ISSN:0148-0227    

DOI:10.1029/92JA02068

年代:1992

数据来源: WILEY

摘要:

The linear and nonlinear properties of the electromagnetic ion/ion cyclotron (EMIIC) instability are investigated using linear theory and hybrid simulations. The instability is driven by the relative streaming of two field‐aligned ion beams. In the electrostatic limit it reduces to the well‐known ion beam driven electrostatic ion cyclotron instability, but for finite beta the instability is enhanced and propagates over a broader range of angles with respect to the magnetic field. Linear theory and one‐dimensional hybrid simulations are used to study the characteristics of the instability as a function of beam density, angle of propagation (θ), ion beta, ratio of the streaming velocity to the Alfven speed, electron to ion temperature ratio, and other parameters. Generally, the one‐dimensional calculations show that the instability behaves like its electrostatic counterpart when θ is near 90°, saturating at low levels by heating the ions in the perpendicular direction. At smaller angles the instability is dominated by electromagnetic effects, large‐amplitude waves, and stronger beam coupling. Two‐dimensional hybrid simulations show some evidence for coherent effects due to a narrowing of the wave spectrum during the linear growth stage and a more quasi‐linearlike heating process in the nonlinear phase, eventually yielding similar asymptotic values for plasma parameters. Applications of the EMIIC instability to upstream and downstream waves and ion dissipation at slow shocks in the magnetotail and to ion heating in the plasma sheet boundary layer are b

ISSN:0148-0227    

DOI:10.1029/92JA00902

年代:1992

数据来源: WILEY

摘要:

Using a one‐dimensional electromagnetic hybrid code (particle ions, fluid electrons), the structure of and ion heating at slow magnetosonic shocks are investigated in the low beta regime. In particular, the effects of the waves generated at or upstream of the shock by the electromagnetic ion/ion cyclotron (EMIIC) instability are studied. To understand the role of shock initialization and boundary conditions on the resulting solutions, three methods of shock formation are utilized. One is a relaxation method, where the shock is started as a structureless discontinuity and allowed to evolve to a possible steady state solution. It is shown that this method of shock formation can lead to spurious results and therefore is not reliable. The other two investigated methods are similar, in that the shock is formed via flow interactions in a dynamic fashion. One of these is the usual piston method, where the plasma is injected from one side of the simulation box and reflected from the other side. In a new, third method (flow‐flow) the plasma is injected from both boundaries, which results in the formation of a pair of slow shocks. Both of these shock formation methods result in reliable solutions. Using the piston or flow‐flow interactions, it is found that slow magnetosonic shocks have a number of different structures depending on the plasma parameters and the shock normal angle. One set of solutions corresponds to the classical structure where a coherent Alfvén wave train is formed downstream of the shock. However, Alfvén waves generated by the backstreaming ions via the electromagnetic ion/ion cyclotron instability are also observed upstream of the shock. The second class of solutions corresponds to structures with Alfvén turbulence upstream and downstream of the shock. These shocks are found to have a nonsteady behavior, due to transport of upstream waves generated by the EMIIC instability into the downstream region. The nonsteady behavior of the shock prevents the formation of a coherent trailing wave train. A third class of structure is associated with much weaker laminar shocks, where little or no wave activity is present both upstream and downstream of the shock. Using the full electromagnetic dispersion equation, it is shown that large linear damping of Alfvén or slow waves prevents the formation of a wave train downstream of such shocks. A fourth class of solutions is associated with no wave activity in the downstream but has Alfvén waves at and upstream of the shock. The excitation of these waves at the shock leads to a nonsteady behavior, even though their Poynting vector is toward the upstream. By using the average magnetic moment and deviations from its upstream value, the role of the EMIIC instability in ion dissipation has also been investigated. It is shown that ion orbits are adiabatic at weak laminar shocks, leading to anisotropic heating withT∥>T⊥. On the other hand, when upstream Alfvén waves are present, they lead to incident ion scattering; before the shock transition region. This scattering, however, does not always lead to complete thermalization immediately downstream of the shock. As such, the length scales associated with the magnetic structure and ion dissipation may vary considerably

ISSN:0148-0227    

DOI:10.1029/92JA00905

年代:1992

数据来源: WILEY

摘要:

Pearl pulsations, with an average repetition period of 60 s, were recorded using the magnetic and electric field experiments on the polar‐orbiting Viking satellite. The wave event occurred on September 30, 1986, during Viking orbit 1212 at 1030 MLT, fromL= 3.6 toL= 4.1, and at an altitude of 13,500 km. Electron density observations obtained from Viking show that the waves were generated at the plasmapause and at lower amplitudes in the plasmasphere. The wave Poynting flux, calculated using the magnetic and electric field, indicated that the waves generally were propagating downward toward the ionosphere, although upward Poynting fluxes were observed. Clear evidence of upward propagating waves, associated with downward propagating waves reflected at the ionosphere, was not observed. Linear convective growth rates suggest that the anisotropic ions which provide the free energy have a perpendicular temperature around 15 keV. The repetition period, calculated using the measured electron density and magnetic field strength at Viking, is consistent with the double‐hop transit time for ion cyclotron waves which propagate along field lines from one hemisphere to the other. However, the absence of upward propagating wave packets implies that the upper limit of the wave ionospheric reflection coefficient is on the order of 10 to 20%. Alternative mechanisms for producing the observed repetition are also investigated and include a periodic generation model of pearl pulsations at the ion bounce per

ISSN:0148-0227    

DOI:10.1029/92JA00838

年代:1992

数据来源: WILEY

摘要:

The structure of the distant magnetotail, where the closed field line region terminates, is investigated in the framework of several models and on the basis of laboratory simulations of the interaction of a flowing magnetized plasma with a dipole field. From our simulations we find that this region for northward “interplanetary” field (IMF) resembles that of a quasi‐static tail structure, in which a flaring tail transits into a nonflaring current sheet configuration at a location where the internal pressure is balanced by the external plasma and magnetic pressures only. This structure is associated with a Y‐type magnetic neutral line. The magnetic tail apparently includes an open field line region, despite the fact that this magnetosphere did not have a history of southward IMF, which could have opened the dipole field through frontside reconnection. This is a strong indication of the occurrence of high‐latitude nightside reconnection, suggested by Dungey (1963). For southward model IMF the closed field line region appears to terminate at an X‐type magnetic neutral line, as in the standard open magnetosphere model of Dungey (1961). This configuration is also in qualitative agreement with the simple superposition of a dipole field and a uniform IMF and with the presence of a plasma mantle which expands through drifts from the magnetosphere boundary toward the plasma sheet and becomes partially trapped through magnetic reconnection at the dis

ISSN:0148-0227    

DOI:10.1029/92JA01145

年代:1992

数据来源: WILEY

摘要:

A method is presented which allows estimation of the variation of the rate of magnetic reconnection at the day side magnetopause. This is achieved using observations of the cusp particle precipitation made by low‐altitude polar‐orbiting spacecraft. In this paper we apply the technique to a previously published example of a cusp intersection by the DMSP F7 satellite. It is shown that the cusp signature in this case was produced by three separate bursts of reconnection which were of the order of 10 min apart, each lasting roughly 1 min. This is similar to the variation of reconnection rate which is required to explain typical flux transfer event signatures at the magnetopa

ISSN:0148-0227    

DOI:10.1029/92JA01261

年代:1992

数据来源: WILEY

摘要:

We present a survey of Prognoz 10 energetic ion (>15 keV) and electron (>30 keV) observations at low and middle latitudes in the dayside magnetosheath. At low latitudes, peak fluxes are observed inside the magnetopause, whereas at middle latitudes the peak fluxes are generally observed in the magnetosheath at some distance from the magnetopause. Both electron and ion fluxes tend to be greater outside the dawnside magnetopause than outside the duskside magnetopause. The flux of energetic particles in the outer magnetosheath is almost invariably less than that within the inner magnetosheath. Although the particle flux exhibits a clear dependence upon geomagnetic activity, no such dependence upon the local magnetosheath magnetic field orientation was noted. We interpret the observations as indicating that leakage of magnetospheric particles is the dominant source of energetic particles in the magnetosheath, but do not rule out Fermi acceleration at the bow shock as a possible subsidiary contributor to the population of ions with energies of about 15 keV.

