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Kinetic Alfven waves and plasma transport at the magnetopause

Description: Large amplitude compressional type waves, with frequencies ranging from 10--500 mHz, are nearly always found in the magnetosheath near the magnetopause where there are large gradients in density, pressure and magnetic field. As compressional waves propagation to the magnetopause, there gradients efficiently couple them with shear/kinetic Alfven waves near the Alfven field-line resonance location ({omega} = k{sub {parallel}} v{sub A}). The authors present a solution of the kinetic-MHD wave equations for this process using a realistic equilibrium profile including full ion Larmor radius effects and wave-particle resonance interactions for electrons and ions to model the dissipation. For northward IMF a KAW propagates backward to the magnetosheath. For southward IMF the wave remains in the magnetopause but can propagate through the k{sub {parallel}} = 0 location. The quasi-linear theory predicts that KAWs produce plasma transport with a diffusion coefficient D{sub {perpendicular}} {approximately} 10{sup 9} m{sup 2}/s and plasma convection on the order of 1 km/s. However, for southward IMF additional transport can occur because magnetic islands form at the k{sub {parallel}} = 0 location. Due to the broadband nature of the observed waves these islands can overlap leading to stochastic transport which is much larger than that due to quasilinear effects.
Date: May 1, 1997
Creator: Johnson, J.R. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department

Magnetic reconnection in space plasmas

Description: This is the final report of a three-year, Laboratory-Directed Research and Development (LDRD) project at the Los Alamos National Laboratory (LANL). Magnetic reconnection produces fundamental changes in the magnetic field topology of plasmas and leads ultimately to substantial plasma heating and acceleration. The transfer of stored magnetic field energy to the plasma occurs primarily at thin conversion layers that extend outward from the reconnection site. We performed a comparative study of the structure and nature of these conversion layers as observed during reconnection at Earth`s magnetopause and in the geomagnetic tail. Our research utilized plasma and magnetic field data from the Earth-orbiting ISEE satellites during crossings of the conversion layers at the magnetopause and in the geomagnetic tail, as well as data obtained during a long-duration balloon flight in Antarctica and simultaneously from satellites in geosynchronous orbit. We have found that the reconnection layer at the magnetopause usually does not contain a slow mode shock, contrary to earlier theoretical expectations. Through a coordinated analysis of data obtained from balloon altitudes and at geosynchronous orbit, we obtained evidence that reconnection can occur simultaneously in both hemispheres at the magnetopause above the polar caps. The final year of our study was oriented primarily towards the question of determining the magnetic topology of disturbances in the solar wind associated with coronal mass ejections (CMEs) and understanding how that topology is affected by magnetic reconnection occurring near the Sun.
Date: April 1, 1996
Creator: Gosling, J.; Feldman, W. & Walthour, D.
Partner: UNT Libraries Government Documents Department

Stochastic Ion Heating at the Magnetopause due to Kinetic Alfven Waves

Description: The magnetopause and boundary layer are typically characterized by large amplitude transverse wave activity with frequency below the ion cyclotron frequency. The signatures of the transverse waves suggest that they are kinetic Alfven waves with wavelength on the order of the ion gyroradius. We investigate ion motion in the presence of large amplitude kinetic Alfven waves with wavelength the order of rho(subscript ''i'') and demonstrate that for sufficiently large wave amplitude (delta B(subscript ''perpendicular'')/B(subscript ''0'') > 0.05) the particle orbits become stochastic. As a result, low energy particles in the core of the ion distribution can migrate to higher energy through the stochastic sea leading to an increase in T(subscript ''perpendicular'') and a broadening of the distribution. This process can explain transverse ion energization and formation of conics which have been observed in the low-latitude boundary layer.
Date: August 10, 2001
Creator: Johnson, Jay R. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department

Signatures of mode conversion and kinetic Alfven waves at the magnetopause

Description: It has been suggested that resonant mode conversion of compressional MHD waves into kinetic Alfven waves at the magnetopause can explain the abrupt transition in wave polarization from compressional to transverse commonly observed during magnetopause crossings. The authors analyze magnetic field data for magnetopause crossings as a function of magnetic shear angle (defined as the angle between the magnetic fields in the magnetosheath and magnetosphere) and compare with the theory of resonant mode conversion. The data suggest that amplification in the transverse magnetic field component at the magnetopause is not significant up to a threshold magnetic shear angle. Above the threshold angle significant amplification results, but with weak dependence on magnetic shear angle. Waves with higher frequency are less amplified and have a higher threshold angle. These observations are qualitatively consistent with theoretical results obtained from the kinetic-fluid wave equations.
Date: July 21, 2000
Creator: Johnson, Jay R. & Cheng, C. Z.
Partner: UNT Libraries Government Documents Department

