摘要:

In a companion paper we have reported results from a survey of strong energetic ion (E>25 keV) anisotropies in the central geomagnetic tail based on 2 years of data from the ISEE 2 spacecraft. Here we extend this study with particular emphasis on the relation of high ion anisotropies to recent models of geomagnetic substorms and include magnetic field and ground‐based data. Beyond 16REdowntail distance, we find a correlation between the magnetic field north‐south component (Bz) and the streaming direction of the energetic ions. Earthward ion streaming is observed almost exclusively whenBzis positive while tailward streaming tends to occur with negativeBz. Strong ion anisotropies are magnetic field aligned rather than perpendicular to it suggesting that most of them are observed at the plasma sheet boundary. However, they are also found in the central plasma sheet. A superposed epoch analysis using theALindex yields a relation between substorm phases and the observation of high ion anisotropies. Earthward streaming at larger distances from the Earth and/or the neutral sheet usually occurs during substorm recovery while earthward streaming closer to the Earth and tailward streaming events are related to the substorm expansion phase. Three events characterized by strong tailward ion and plasma streaming detected first changing to earthward streaming later on are presented in some detail. The overall features of two of these events strongly suggest the formation of a magnetic neutral line earthward of the satellite at substorm onset moving tailward around recovery. While most observations during the third event also indicate such a sequence, there are some puzzling details which do not match this interpretation. Although a certainBysignature proposed by numerical simulations of near‐Earth reconnection could not be found, we conclude that the statistical properties of strong ion anisotropies can consistently be explained in terms of the near‐Earth neutral line model of substorms if the substorm neutral line sometimes forms well inside the ISEE orbit. Other recently published models of substorm onset such as the thermal catastrophe model and the current disruption model can also account for part of the observations but still require further ref

ISSN:0148-0227    

DOI:10.1029/92JA01657

年代:1993

数据来源: WILEY

摘要:

The existence in the Earth's magnetotail of magnetic field and plasma density inhomogeneities can influence the propagation of electromagnetic waves in the lobe and plasma sheet. This is important in understanding the origin of magnetic noise bursts that are observed near the neutral sheet since the waves may not necessarily be generated locally but can be excited in other regions of the tail and propagate towards the central plasma sheet due to the presence of the global inhomogeneities. In this paper the generation and propagation of low‐frequency whistler waves excited by ion beams in the plasma sheet boundary layer is examined. The trajectories of waves that are generated inside the plasma sheet boundary layer are followed using standard ray tracing techniques assuming a Harris‐type neutral sheet magnetic field profile and with the density chosen such that the total pressure remains constant. The wave energy is calculated along the wave path taking into account growth due to the ion beam while the wave is in the plasma sheet boundary layer, and damping due to resonant interactions with the background plasma. Results show that the electromagnetic waves are guided towards the central plasma sheet by the plasma gradients but are damped out before reaching the neutral plane itself due to Cerenkov resonance with the hot central plasma sheet background ions. The amplitudes and final positions of the waves depend on the temperature of the background plasma, the ion beam drift speed, and the ion beam thermal sp

ISSN:0148-0227    

DOI:10.1029/92JA01835

年代:1993

数据来源: WILEY

摘要:

Convection of closed magnetic tubes in the central part of the Earth's magnetospheric tail reduces their length and compresses the plasma contained in them. In a reasonable approximation the problem of plasma compression in a shrinking tube reduces to a classical hydrodynamical problem of a piston being pushed into a cylinder which has contained the gas at rest. In this case the top of the flux tube plays the role of the piston. As the piston is pushed in, a shock wave is known to separate from it. Between the piston and the shock front the gas is compressed and heated, while the gas ahead of the front is undisturbed. Similar processes occur in the plasma contained in the contracting flux tube and are accompanied by the formation in the tail of two shock waves which have separated from the neutral plane in the northward and southward directions, between which the plasma is hotter and denser than in the tail lobes. In the case of stationary magnetospheric convection, shock waves are also stationary. The distinctive plasma characteristics in the region between the shock waves and also the spatial position of this region suggest that in the real magnetosphere this region corresponds to the plasma sheet. We assess the validity of this hypothesis and its effectiveness in describing geophysical effects. We show that the plasma particle energy expected from this model corresponds to experimentally derived values. At the same time, within the framework of the MHD theory with isotropic pressure it is impossible to obtain the required high degree of compression during the transition of the plasma across the shock front (100 times or more). By analyzing the constraints imposed by the laws of conservation of mass, energy, and momentum on plasma parameter jumps in a collisionless shock wave, we find that in order to achieve the observed degree of compression it is sufficient to have a small excess of transverse pressure in the plasma sheet over longitudinal pressure, which is consistent with observations. We have numerically simulated the motion of shock waves in convecting magnetic flux tubes. By postulating the relationship between the shock wave velocity and plasma parameters, we have calculated the form of the shock surface, which we interpret as the plasma sheet boundary. The line of intersection of the shock surface with the ionosphere is taken here as the polar boundary of the auroral oval. Results of calculations agree with the present‐day understanding of the structure of the plasma sheet and of the auroral zone. The applications we consider of this hypothesis lead us to conclude that this approach to describing phenomena associated with the plasma sheet boundary and with the auroral region is promisin

