摘要:

Ulysses observations during the inbound leg of the February 1992 encounter with Jupiter offer the opportunity to evaluate both the hinged magnetodisc and the magnetic anomaly models currently used to describe the Jovian magnetospheric corotation lag. Particle and field detectors on board Ulysses measure signatures of the current and plasma sheets. The locations of these sheets in radius and system III longitude are then used as inputs to the models. It is shown that the hinged magnetodisc model is able to explain some characteristics of the data but unable to explain others. The magnetic anomaly model appears to account for the overall nature of the magnetospheric corotational lag; however, it, too, cannot fully account for the Ulysses data. A modification of the magnetic anomaly model is offered which adequately explains the Ulysses data and, with the assumption of a sufficiently significant, time‐dependent Iogenic plasma source, explains data from previous Jupiter encounters as wel

ISSN:0148-0227    

DOI:10.1029/96JA02042

年代:1996

数据来源: WILEY

摘要:

An orbit tracing technique was used to generate current sheets for three magnetotail models. Groups of ions were followed to calculate the resulting cross‐tail current. Several groups then were combined to produce a current sheet. The goal is a model in which the ions and associated electrons carry the electric current distribution needed to generate the magnetic fieldBin which ion orbits were traced. The region −20RE

ISSN:0148-0227    

DOI:10.1029/96JA01945

年代:1996

数据来源: WILEY

摘要:

A 2½‐dimensional, steady state model of the low‐latitude boundary layer (LLBL) on closed field lines has been developed. Coupling to the dayside auroral ionosphere via region 1 field‐aligned currents and associated finite field‐aligned potential drops is included. The boundary layer (BL) flow is influenced by viscous forces, inertia forces, pressure forces, andj×Bforces. The BL approximation and kinematic treatment of the plasma motion perpendicular to the equatorial plane greatly simplify the model. The model has three modules: an equatorial LLBL module, a force‐free coupling module, and an ionospheric module. It includes nonconstant magnetosphere‐ionosphere mapping factors, describing the increasing stretching of magnetic field lines with increasing distance away from the subsolar region, as well as variations in mapping factors across the BL. In this paper, both the BL height and the ionospheric conductivities are assumed constant. The effects of finite field‐aligned potential drops and nonconstant mapping factors on the BL flow, on the plasma density and temperature, on the magnetic field line bending, and on the field‐aligned current distribution are presented. The effects of varying the magnitude of the (constant) ionospheric Pedersen conductivity are also discussed. In addition, the effects of viscosity, including varying its magnitude and allowing it to be a function of location both along and across the LLBL, are examined. A number of qualitative features of the simulation results are found to be consistent with var

ISSN:0148-0227    

DOI:10.1029/96JA02000

年代:1996

数据来源: WILEY

摘要:

VLF chorus, consisting of narrowband rising frequency emissions, has often been observed in association with microburst electron precipitation. We present the first simultaneous rocket observations of these two phenomena, with emphasis on understanding the source of the VLF emissions. The rocket experiment was launched on May 6, 1993, from Poker Flat, Alaska (L= 5.6). In this work, the observed 1–4 kHz chorus emissions are interpreted in terms of a cyclotron resonance interaction. The frequency range of the risers and the observed electron energy range agree with those required for this interaction. Using a criterion derived from the conservation of energy during an interaction, it is shown that a cold plasma cyclotron resonance interaction can produce the lower‐frequency portions of the observed chorus risers, from ≈1000 Hz to ≈2500 Hz, while a warm plasma model is required to produce frequencies>2500 Hz. The warm plasma model assumes a two‐component plasma, with an isotropic cold component and a bi‐Maxwellian warm component. The effect of the warm component is to change the wave dispersion relation, allowing the production of the higher‐frequency risers. A portion of the anisotropy required to produce the high‐frequency emissions can also be provided by a loss cone distribution. The chorus source is estimated from this cyclotron resonance theory to be located near the e

ISSN:0148-0227    

DOI:10.1029/96JA02007

年代:1996

数据来源: WILEY

摘要:

We discuss the ponderormotive redistribution of ions along geomagnetic field lines due to the action of Pc 1 pulsations, using a simple diffusion equilibrium model. The field‐aligned component of the ponderomotive force is derived for Alfvén waves as well as for ion cyclotron waves propagating in a multicomponent plasma. Our theory gives a possibility for a better understanding of the problem and a convenient way to make rough estimates of the ponderomotive efficiency of Pc 1 waves under concrete magnetospheric conditions. Qualitative analysis of equations and satellite information on the intensity of wave fields show that the ponderomotive forces can have a pronounced effect on plasma. Ground‐based observations of Pc 1 pulsations also suggest a ponderomotive redistribution of the background pl

ISSN:0148-0227    

DOI:10.1029/96JA01750

年代:1996

数据来源: WILEY

摘要:

We present evidence for the acceleration of magnetospheric ions by reflection off two Alfvén waves, launched by the reconnection site into the inflow regions on both sides of the reconnecting magnetopause. The “exterior” wave stands in the inflow from the magnetosheath and is the magnetopause, in the sense that the majority of the field rotation occurs there. The other, “interior” wave stands in the inflow region on the magnetospheric side of the boundary. The population reflected by the interior wave is the more highly energized of the two and appears at low altitudes on open field lines, immediately equatorward of the cusp precipitation. In addition, we identify the population of magnetosheath ions transmitted across the exterior Alfvén wave, as well as a population of magnetospheric ions which are accelerated, after transmission through the interior wave, by reflection off the exterior wave. The ion populations near the X line are modeled and, with allowance for time‐of‐flight effects, are also derived from observations in the dayside auroral ionosphere. Agreement between observed and theoretical spectra is very good and the theory also explains the observed total fluxes and average energies of the precipitations poleward of the open/closed field line boundary. The results offer a physical interpretation of all the various classifications of precipitation into the dayside ionosphere (central plasma sheet, dayside boundary plasma sheet, void, low‐latitude boundary layer, cusp, and mantle) and allow the conditions in both the magnetosphere and the magnetosheath adjacent to the X lin