Global structure of mirror modes in the magnetosheath

Description: A global stability analysis of mirror modes in the magnetosheath is presented. The analysis is based upon the kinetic-MHD formulation which includes relevant kinetic effects such as Landau resonance and gradient drift effects related to inhomogeneities in the background density, temperature, pressure and its anisotropy, magnetic field, and plasma flow velocity. Pressure anisotropy provides the free energy for the global mirror mode. The local theory of mirror modes predicts purely growing modes confined in the unstable magnetosheath region; however, the nonlocal theory that includes the effects of gradients and plasma flow predicts modes with real frequencies which propagate with the flow from the magnetosheath toward the magnetopause boundary. The real frequency is on the order of a combination of the diamagnetic drift frequency and the Doppler shift frequency associated with plasma flow. The diamagnetic drift frequency provides a wave phase velocity in the direction of the magnetopause so that wave energy accumulates against the magnetopause boundary, and the amplitude is skewed in that direction. On the other hand, plasma flow also gives rise to a real phase velocity, but the phase velocity is smaller than the flow velocity. As a result, the wave amplitude is increased in the wake of the plasma flow and piles up against the bow shock boundary.
Date: November 1, 1996
Creator: Johnson, J.R. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department

Magnetopause structure and dynamics: Issues for GEM (geospace environment modeling)

Description: Recent multi-spacecraft observations of the magnetopause have allowed us to establish its structure and dynamical behavior. The magnetopause current sheet is thicker than expected, often ten magnetosheath ion gyroradii or more. One very important result has been the confirmation of magnetic reconnection in both its quasi- steady and transient forms. A boundary layer of magnetosheath-like plasma is often, but not always, observed earthward of the magnetopause current layer. There is considerable small-scale magnetic structure within the current layer, suggesting the presence of filamentary currents much smaller than an ion gyroradius. Such micro-structure may be important in particle diffusion and, hence, reconnection. There are many outstanding questions, among them: How does the low latitude boundary layer form. Why is the magnetopause current layer so thick. What is the detailed structure and topology of FTEs. How are quasi-steady and transient reconnection related. The GEM program may help us address these issues. 18 refs., 9 figs.
Date: January 1, 1989
Creator: Elphic, R.C.
Partner: UNT Libraries Government Documents Department

Proxy and in-situ studies of dayside magnetopause reconnection

Description: The functional dependence of magnetic reconnection on solar wind parameters is examined utilizing the am geomagnetic index and satellite observations at the magnetopause. Several parameters in the solar wind are found to control geomagnetic activity. Reconnection is found to be most efficient when the interplanetary magnetic field is southward, although some activity remains when the IMF is horizontal and slightly northward. The reconnection efficiency increases with the solar wind dynamic pressure but decreases when the Mach number is greater than 7.5. These results are compared with the functional dependencies found by correlating solar wind and magnetosheath measurements with observations of accelerated tows at the magnetopause. Accelerated tows are found to occur most often when the interplanetary magnetic field is directed southward. However, accelerated flows do occur when the IMF is horizontal and northward. Accelerated flows are also affected by the magnetosheath beta such that higher beta inhibits their occurrence. The location of accelerated tows indicates that reconnection occurs mainly at the subsolar point.
Date: January 1, 1992
Creator: Scurry, L.; Russell, C.T. (California Univ., Los Angeles, CA (United States). Inst. of Geophysics and Planetary Physics) & Gosling, J.T. (Los Alamos National Lab., NM (United States))
Partner: UNT Libraries Government Documents Department

Magnetopause layer and plasma boundary layer of the magnetosphere. [Configuration, kinetics]

Description: Due to recent availability and analyses of high time resolution satellite data (including IMP 6 and the ISEE-1 and -2 satellite pair), the study of the magnetopause and boundary layer has entered a period of renewed activity. Plasma observations from the VELA satellites first established the presence of magnetosheath-like plasma with reduced density and flow velocity in a relatively thin (approx.<1 R/sub E/) layer bordering the plasma sheet at low latitudes and bordering the lobe environment at high latitudes. Recent analyses of HEOS 2, Explorer 33 and IMP 6 data have established the presence of this ''plasma boundary layer'' (PBL) over the entire sunward magnetosphere near the magnetopause. This review gives a brief summary of recent published results on two distinct regions of the PBL: that bordering open field line regions and that bordering closed field line regions. The magnetopause layer (i.e., current layer) can usually be identified by a change in magnetic field direction and cannot be uniquely identified by any other field or plasma parameters. Immediately earthward of this magnetopause current layer, a PBL of magnetosheath-like plasma is usually observed that has dominantly magnetosheath-like energy spectra and flow characteristics. Observed plasma boundary layer thicknesses are highly variable and are generally much larger than the magnetopause layer thicknesses even near the subsolar region. Several suggested source mechanisms for the plasma boundary layer are discussed and compared.
Date: January 1, 1978
Creator: Eastman, T.E. & Hones, E.W. Jr.
Partner: UNT Libraries Government Documents Department

