnetopause are obtained. The most important feature of the event is that the dynamic pressure and the IMF in the solar wind do not arrive at the magnetopause at the same time, which will direct us to find out how the variation in the solar wind dynamic pressure is transported from the bow shock to the magnetopause. Another significant feature is that this event presents a shorter intrinsic time, similar to 2 min, for reconnection onset at the dayside magnetopause than that given by the previous work of Le et al. (1993) and Russell et al. (1997).

FAN-SHAPED JETS IN THREE-DIMENSIONAL RECONNECTION SIMULATION AS A MODEL OF UBIQUITOUS SOLAR JETS

Magnetic reconnection is a fundamental process in space and astrophysical plasmas in which oppositely directed magnetic fields changes its connectivity and eventually converts its energy into kinetic and thermal energy of the plasma. Recently, ubiquitous jets (for example, chromospheric anemone jets, penumbral microjets, umbral light bridge jets) have been observed by Solar Optical Telescope on board the satellite Hinode. These tiny and frequently occurring jets are considered to be a possible evidence of small-scale ubiquitous reconnection in the solar atmosphere. However, the details of three dimensional magnetic configuration are still not very clear. Here we propose a new model based on three dimensional simulations of magnetic reconnection using a typical current sheet magnetic configuration with a strong guide field. The most interesting feature is that the jets produced by the reconnection eventually move along the guide field lines. This model provides a fresh understanding of newly discovered ubiquitous jets and moreover a new observational basis for the theory of astrophysical magnetic reconnection.

On nonlinear decay of kinetic Alfvén waves and application to some processes in space plasmas

This paper considers the nonlinear decay of the kinetic Alfvén waves (KAW) in the space plasmas. By using a two‐fluid model, we obtain a nonlinear equation to investigate the resonant interaction among three kinetic Alfvén waves. It is shown that the parametric instability of the kinetic Alfvén wave becomes important when its perpendicular wavelength is the order of the ion acoustic gyroradius or the electron inertial length. We give a detailed discussion for the KAW decay in the plasma inertial range and show that (1) the reverse decay of the kinetic Alfvén wave is stronger than its parallel decay for the arbitrary wavelength range; (2) the reverse decay is lager than the parallel decay for small angles of two perpendicular wave vectors of the decay waves, and these two decays are zero for large angles; (3) both growth rates depend on the choice of the wave number range of the decay waves; and (4) there exists two forbidden regions for the KAW decay. In this paper, we also discuss the nonlinear decay of the kinetic Alfvén waves in the auroral zone and show that the parametric instability can occur there and may play an important role in forming two reverse electron streaming fluxes in the electron acceleration region.

COMBINED MODELING OF ACCELERATION, TRANSPORT, AND HYDRODYNAMIC RESPONSE IN SOLAR FLARES. I. THE NUMERICAL MODEL

Acceleration and transport of high-energy particles and fluid dynamics of atmospheric plasma are interrelated aspects of solar flares, but for convenience and simplicity they were artificially separated in the past. We present here self-consistently combined Fokker-Planck modeling of particles and hydrodynamic simulation of flare plasma. Energetic electrons are modeled with the Stanford unified code of acceleration, transport, and radiation, while plasma is modeled with the Naval Research Laboratory flux tube code. We calculated the collisional heating rate directly from the particle transport code, which is more accurate than those in previous studies based on approximate analytical solutions. We repeated the simulation of Mariska et al. with an injection of power law, downward-beamed electrons using the new heating rate. For this case, a ~10% difference was found from their old result. We also used a more realistic spectrum of injected electrons provided by the stochastic acceleration model, which has a smooth transition from a quasi-thermal background at low energies to a nonthermal tail at high energies. The inclusion of low-energy electrons results in relatively more heating in the corona (versus chromosphere) and thus a larger downward heat conduction flux. The interplay of electron heating, conduction, and radiative loss leads to stronger chromospheric evaporation than obtained in previous studies, which had a deficit in low-energy electrons due to an arbitrarily assumed low-energy cutoff. The energy and spatial distributions of energetic electrons and bremsstrahlung photons bear signatures of the changing density distribution caused by chromospheric evaporation. In particular, the density jump at the evaporation front gives rise to enhanced emission, which, in principle, can be imaged by X-ray telescopes. This model can be applied to investigate a variety of high-energy processes in solar, space, and astrophysical plasmas.