ISSN:0148-0227    

DOI:10.1029/92JA01134

年代:1992

数据来源: WILEY

摘要:

ULF pulsations were observed by DE 1 between 1600 and 1830 UT, October 31, 1982, during a magnetically disturbed interval. Ground observations suggested that the pulsations were excited by a sudden increase in the solar wind velocity and pressure. During the pulsation interval DE 1 traveled near apogee from −55 to −20° geomagnetic latitude and fromL∼ 13 toL∼ 4 at about 0900 LT. The waves observed were azimuthal oscillations preceded by gradually decaying long period compressional waves which lasted for more than 1 hour. Phase relations between magnetic and electric field oscillations and calculated Poynting flux indicate that in the outer magnetosphere (L>8) DE 1 observed propagating waves which contained strong poloidal components, while the quasi‐sinusoidal toroidal waves seen later forL<10.3 were standing along field lines. The toroidal waves appeared as four wave packets, each of which corresponded to a region with a distinct plasma distribution. The observed wave periods decreased withLover an extended magnetospheric region. The seemingly weak interaction between magnetic shells suggests that the source was a broadband one. Magnetometer data from several high latitude observatories located near the footpoints of the magnetic shells crossed by DE 1 were also examined. The magnetic pulsations on the ground contained many frequency components, and the waves seen most strongly in space were often not the strongest signals seen on the ground near the same field lines. The broadband nature of the ground pulsations indicates that the stations also detected oscillations of the adjacent field lines. The major frequencies seen at ground stations seemed to be roughly constant for about 2 hours butLdependent. This suggests that the changing periods seen in space by DE 1 were clearlyLrelated and not temporal

ISSN:0148-0227    

DOI:10.1029/92JA00315

年代:1992

数据来源: WILEY

摘要:

Systems for imaging optical emissions all have a detection sensitivity which, be it uniform or not, can make image interpretation very difficult. A time differential technique involving pixel by pixel subtraction of successive images has been developed to overcome this problem. The technique has been applied to images of pulsating aurorae recorded with a low‐light‐level television system. Individual auroral forms exhibit within a short interval of time a diversity of pulsation modes not evident by direct visual inspection of the television images. The results are important for possible source mechanisms or any meaningful classification scheme for pulsating auro

ISSN:0148-0227    

DOI:10.1029/92JA00447

年代:1992

数据来源: WILEY

摘要:

The majority of current ground‐based radar experiments measuring ionospheric convection flows derive the ionospheric flow vectors from line‐of‐sight (l‐o‐s) velocity measurements made from a single radar site. In order to deduce vector information from such a set of scalar measurements, assumptions have to be made about the spatial and temporal structure of the flow pattern. These assumptions involve an actual or effective beam‐swinging technique. In this study the flow direction estimates from the application of sophisticated beam‐swinging software, developed by the Applied Physics Laboratory of the Johns Hopkins University, to over 20,000 estimates of the l‐o‐s ionosphericEregion mean irregularity drift velocity from the Wick radar of the Sweden And Britain Radar‐auroral Experiment (SABRE) system, are compared with merged velocity vectors derived from data from both SABRE radar sites. The application of the beam‐swinging software to VHFEregion backscatter data is found to lead to flow direction estimations which are in error a significant amount of the time, with beam‐swinging‐derived drift velocities lying outside a cone of half width 30° around the merged SABRE drift velocities in 30% or more of cases. Both curvatures and gradients in the ionospheric flow pattern measured by SABRE cause distortions of the beam‐swinging‐evaluated flow patterns. Moreover, from an examination of the Wick l‐o‐s velocity data and the data fitting calculations alone, it is often not possible to distinguish between accurate and inaccurate beam‐swinging flow direction estimates. Criteria are discussed which can be employed to increase confidence in the beam swinging results. Although the present comparison is confined toEregion irregularity drift velocity measurements, these criteria probably have implications for all monostatic experiments employing a beam‐swinging technique (e.g., coherent and incoherent radars and optical instruments). It is concluded that it is highly desirable for the next generation of ionospheric radars to be capable of making common‐volume me

ISSN:0148-0227    

DOI:10.1029/92JA01191

年代:1992

数据来源: WILEY