Kinetic Alfven Waves at the Magnetopause--Mode Conversion, Transport and Formation of LLBL

Description: At the magnetopause, large amplitude, low-frequency (ULF), transverse MHD waves are nearly always observed. These waves likely result from mode conversion of compressional MHD waves observed in the magnetosheath to kinetic Alfven waves at the magnetopause where there is a steep gradient in the Alfven velocity [Johnson and Cheng, Geophys. Res. Lett. 24 (1997) 1423]. The mode-conversion process can explain the following wave observations typically found during satellite crossings of the magnetopause: (1) a dramatic change in wave polarization from compressional in the magnetosheath to transverse at the magnetopause, (2) an amplification of wave amplitude at the magnetopause, (3) a change in Poynting flux from cross-field in the magnetosheath to field-aligned at the magnetopause, and (4) a steepening in the wave power spectrum at the magnetopause. We examine magnetic field data from a set of ISEE1, ISEE2, and WIND magnetopause crossings and compare with the predictions of theoretical wave solutions based on the kinetic-fluid model with particular attention to the role of magnetic field rotation across the magnetopause. The results of the study suggest a good qualitative agreement between the observations and the theory of mode conversion to kinetic Alfven waves. Because mode-converted kinetic Alfven waves readily decouple particles from the magnetic field lines, efficient quasilinear transport (D {approx} 109m2/s) can occur. Moreover, if the wave amplitude is sufficiently large (Bwave/B0 &gt; 0.2) stochastic particle transport also occurs. This wave-induced transport can lead to significant heating and particle entry into the low latitude boundary layer across closed field lines.At the magnetopause, large amplitude, low-frequency (ULF), transverse MHD waves are nearly always observed. These waves likely result from mode conversion of compressional MHD waves observed in the magnetosheath to kinetic Alfven waves at the magnetopause where there is a steep gradient in the Alfven velocity [Johnson and Cheng, Geophys. Res. Lett. ...
Date: May 31, 2002
Creator: Johnson, Jay R. & Cheng, C.Z.
Partner: UNT Libraries Government Documents Department

Plasma boundary layer and magnetopause layer of the earth's magnetosphere

Description: IMP 6 observations of the plasma boundary layer (PBL) and magnetopause layer (MPL) of the earth's magnetosphere indicate that plasma in the low-latitude portion of the PBL is supplied primarily by direct transport of magnetosheath plasma across the MPL and that this transport process is relatively widespread over the entire sunward magnetospheric boundary.
Date: June 1, 1979
Creator: Eastman, T.E.
Partner: UNT Libraries Government Documents Department

Field-aligned currents near the magnetosphere boundary

Description: This paper reviews present thinking about the structure of magnetospheric boundary layers and their roles in the generation of the field-aligned currents that are observed in the polar regions. A principal effect of the momentum loss by magnetosheath plasma to the magnetosphere boundary regions just within the magnetopause, whether it be by a diffusive process or by magnetic reconnection, is the tailward pulling of surface flux tubes relative to those deeper below the surface. The dayside region 1 currents at low altitudes flow along field lines in the resulting regions of magnetic shear. The direction of the shear and its magnitude, measured in the boundary region, confirm tht the polarities and intensities of the dayside region 1 currents can be accounted for by this process. The low latitude boundary layer, formerly thought to be threaded entirely by closed field lines, now appears to contain at least some open field lines, newly reconnected, that are in the process of being swept into the high latitude tail to form the plasma mantle. The open flux tubes of the flux transfer events, thought to be the product of patchy reconnection have a spiral magnetic structure whose helicity is such as to suggest currents having the polarities of the region 1 currents.
Date: January 1, 1983
Creator: Hones, E.W. Jr.
Partner: UNT Libraries Government Documents Department