Electron trapping around a magnetic null

Magnetic reconnection is an important process in astrophysical, space and laboratory plasmas. The magnetic null pair structure is theoretically suggested to be a crucial feature of the three‐dimensional magnetic reconnection. The physics around the null pair, however, has not been explored in combination with the magnetic field configuration deduced from in situ observations. Here, we report the identification of the configuration around a null pair and simultaneous electron dynamics near one null of the pair, observed by four Cluster spacecraft in the geo‐magnetotail. Further, we propose a new scenario of electron dynamics in the null region, suggesting that electrons are temporarily trapped in the central reconnection region including electron diffusion region resulting in an electron density peak, accelerated possibly by parallel electric field and electron pressure gradient, and reflected from the magnetic cusp mirrors leading to the bi‐directional energetic electron beams, which excite the observed high frequency electrostatic waves.

Design and operations of a multi-satellite system for the Cross-Scale mission concept

The ESA-funded “Cross-Scale Technology Reference Study” has been carried out with the primary aim to identify and analyse a mission concept for the investigation of fundamental space plasma processes that involve dynamical non-linear coupling across multiple length scales. To fulfil this scientific mission goal, a constellation of spacecraft is required, flying in loose formations around the Earth and sampling three characteristic plasma scale distances simultaneously, with at least two satellites per scale: electron kinetic (∼10km), ion kinetic (∼100–2000km) and magnetospheric fluid (∼3000–15000km).The key Cross-Scale (CS) mission drivers identified are the number of S/C and the space segment configuration, the reference orbit design, the transfer and deployment strategy, the S/C and payload design philosophy, the inter-satellite localisation and synchronisation process, as well as the mission operations.This paper presents a comprehensive overview of the mission design and analysis for the CS concept and outlines a technically feasible system architecture for a multi-dimensional investigation of space plasma phenomena. The main effort has been devoted to apply a thorough system-level trade-off approach and to accomplish an exhaustive analysis, so as to allow the characterisation of a wide range of mission requirements and design solutions.

Effects of ion temperature anisotropy on diffusion process in plasma with pump

On the basis of kinetic fluctuation theory, we consider the parametric interaction of lower and upper hybrid waves with ion-cyclotron oscillations in a plasma where the ion velocity distribution function is anisotropic. The turbulent diffusion coefficient is found to depend significantly on the ratio between the parallel and perpendicular ion temperatures. Our results can be of interest for the investigation of anomalous transport processes in space and laboratory plasma with ion temperature anisotropy.

Laser Plasma Experiments to Simulate Coronal Mass Ejections During Giant Solar Flare and Their Strong Impact on Magnetospheres

Giant solar flares are the most powerful phenomenon in the solar system, which can strongly affect various geospheres and technical systems in the near Earth's space or its surface. During the space era, only few events with a total energy of more than 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">34</sup> erg happened, and probably, only one of these ldquowas directedrdquo to the Earth (August 4, 1972). In this paper, we report on the first attempts to simulate in a laboratory both the initial (at the Sun) and final (near the Earth) stages of relevant interaction processes between the plasma flows and magnetic fields. By using laser-produced plasmas and intense magnetic dipole, we performed two types of simulation experiments: 1) on the interaction of ejected solar plasma flows with/in dipole magnetic field and 2) on the extreme (three fold) compression of the Earth's magnetopause by giant coronal mass ejections from the Sun. General physical conditions of these phenomena are briefly described, and the developed methods of laboratory simulation and numerical modeling of various explosive processes in collisionless space plasmas are discussed on the basis of relevant dimensionless criteria of the problems.

On interrelating laboratory experiments and geoplasma observations

The history, benefits, suitability and limitations of laboratory simulation of space plasma processes are reviewed. Aspects of waves, instabilities, nonlinearities, particle transport, reconnection and hydrodynamics are addressed in terms of interrelated experiments performed in space and in the laboratory. Over time, the degree to which the interrelated experiments can be compared has increased, thanks to improved diagnostic techniques in space and closer attention to matching dimensionless space parameters in the laboratory.

