MHD model of magnetosheath flow: comparison with AMPTE/IRM observations on 24 October, 1985 (original) (raw)
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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.
Plasma penetration of the dayside magnetopause
Physics of Plasmas, 2012
Data from the Cluster spacecraft during their magnetopause crossing on 25 January 2002 are presented. The magnetopause was in a state of slow non-oscillatory motion during the observational period. Coherent structures of magnetosheath plasma, here typified as plasmoids, were seen on closed magnetic field lines on the inside of the magnetopause. Using simultaneous measurements on two spacecraft, the inward motion of the plasmoids is followed from one spacecraft to the next, and it is found to be in agreement with the measured ion velocity. The plasma characteristics and the direction of motion of the plasmoids show that they have penetrated the magnetopause, and the observations are consistent with the concept of impulsive penetration, as it is known from theory, simulations, and laboratory experiments. The mean flux across the magnetopause observed was 0.2%-0.5% of the solar wind flux at the time, and the peak values of the flux inside the plasmoids reached approximately 20% of the solar wind flux. V
Advances in Space Research, 2006
A problem of the solar wind flow around the magnetopause is rather complex but a combination of the numerical modeling and experimental observations can bring a progress in magnetosheath studies. In this contribution, we have used the MHD BATS-R-US model for prediction of magnetosheath ion flux and magnetic field profiles under specific upstream conditions. The comparison is based on two INTERBALL-1 passes through the magnetosheath at low and high latitudes. We discuss the influence of the EarthÕs dipole tilt, the IMF direction, and latitude dependence on profiles in both MHD simulations and experimental investigations.
Journal of Geophysical Research, 2012
Here, we present a case study of THEMIS and ground-based observations on the dayside magnetopause, and geomagnetic field perturbations related to the interaction of an interplanetary directional discontinuity (DD), as observed by ACE, within the magnetosphere on 16 June 2007. The interaction resulted in a large-scale local magnetopause distortion of an "expansion-compressionexpansion" (ECE) sequence that lasted for ~15 min. The compression was caused by a very dense, cold, and fast high-β magnetosheath plasma flow, a so-called plasma jet, whose kinetic energy was approximately three times higher than the energy of the incident solar wind. The plasma jet resulted in the effective penetration of the magnetosheath plasma inside the magnetosphere. A strong distortion of the Chapman-Ferraro current in the ECE sequence generated a tripolar magnetic pulse "decrease-peak-decrease" (DPD) that was observed at low and middle latitudes by the INTERMAGNET network of ground-based magnetometers. The characteristics of the ECE sequence and the spatial-temporal dynamics of the DPD pulse were found to be very different from any reported patterns of DD interactions within the magnetosphere. The observed features only partially resembled structures such as FTE, hot flow anomalies, and transient density events. Thus, it is difficult to explain them in the context of existing models.
Simulated magnetopause losses and Van Allen Probe flux dropouts
Geophysical Research Letters, 2014
Three radiation belt flux dropout events seen by the Relativistic Electron Proton Telescope soon after launch of the Van Allen Probes in 2012 (Baker et al., 2013a) have been simulated using the Lyon-Fedder-Mobarry MHD code coupled to the Rice Convection Model, driven by measured upstream solar wind parameters. MHD results show inward motion of the magnetopause for each event, along with enhanced ULF wave power affecting radial transport. Test particle simulations of electron response on 8 October, prior to the strong flux enhancement on 9 October, provide evidence for loss due to magnetopause shadowing, both in energy and pitch angle dependence. Severe plasmapause erosion occurred during~14 h of strongly southward interplanetary magnetic field B z beginning 8 October coincident with the inner boundary of outer zone depletion. HUDSON ET AL.