Structure of the magnetopause current layer at the subsolar point

Description: A one-dimensional electromagnetic particle simulation model developed for the magnetopause current layer between the shocked solar wind and the dipole magnetic field at the subsolar point has been extended to include the interplanetary magnetic field (IMF) in the solar wind. Interaction of the solar wind with the vacuum dipole field as well as the dipole field filled with a low density magnetospheric plasma are studied. It is found that the width and the structure of the magnetopause current layer differ markedly depending on the direction of the IMF. When the IMF is pointing southward, the current layer between the solar wind and the dipole field is narrow and the magnetic field has a single ramp structure caused by the reflection of the solar wind at that point. The current layer becomes several times wider and the magnetic field developes a multiple ramp structure when the IMF is northward. This broadening of the current layer is caused by the multiple reflection of the solar wind by the magnetic field. For the northward IMF, the magnetic field does not change its sign across the current layer so that the E {times} B drift of the solar wind electrons remains the same direction while for the southward IMF, it reverses the sign. This results in a single reflection of the solar wind for the southward IMF and multiple reflections for the northward IMF. When a low density mangetospheric plasma is present in the dipole magnetic field, a small fraction of the solar wind ions are found to penetrate into the dipole magnetic field beyond the reflection point of the solar wind electrons. The width of the ion current layer is of the order of the solar wind ion gyroradius, however, the current associated with the ions remains much smaller than the electron current ...
Date: December 1, 1991
Creator: Okuda, H.
Partner: UNT Libraries Government Documents Department

Characteristics of the magnetospheric boundary layer and magnetopause layer in high time resolution

Description: A basic problem of magnetospheric physics is to determine how and where solar wind mass, momentum and energy is transferred to the magnetosphere. IMP 6 plasma and magnetic field data indicate that the closed magnetospheric boundary layer is always present adjacent to and earthward of the magnetopause layer at all locations equatorward of the entry layer and plasma mantle. All IMP 6 crossings show some magnetosheath-like plasma earthward of the magnetopause layer and the boundary layer electron spectra are often indistinguishable from the nearby magnetosheath electron spectra. In 24 out of 40 crossings, no change in density or electron spectra is observed near the magnetopause layer. In most remaining cases, changes in the plasma parameters occur primarily in the boundary layer. Observed boundary layer bulk plasma flow always has an anti-sunward component and often has a cross-field component. Energetic electron pitch angle distributions indicate that the boundary layer is on closed field lines. During six IMP 6 crossings near local noon, equatorward of the cusp regions, boundary layer plasma flow was observed to have an anti-sunward component consistent with flow away from the subsolar region or the nearby magnetosheath. The observations are not consistent with a primary source in the cusp region or any other nonlocal source. It is concluded that the boundary layer is populated primarily by direct plasma transport through the magnetopause layer.
Date: January 1, 1978
Creator: Eastman, T.E. & Hones, E.W. Jr.
Partner: UNT Libraries Government Documents Department

Large scale instabilities and dynamics of the magnetotail plasma sheet

Description: The stability properties of the magnetotail current sheet against large scale modes is reviewed in the framework of ideal MHD, resistive MHD, and collisionless Vlasov theory. It appears that the small deviations from a plane sheet pinch (in particular a magnetic field component normal to the sheet) are important to explain the transition of the tail from a quiet stable state to an unstable dynamic state. It is found that the tail is essentially stable in ideal MHD, but unstable in resistive MHD, while both stable and unstable configurations are found within collisionless theory. The results favor an interpretation where the onset of magnetotail dyanmics leading to a sudden thinning of the plasma sheet and the ejection of a plasmoid is caused by the onset of a collisionless instability that either directly leads to the growth of a collisionless tearing mode or via microscopic turbulence to the growth of a resistive mode. The actual onset conditions are not fully explored yet by rigorous methods. The onset may be triggered by local conditions as well as by boundary conditions at the ionosphere or at the magnetopause (resulting from solar wind conditions). 53 refs., 5 figs.
Date: January 1, 1986
Creator: Birn, J. & Schindler, K.
Partner: UNT Libraries Government Documents Department

Numerical simulations on the magnetopause current layer

Description: One-dimensional particle simulations are carried out in order to study the current layer between a plasma and magnetic field such as seen at the magnetopause boundary layer. When a subsonic solar wind plasma flow impinges upon a vacuum dipole magnetic field, the width of the current layer is found much smaller than the ion gyroradius and is close to theoretically predicted geometric mean of the ion and electron gyroradii. The width remains essentially the same when the magnetic field is filled with a thermal plasma whose density is smaller than the incoming solar wind density. The width, therefore, remains much smaller than the ion gyroradius. It is found that a similar sharp current layer develops in a plasma confined in a magnetic field such as seen in laboratory and space plasmas. 15 refs., 11 figs.
Date: December 1, 1990
Creator: Okuda, H.
Partner: UNT Libraries Government Documents Department