The Double Star magnetic field investigation: instrument design, performance and highlights of the first year's observations

Abstract. One of the primary objectives of the Double Star mission is the accurate measurement of the magnetic field vector along the orbits of the two spacecraft. The magnetic field is an essential parameter for the understanding of space plasma processes and is also required for the effective interpretation of data from the other instruments on the spacecraft. We present the design of the magnetic field instrument onboard both of the Double Star spacecraft and an overview of the performance as measured first on-ground and then in-orbit. We also report the results of in-flight calibration of the magnetometers, and the processing methods employed to produce the final data products which are provided to Double Star investigators, and the wider community in general. Particular attention is paid to the techniques developed for removing magnetic interference generated by the solar arrays on the first (equatorial orbiting) spacecraft. Results from the first year of operations are reviewed in the context of combined observations by Double Star and Cluster, and examples given from the different regions visited by the spacecraft to date.

Simulation of Space Plasma Allowing for the Hall Effect: Magnetic Reconnection

This is a survey of literature on simulation of magnetic reconnection in the presence of the Hall effect. We give a brief description of the Earth's magnetosphere, the ideal and the Hall MHD models, as well as the “traditional” and the “modified” electromagnetic hybrid models. For the Hall MHD we analyze the Hall term in the generalized Ohm's law. We review publications on the modeling of reconnection processes in space plasma that use Hall MHD, PIC simulation, and hybrid simulation. The main focus is on the manifestation of Hall currents. Simulation results using different models are compared. A separate section examines studies on the simulation of current sheet dynamics, which is closely linked with magnetic reconnection.

The role of Hall currents on incompressible magnetic reconnection

Magnetic reconnection is one of the most important energy conversion processes in space plasmas. Theoretical models of magnetic reconnection have been traditionally developed within the framework of magnetohydrodynamics (MHD). However, in low density astrophysical plasmas as those found in the magnetopause and the magnetotail, the current sheet thickness can be comparable to the ion inertial scale and therefore the Hall electric field becomes non-negligible. The role of the Hall current is to increase the reconnection rate with respect to MHD predictions, which therefore poses a promising mechanism for fast reconnection. We present results from parallel simulations of the incompressible Hall MHD equations in 2 1 2 dimensions. We quantitatively evaluate the relevance of the Hall current in the reconnection process by performing a set of simulations with different values of the Hall parameter. We compute the corresponding reconnection rates as a function of time, and explore the spatial structure of the fields in the surroundings of the diffusion region. We quantify the increase of the reconnection rate as a function of the Hall parameter, and confirm the presence of a quadrupolar structure for the out-of-plane magnetic field.

Perspectives in space plasma theory

The past half century has been characterized by the birth of space plasma physics and its rise to maturity which has led into a certain mood of satisfaction and partial saturation with its achievements. Indeed, on the global scale our knowledge about the magnetosphere, the solar wind from the Sun out to its boundaries, even of the conditions on the surface and in the atmosphere of the Sun and the Sun’s interior has to a high degree become complete. The degree of completion is indeed such that it allows predicting up to a certain precision their behavior. New names like solar seismology, solar meteorology, and space weather have been coined indicating the transition of some of the sub-field of Space Plasma Physics into the domain of an industry of monitoring and predicting in the interest of the needs of a general society. This is an entirely healthy evolution of a scientific field that is on the path of completion. Still there is a large number of questions which should and can hopefully be answered by space plasma theory. Some of them will be listed here in view of what has been achieved and in which direction future theory could proceed in order to contribute to their resolution when addressing the already operating and the new and upcoming space missions like Cluster, the planned Magnetospheric Multi-Scale mission MMS and others, respectively, most of which are designed to become multi-spacecraft enterprizes and also in view of the new theoretical and computational techniques and methods that have been developed during the past decades. These directions reach from the detailed understanding of the processes in the solar atmosphere and solar wind through the detailed physics of the formation of collisionless shocks, magnetosheaths and turbulence, the microphysics of reconnection and particle acceleration, and the substorm mechanism to the detailed understanding of planetary magnetospheres, the outer heliosphere with its heliospheric termination shock, heliosheath and boundary layer. All this research though already in flow has by far not been completed yet. It requires new efforts as it implies resolution of the smallest though still non-atomic scales, but it has high implications for the understanding of the processes in space plasma and, in extension, of processes in the remote astrophysical systems which can be understood only by observation of their radiation in all wavebands.