ISSN:0148-0227    

DOI:10.1029/91JA02515

年代:1993

数据来源: WILEY

摘要:

The development of the ion Weibel instability in the Earth's neutral sheet is examined by quasi‐linear analysis for waves propagating along the ambient magnetic field. For ion drift speed reaching a sizable fraction (∼0.5 to 1) of the ion thermal speed, numerical solution of the quasi‐linear kinetic equations shows the wave growth reaching the nonlinear stage in less than one ion gyroperiod. The saturation level is attained with the ion drift speed reducing by ∼15 to 28% of its initial value, wave amplitudes ranging from ∼0.2 to 0.8 of the ambient magnetic field, and the ion temperature along the magnetic field increasing by ∼25 to 90%. This instability thus provides a means for non‐adiabatic heating of ions during substorms. The resulting anomalous resistivity is estimated to be ∼1×10−7to 1×10−6s, about 11 to 12 orders of magnitude above the classical Coulomb resistivity. These predictions may be regarded as lower limits on the effects of the nonlinear stage of this instability since only parallel propagating waves are considered. Nevertheless, the calculated reduction in the ion drift speed compares reasonably well with the inferred amount of current reduction from observations of current disruption events, and the anomalous resistivity is within the range of values estimated for active magnetotail regions during substorm expansion. The difference in the nonlinear evolution with different initial ion drift speeds suggests that a current system in the same sense as the substorm current wedge is expected to be established in the midnight region as a result of this instability occurring in the nightside neutral sheet. For a typical amount of current reduction, it is estimated that the plasma in this region is subjected to a substantial net force i

ISSN:0148-0227    

DOI:10.1029/92JA02034

年代:1993

数据来源: WILEY

摘要:

We investigate single charged particle dynamics in the Earth's magnetotail using a simple, scale free magnetic field model which is explicitly time dependent, with a corresponding induction electric field. The time‐dependent Hamiltonian of particle motion in the simple model describes a system of two coupled oscillators which is driven. When the (time‐dependent) ratio of the oscillation frequencies is different from unity, the motion is regular, but if it approaches unity at some point on the trajectory, the motion becomes chaotic. A parameter α is found which characterizes the adiabaticity of the system, and the transition time for behavior in the system is given by at αt→ 1. The condition at αt≈ 1 is equivalent to κ ≈ 1 in the static parabolic field model discussed by previous authors (Buchner and Zelenyi, 1989). The explicit time dependence produces two possible classes of motion, ordered by α. If the reversal is thick and is folding slowly, so that α ≪ 1, the motion is a transition in behavior, from regular μ conserving to chaotic “cucumberlike” trajectories, whent≈ 1/α. If on the other hand, the reversal is thin and folds quickly, so that α ≫ 1, the particles execute regular “ring type” trajectories oncet>1/α. Simple estimates of presubstorm magnetotail parameters indicate that electrons in a slowly thinning (15‐min time scale), thick (1RE) sheet have α ≪ 1, whereas protons in a thin (several hundreds of kilometers) sheet which thins on a 5‐min time scale have α ≫ 1. Hence the behavior of the particles, and by implication, the field reversal which they support, will depend upon the adiabaticity of the system α as well as the “chaotization” parameter αt; this is shown only to be the case in a model which is explicitly time dependen

ISSN:0148-0227    

DOI:10.1029/92JA01548

年代:1993

数据来源: WILEY

摘要:

High‐resolution data (covering up to 8 Hz) from the flux gate magnetometers on the two ISEE spacecraft are used to analyze ultralow‐frequency (ULF) fluctuations observed at the magnetopause and in the adjacent layers. Intersatellite correlations are computed to show that the same structure can be identified in the fluctuations observed on both spacecraft when the interspacecraft distance is small. Then the possibility of deducing the velocity of the structure from two‐point measurements is discussed; it is shown that it can be estimated only in certain

ISSN:0148-0227    

DOI:10.1029/92JA01668

年代:1993

数据来源: WILEY

摘要:

Strong slow mode waves in the Pc 3–4 frequency range are found in the magnetosheath close to the magnetopause. We have studied these waves at one of the ISEE subsolar magnetopause crossings using the magnetic field, electric field, and plasma measurements. We use the pressure balance at the magnetopause to calibrate the Fast Plasma Experiment data versus the magnetometer data. When we perform such a calibration and renonnalization, we find that the slow mode structures are not in pressure balance and small scale fluctuations in the total pressure still remain in the Pc 3–4 range. Energy in the total pressure fluctuations can be transmitted through the magnetopause by boundary motions. The Poynting flux calculated from the electric and magnetic field measurements suggests that a net Poynting flux is transmitted into the magnetopause. The two independent measurements show a similar energy transmission coefficient. The transmitted energy flux is about 18% of the magnetic energy flux of the waves in the magnetosheath. Part of this transmitted energy is lost in the sheath transition layer before it enters the closed field line region. The waves reaching the boundary layer decay rapidly. Little wave power is transmitted into the magnetosph

ISSN:0148-0227    

DOI:10.1029/92JA01534

年代:1993

数据来源: WILEY

摘要:

In order to identify the generation mechanisms of low‐latitude Pc3–4 geomagnetic pulsations, data were obtained from a meridional chain of induction magnetometers spanning L values from 1.4 to 2.7 (−30° to −52° geomagnetic latitude). The spatial structure of Pc3–4 signal parameters was examined by means of spectral, polarization and interstation phase analysis. This paper describes three typical individual events whose spectral, polarization and phase characteristics indicate the existence of field line resonances at low latitudes within the plasmasphere. The spatial phase structure shows a local minimum and indicates phase motion toward the resonance region. Resonance region widths of ΔL = 0.2 to ΔL = 0.8, corresponding to north–south ionospheric scale lengths of 250 and 1500 km or more, respectively, are seen. The coupling of field line resonances to global compressional modes is considered to be a likely generation mechanism of t

ISSN:0148-0227    

DOI:10.1029/92JA01433

年代:1993

数据来源: WILEY

摘要:

Radial transport of energetic ions for the development of the main phase of geomagnetic storms is investigated with data from the medium energy particle analyzer (MEPA) on the Charge Composition Explorer (CCE) spacecraft, which monitored protons (Ep>56 keV), helium ions (EHe>72 keV), and the carbon‐nitrogen‐oxygen group, which is mostly dominated by oxygen ions (E0>137 keV). From a study of four geomagnetic storms, we show that the flux increase of these ions in the inner ring current region (L≲ 5) can be accounted for by an inward displacement of the ring current population by ∼0.5 to 3.5RE. There is a general trend that a larger inward displacement occurs at higherLshells than at lower ones. These results are in agreement with previous findings. The radially injected population consists of the prestorm population modified by substorm injections which occur on a much shorter time scale than that for a storm main phase. It is also found that the inward displacement is relatively independent of ion mass and energy, suggesting that the radial transport of these energetic ions is effected primarily by convective motion from a large electric field or by diffusion resulting from magnetic field fluct

ISSN:0148-0227    

DOI:10.1029/92JA02079

年代:1993

数据来源: WILEY

摘要:

The behavior of ring current ions in low‐frequency geomagnetic pulsations is investigated analytically and numerically. We focus primarily on ring current O+ions, whose flux increases dramatically during geomagnetic storms and decays at a rate which is not fully explained by collisional processes. This paper presents a new loss mechanism for the O+ions due to the combined effects of convection and corotation electric fields and interaction with Pc 5 waves (wave period: 150–600 s) via a magnetic drift‐bounce resonance. A test particle code has been developed to calculate the motion of the ring current O+ions in a time‐independent dipole magnetic field, and convection and corotation electric fields, plus Pc 5 wave fields, for which a simple analytical model has been formulated based on spacecraft observations. For given fields, whether a particle gains or loses energy depends on its initial kinetic energy, pitch angle at the equatorial plane, and the position of its guiding center with respect to the azimuthal phase of the wave. The ring current O+ions show a dispersion in energies andLvalues with decreasing local time across the day side, and a bulk shift to lower energies and higherLvalues. The former is due to the wave‐particle interaction causing the ion to gain or lose energy, while the latter is due to the convection electric field. Our simulations show that, due to interaction with the Pc 5 waves, the particle's kinetic energy can drop below that required to overcome the convection potential and the particle will be lost to the dayside magnetopause by a sunwardE×Bdrift. This may contribute to the loss of O+ions at intermediate energies (tens of keV) observed during the recovery phase of geomagne

ISSN:0148-0227    

DOI:10.1029/92JA01540

年代:1993

数据来源: WILEY