Observations of flux transfer events: Are FTEs flux ropes, islands, or surface waves

Description: Flux transfer events (FTEs) are widely regarded as a signature of transient magnetic reconnection between the solar wind and magnetospheric plasmas. However, there is disagreement on what form this reconnection takes: Are FTEs tearing islands, or time-varying single x-line reconnection We reexamine the evidence that first led to the suggestion that FTEs are related to a non-time-stationary reconnection process. In particular we discuss how the combination of field and plasma variations suggest that FTEs are magnetic flux ropes. Both time-varying single x-line reconnection and multiple x-line merging can produce a signature which 'mimics' that of a flux rope, but without the flux rope topology. Finally, we review the evidence that FTEs cannot be merely surface waves: their occurrence during southward IMF, mixture of solar wind and magnetospheric plasmas, leakage of energetic particles, accelerated plasma flows and peculiarities of the magnetic signature all point to a reconnection-related phenomenon. 38 refs., 16 figs.
Date: January 1, 1989
Creator: Elphic, R.C.
Partner: UNT Libraries Government Documents Department

A two-dimensional particle simulation of the magnetopause current layer

Description: We have developed a 2/1/2/-D (x, y, v/sub x/, v/sub y/, v/sub z/) electromagnetic code to study the formation and the stability of the magnetopause current layer. This code computes the trajectories of ion and electron particles in their self-consistently generated electromagnetic field and an externally imposed 2-D vacuum dipolar magnetic field. The results presented here are obtained for the simulation of the solar wind-magnetosphere interaction in the subsolar region of the equatorial plane. We observe the self-consistent establishment of a current layer resulting from both diamagnetic drift and E /times/ B drift due to the charge separation. The simulation results show that during the establishment of the current layer, its thickness is of the order of the hybrid gyroradius /rho//sub H/ = ..sqrt../rho//sub i//rho//sub e/ predicted by the Ferraro-Rosenbluth model. However, diagnostics indicate that the current sheet is subject to an instability which broadens the width of the current layer. Ripples with amplitudes of the order of the ion gyroradius appear at the interface between the field and the particles. These pertubations are observed both on the electrostatic field and on the compressional component of the magnetic field. This instability has a frequency of the order of the local ion cyclotron frequency. However, the modulation propagates in the same direction as the electron diamagnetic drift which indicates that the instability is not a classical gradient-driven instability, such as the lower hybrid or ion drift cyclotron instabilities. The nonlinear phase of the instability is characterized by the filamentation of the current layer which causes anomalous diffusion inside the central current sheet. 79 refs., 7 figs.
Date: November 1, 1988
Creator: Berchem, J. & Okuda, H.
Partner: UNT Libraries Government Documents Department

Ion anisotropy driven waves in the earth's magnetosheath and plasma depletion layer

Description: Recent studies of low frequency waves ([omega][sub r] [le] [Omega][sub p], where [Omega][sub p] is the proton gyrofrequency) observed by AMPTE/CCE in the plasma depletion layer and magnetosheath proper arereviewed. These waves are shown to be well identified with ion cyclotron and mirror mode waves. By statistically analyzing the transitions between the magnetopause and time intervals with ion cyclotron and mirror mode waves, it is established that the regions in which ion cyclotron waves occur are between the magnetopause and the regions where the mirror mode is observed. This result is shown to follow from the fact that the wave spectral properties are ordered with respect to the proton parallel beta, [beta][sub [parallel]p]. The later result is predicted by linear Vlasov theory using a simple model for the magnetosheath and plasma depletion layer. Thus, the observed spectral type can be associated with relative distance from the magnetopause. The anisotropy-beta relation, A[sub p] [triple bond] (T[perpendicular]/T[sub [parallel]])[sub p] [minus] 1 = 0.50[beta][sub [parallel]p][sup [minus]0.48] results from the fact that the waves pitch angle scatter the particles so that the plasma is near marginal stability, and is a fundamental constraint on the plasma.
Date: January 1, 1993
Creator: Denton, R.E.; Hudson, M.K. (Dartmouth Coll., Hanover, NH (United States). Dept. of Physics and Astronomy); Anderson, B.J. (Johns Hopkins Univ., Laurel, MD (United States). Applied Physics Lab.); Fuselier, S.A. (Lockheed Palo Alto Research Labs., CA (United States)) & Gary, S.P. (Los Alamos National Lab., NM (United States))
Partner: UNT Libraries Government Documents Department