Strong space plasma magnetic barriers and Alfvénic collapse (original) (raw)
Related papers
Critical Issues on Magnetic Reconnection in Space Plasmas
Space Science Reviews, 2005
The idea of expedient energy transformation by magnetic reconnection (MR) has generated much enthusiasm in the space plasma community. The early concept of MR, which was envisioned for the solar flare phenomenon in a simple two-dimensional (2D) steady-state situation, is in dire need for extension to encompass three-dimensional (3D) non-steady-state phenomena prevalent in space plasmas in nature like in the magnetosphere. A workshop was organized to address this and related critical issues on MR. The essential outcome of this workshop is summarized in this review. After a brief evaluation on the pros and cons of existing definitions of MR, we propose essentially a working definition that can be used to identify MR in transient and spatially localized phenomena. The word "essentially" reflects a slight diversity in the opinion on how transient and localized 3D MR process might be defined. MR is defined here as a process with the following characteristics:
Impulsive plasma transport through the magnetopause
Geophysical Research Letters, 1982
A localized plasma cloud of magnetosheath plasma with some excess momentum is assumed to distort the surface of the magnetopause, and its associated currents, inducing an electric field that can be of the order of lmV/m. This induction electric field by itself is just what is needed so that the plasma can follow the moving magnetopause. A normal component of the magnetic field B n through the magnetopause will permit a small field-aligned polarization current; this current will deliver charge that will create an electrostatic field. The normal component of the total electric field will be reduced (perhaps to zero) while the tangential component will be enhanced. This enhancement will allow the cloud to continue moving toward the moving magnetopause. At the same time the plasma particles will be slightly energized, and being propelled by the mirror force-•VB they will become more field aligned as they go through the magnetopause. Energy for these events comes from the excess momentum Via the induction electric field. Once inside the moving magnetopause, the cloud can go across field lines (either open or closed) until it loses its excess momentum. A cross-sectional slice of the plasma cloud (at the inner edge of the magnetopause current) acts as a generator; the whole process can be regarded as an electric circuit, with a generator preceding the load, the trailing portion of the current. Losses of partic]es, momentum, and energy will occur; the mechanism described is one possible form of "viscous interaction" between the shocked solar wind and the magnetosphere. The total amount of power going into the plasma is likely to be much less than 5x10 • watts, and may even be negative, indicating the futility of searching for dissipation of this magnitude. The discovery of the entry layer connected to the dayside cusps by the satellite HEOS-2 (Paschmann et el., 1976) was a turning point for magnetospheric physics. Until then two-dimensional time-independent theories of magnetic reconnection (Vasy]iunas, 1975;' Sonnerup, 1979) had been strongly favored as the main process for the interaction of solar wind plasma with the magnetosphere. This process depends on an open magnetosphere and an X-line in the subsolar region; if there is a dawn-dusk electric field along the X-type neutral line (Figure la), plasma would be convected towards the separatrix from both sides in the equatorial plane. An eastward current sheet would be formed, constituting the magnetopause; this current J together with the norma] component of the magnetic field B through the magnetopause would provide a plasma force F=J x B producing a jet of plasma toward higher latitudes on open magnetic field lines. At the same time E ' J is positive, showing that the plasma gains energy in the process. An important fact about the entry layer is that at least part of it seems to be on closed lines. This is implied by two observations; first, that the magnetic field is northward for any orientation of the IMF, and second, and most important, that energetic electrons showed a trapped pitch angle distribution. In contrast, the high speed jets predicted by reconnection theories would be on open field lines. Until recently, evidence for these jets of energized plasma had been missing, suggesting that the electric field along the magnetopause is very small (Heikkila, 1975). Now the ISEE satellites have provided new evidence on the plasma jets (Paschmann et el., 1979; Sonnerup et el., 1981), and also on the tangential component of the electric field (Mozer, et el., 1979). However, the problem is far from being settled. The measurement of the electric field is very difficult, especially at the magnetopause with its turbulent behavior; moreover, on the pass reported by Mozer there was no evidence for plasma energization or for high speed jets. The plasma jets are more convincing, but Johnstone (1979) in a comment on the paper by Paschmann et el. (1979) has asked the question: "If the effect is so easily detected in this crossing, why has it
Magnetospheric plasma boundaries: a test of the frozen-in magnetic field theorem
Annales Geophysicae, 2005
The notion of frozen-in magnetic field originates from H. Alfvén, the result of a work on electromagnetichydrodynamic waves published in 1942. After that, the notion of frozen-in magnetic field, or ideal MHD, has become widely used in space plasma physics. The controversy on the applicability of ideal MHD started in the late 1950s and has continued ever since. The applicability of ideal MHD is particularly interesting in regions where solar wind plasma may cross the magnetopause and access the magnetosphere. It is generally assumed that a macroscopic system can be described by ideal MHD provided that the violations of ideal MHD are sufficiently small-sized near magnetic x-points (magnetic reconnection). On the other hand, localized departure from ideal MHD also enables other processes to take place, such that plasma may cross the separatrix and access neighbouring magnetic flux tubes. It is therefore important to be able to quantify from direct measurements ideal MHD, a task that has turned out to be a major challenge.