ISSN:0148-0227    

DOI:10.1029/96JA01948

年代:1996

数据来源: WILEY

摘要:

We have analyzed complete spectrum of waves (0–4 MHz) and electron distributions during events of modulated Langmuir waves observed by Freja in the topside polar ionosphere. Modulated Langmuir waves are observed with amplitudes 1–1000 mV/m in association with beams of electrons with energies 50–5000 eV streaming through cold ionospheric plasma. We find overwhelming evidence for interactions of Langmuir (L) waves with lower hybrid (LH) waves. The interactions are consistent with parametric decay of beam inducedLwaves (L↝L′ +LH) as well as with scattering ofLwaves on preexistingLHwaves (L+LH↝L′). These interactions are observed at all wave amplitudes ∼ 1 – 1000 mV/m but are sensitive to thekvectors of the primary waves (i.e., electron beam energy). In association with modulated Langmuir waves we also observe ion acoustic‐like signals which appear to be produced by rectification of the high‐frequency waves in the probe sheath through nonlinearities in the current‐voltage characteristic. The Langmuir wave events occur frequently in localized regions which are usually related to the phase of magnetic perturbation of an Alfvén wave. The observations show that Langmuir waves with amplitudes>50 mV/m are very common in the auroral zone, and they represent the strongest electrostatic wave mode at altitudes of ∼ 1500 km. Langmuir waves appear to be an important element in the energy exchange chain in which part of the energy carried by Alfvén waves is transferred to energetic electrons and finally deposited to thermal plasma in a sequence: Alfvén wave ↝ electron beam ↝ Langmuir wave ↝LH

ISSN:0148-0227    

DOI:10.1029/96JA01747

年代:1996

数据来源: WILEY

摘要:

We present a study of the proton cyclotron instability in the Earth's outer magnetosphere,L>7, using Active Magnetosphere Particle Tracer Explorers/Charge Composition Explorer (AMPTE/CCE) magnetic field, ion, and plasma wave data. The analysis addresses the energy of protons that generate the waves, the ability of linear theory to predict both instability and stability, comparison of the predicted wave properties with the observed wave polarization and frequency, and the temperature anisotropy/parallel beta relation. The data were obtained during 24 intervals of electromagnetic ion cyclotron (EMIC) wave activity (active) and 24 intervals from orbits without EMIC waves (quiet). This is the same set of events used byAnderson and Fuselier[1994]. The active events are drawn from noon and dawn local times for which the wave properties are significantly different. For instability analysis, magnetospheric hot proton distributions required the use of multiple populations to analytically represent the data. Cyclotron waves are expected to limit the proton temperature anisotropy,Ap=T⊥p/T‖p− 1, according toAp

ISSN:0148-0227    

DOI:10.1029/96JA01251

年代:1996

数据来源: WILEY

摘要:

Observations of lower hybrid wave packets in density depletions at altitudes of 500 to 13,000 km are compared with predictions from the theory of lower hybrid wave nucleation and collapse. It is shown that a widely accepted interpretation of these cavities as collapsing lower hybrid wave packets localized in self‐consistent density wells produced by their ponderomotive force is only partly consistent with observations. Observed potentials match or exceed those required for collapse and most observed length scales and aspect ratios lie in the theoretically predicted range. However, associated density perturbations appear to be almost always negative, contrary to theory based on ponderomotive force balance, which predicts them to be positive and negative equally often; moreover, they are typically 100 times greater in magnitude than this theory predicts. It is concluded that the electric‐field and length‐scale data are consistent with the occurrence of lower hybrid collapse, but that the lower hybrid waves either accumulate in preexisting density depressions, or produce the depressions ponderomotively via multiple collapses at a single site, or by means other than their ponderomotive force, as suggested by some previous authors. This work also suggests a possible resolution of conflicting results relating to the dominance of electron or ion heating by collapsing lower hybrid wave packets, with electron heating dominating for large ratios of electron temperature to ion temperature and ion heating prevailing in the opposite

ISSN:0148-0227    

DOI:10.1029/96JA01857

年代:1996

数据来源: WILEY

摘要:

The terrestrial plasma depletion layer (PDL) is characterized by electromagnetic ion cyclotron waves (EICWs) [Anderson et al., 1991, 1994;Anderson and Fuselier, 1993]. These are of two types, continuous (CON), where the wave power transverse to the local mean field decreases continuously with frequency, and bifurcated (BIF), where a diminution in wave activity occurs between two activity peaks [Anderson et al., 1994]. Linear kinetic theory of plasma waves has been applied successfully to explain many features of the observed spectra [Denton et al., 1993; Gary et al., 1993]. In this paper we complement these theoretical studies by examining the effects a possible relative motion between the thermal H+and He2+populations has on the ion cyclotron activity, illustrating theoretical predictions by comparison with observed CON and BIF spectra. We find that even a modest differential speed (Uα≡ |vα| − |vp|∼ 0.1 vA, wherevAis the local Alfvén speed) can regulate the presence, location, and strength of the diminution in wave power often observed between the upper (∼ 0.55fp

ISSN:0148-0227    

DOI:10.1029/96JA01745

年代:1996

数据来源: WILEY