Annales Geophysicae, 2005
An event of strong flux pile-up and plasma depletion at the high latitude magnetopause tailward of the cusp has been analyzed based on observations by the suite of Cluster spacecraft. The multi-satellite analysis facilitates the separation of temporal and spatial features and provides a direct estimate for the strength of the plasma depletion layer for this event. A doubling of the magnetic field strength and a forty percent reduction of the density are found. Our analysis shows that roughly half of the total magnetic field increase occurs within 0.6 R E of the magnetopause and another quarter within a distance of 1.2 R E . In addition, the plasma depletion signatures exhibit temporal variations which we relate to magnetopause dynamics.
MHD model of magnetosheath flow: comparison with AMPTE/IRM observations on 24 October, 1985
Annales Geophysicae, 1998
We compare numerical results obtained from a steady-state MHD model of solar wind¯ow past the terrestrial magnetosphere with documented observations made by the AMPTE/IRM spacecraft on 24 October, 1985, during an inbound crossing of the magnetosheath. Observations indicate that steady conditions prevailed during this about 4 hour-long crossing. The magnetic shear at spacecraft entry into the magnetosphere was 15. A steady density decrease and a concomitant magnetic ®eld pileup were observed during the 40 min interval just preceding the magnetopause crossing. In this plasma depletion layer (1) the plasma beta dropped to values below unity; (2) the¯ow speed tangential to the magnetopause was enhanced; and (3) the local magnetic ®eld and velocity vectors became increasingly more orthogonal to each other as the magnetopause was approached (Phan et al., 1994). We model parameter variations along a spacecraft orbit approximating that of AMPTE/IRM, which was at slightly southern GSE latitudes and about 1.5 h postnoon Local Time. We model the magnetopause as a tangential discontinuity, as suggested by the observations, and take as input solar wind parameters those measured by AMPTE/IRM just prior to its bow shock crossing. We ®nd that computed ®eld and plasma pro®les across the magnetosheath and plasma depletion layer match all observations closely. Theoretical predictions on stagnation line¯ow near this low-shear magnetopause are con®rmed by the experimental ®ndings. Our theory does not give, and the data on this pass do not show, any localized density enhancements in the inner magnetosheath region just outside the plasma depletion layer.
Plasma Physics and Controlled Fusion, 2008
Here we describe a new experiment to test the shielding concept of a dipolelike magnetic field and plasma, surrounding a spacecraft forming a 'mini magnetosphere'. Initial laboratory experiments have been conducted to determine the effectiveness of a magnetized plasma barrier to be able to expel an impacting, low beta, supersonic flowing energetic plasma representing the solar wind. Optical and Langmuir probe data of the plasma density, the plasma flow velocity and the intensity of the dipole field clearly show the creation of a narrow transport barrier region and diamagnetic cavity virtually devoid of energetic plasma particles. This demonstrates the potential viability of being able to create a small 'hole' in a solar wind plasma, of the order of the ion Larmor orbit width, in which an inhabited spacecraft could reside in relative safety. The experimental results have been quantitatively compared with a 3D particle-incell 'hybrid' code simulation that uses kinetic ions and fluid electrons, showing good qualitative agreement and excellent quantitative agreement. Together the results demonstrate the pivotal role of particle kinetics in determining generic plasma transport barriers.
2005
Within the framework of a two-fluid description possible pathways for the generation of fast flows (dynamical as well as steady) in the lower solar atmosphere is established. It is shown that a primary plasma flow (locally sub-Alfvénic) is accelerated when interacting with emerging/ambient arcade--like closed field structures. The acceleration implies a conversion of thermal and field energies to kinetic energy of the flow. The time-scale for creating reasonably fast flows ($\gtrsim 100$ km/s) is dictated by the initial ion skin depth while the amplification of the flow depends on local beta\beta beta. It is shown, for the first time, that distances over which the flows become "fast" are sim0.01Rs\sim 0.01 R_ssim0.01Rs from the interaction surface; later the fast flow localizes (with dimensions lesssim0.05RS\lesssim 0.05 R_Slesssim0.05RS) in the upper central region of the original arcade. For fixed initial temperature the final speed ($\gtrsim 500 km/s$) of the accelerated flow, and the modification of the field structure are independent of the time-duration (life-time) of the initial flow. In the presence of dissipation, these flows are likely to play a fundamental role in the heating of the finely structured Solar atmosphere.
Plasma and energetic particle behaviors during asymmetric magnetic reconnection at the magnetopause
2014), Plasma and energetic particle behaviors during asymmetric magnetic reconnection at the magnetopause, Abstract The factors controlling asymmetric reconnection and the role of the cold plasma population in the reconnection process are two outstanding questions. We present a case study of multipoint Cluster observations demonstrating that the separatrix and flow boundary angles are greater on the magnetosheath than on the magnetospheric side of the magnetopause, probably due to the stronger density than magnetic field asymmetry at this boundary. The motion of cold plasmaspheric ions entering the reconnection region differs from that of warmer magnetosheath and magnetospheric ions. In contrast to the warmer ions, which are probably accelerated by reconnection in the diffusion region near the subsolar magnetopause, the colder ions are simply entrained by × drifts at high latitudes on the recently reconnected magnetic field lines. This indicates that plasmaspheric ions can sometimes play only a very limited role in asymmetric reconnection, in contrast to previous simulation studies. Three cold ion populations (probably H + , He + , and O + ) appear in the energy spectrum, consistent with ion acceleration to a common velocity.
Journal of Geophysical Research: Space Physics, 1998
We present predictions on the signatures of flux transfer events (FTEs) at the dayside magnetopause by the single X line MHD model based on the subsolar environment: initially in the magnetosheath with background flow northward parallel to the dayside magnetopause. In conformity with our previous investigation [Ku and Sibeck, 1997] we choose realistic parameteres for the simulation with the ratio of Pmsh/Psph-10, Bmsh/Bsp h-0.5, and rmsh/rsp h-0.175 along the magnetosheath and magnetospheric sides. The localized resistivity turns on near the low northern latitude of dayside magnetopause, and the initial magnitude of background magnetosheath plasma flow is set 0.15 VAO (Bsph/4t9msh•O), or equivalent to 30% of magnetosheath Alfv6n velocity. Simulation results produce signatures of FTEs similar to the previous case without the background magnetosheath flow [Ku and Sibeck, 1997] but different in some respects. Events moving opposite to this flow slow down but intensify, whereas events moving in the direction of the magnetosheath flow accelerate but weaken. Similarly, the satellite observes no significant signatures of FTEs in the magnetosphere. All the FTEs exhibit asymmetric bipolar magnetic field signatures normal to the magnetopause. Scatterplots versus the plasma density reveal that steep changes for the temperature kT, the magnetic field B z, and the Alfv6n velocity occur in the magnetospheric side immediately adjacent to the magnetopause.