ISSN:0148-0227    

DOI:10.1029/96JA01263

年代:1996

数据来源: WILEY

摘要:

Resistive MHD simulations of large‐scale magnetotail dynamics demonstrate that the same unstable mode causes plasmoid formation and ejection into the far tail and dipolarization and the formation of the substorm current wedge in the inner tail, consistent with the neutral line model of substorms. However, they have also modified some aspects of the model and added details that could not easily be inferred without the self‐consistent approach. We review recent results that include the externally driven formation of a thin current sheet in the near tail, which eases the onset of instability and leads to a faster dynamic evolution. In contrast to earlier expectations, the field‐aligned current generation and diversion takes place in the inner tail earthward of the reconnection site, resulting from shear and diversion of the earthward flow caused by reconnection farther out. Dipolarization starts most pronounced in the tail‐dipole transition region, propagating both tailward and flankward. Strong electric fields and plasma heating also are most prominent in the inner tail. Three‐dimensional simulations without mirror symmetries have generalized the picture of plasmoid formation and ejection, demonstrating a tangled geometry of helical flux ropes with different connections that change increasingly from the Earth to the magnetosheath. The interconnection with the magnetosheath may also play a role in generating plasmoid flux ropes with strong core fields. Mass, energy, and momentum gain of plasmoids results mainly from the accumulation of already accelerated plasma rather than from a sling shot effect acting on the entire

ISSN:0148-0227    

DOI:10.1029/96JA00611

年代:1996

数据来源: WILEY

摘要:

This paper provides a phenomenological and theoretical framework for understanding magnetospheric substorms based on the boundary layer dynamics model developed byRostoker and Eastman[1987] updated to be consistent with modern observations. The model is designed to account for both the directly driven and storage‐release aspects of substorm activity. The essence of the model is that enhanced frontside reconnection leads to both growth of the directly driven electrojets and storage of energy in the magnetic field and particle drift in the tail. The growth of particularly intense cross‐tail current near the inner edge of the plasma sheet is terminated by an abrupt collapse marking the onset of the expansive phase of the substorm. This collapse normally takes place in the midnight sector inside ∼ −12 RE. The collapse of this cross‐tail current is consistent with the breakdown of shielding associated with region 2 field‐aligned currents. The expansive phase proceeds through antiearthward propagation in the tail of a wave triggered by the collapse. When the wave reaches the neutral line in the tail, it enhances the reconnection rate leading to stronger velocity shear along the interface between the low latitude boundary layer and central plasma sheet. This velocity shear is responsible for activity at the high latitude edge of the expanded auroral oval. The physics of the processes characterizing the model requires a reexamination of the Harang discontinuity as it would appear that there are actually two regions of electric field reversal in the nightside magnetosphere both of which play important roles in the subst

ISSN:0148-0227    

DOI:10.1029/96JA00127

年代:1996

数据来源: WILEY

摘要:

The near‐Earth neutral line (NENL) model of magnetospheric substorms is reviewed. The observed phenomenology of substorms is discussed including the role of coupling with the solar wind and interplanetary magnetic field, the growth phase sequence, the expansion phase (and onset), and the recovery phase. New observations and modeling results are put into the context of the prior model framework. Significant issues and concerns about the shortcomings of the NENL model are addressed. Such issues as ionosphere‐tail coupling, large‐scale mapping, onset triggering, and observational timing are discussed. It is concluded that the NENL model is evolving and being improved so as to include new observations and theoretical insights. More work is clearly required in order to incorporate fully the complete set of ionospheric, near‐tail, midtail, and deep tail features of substorms. Nonetheless, the NENL model still seems to provide the best available framework for ordering the complex, global manifestations of su

ISSN:0148-0227    

DOI:10.1029/95JA03753

年代:1996

数据来源: WILEY

摘要:

Fundamental observational signatures of substorms and other auroral zone disturbances are summarized in this review. It is shown that at least three distinct types of major geomagnetic disturbance have been identified in the auroral zones, and it is suggested that care should be taken in distinguishing substorms from the other types of disturbance when reaching observational conclusions concerning substorms. Substorms can be uniquely identified by the formation of the current wedge and by the associated westward electrojet and region of active aurora that move poleward within the ionosphere during the expansion phase. Periods of enhanced convection have been observed to continue without substorms for as long as the interplanetary magnetic field (IMF) remains stably southward. They can haveALsignatures that look very much like intense substorms, but these signatures result from strongDP2 ionospheric currents driven by enhanced convection rather than from a nightside, westward electrojet. A third type of disturbance is characterized by short intervals of enhanced equatorward flow within the ionosphere and enhanced auroral emissions. These enhancements move equatorward from near the polar cap boundary, and they may be associated with several‐minute‐long enhancements of earthward flow within the plasma sheet. Studies that have examined the triggering of substorm expansions by IMF changes are also reviewed. When considered together, and substorms are distinguished from the other disturbances, these studies imply that most, and perhaps all, expansions are triggered by IMF changes. These changes include both northward turnings and reductions in the magnitude of theycomponent. If most substorms are indeed externally triggered as suggested here, then substorms must generally not be the result of an internal magnetospheric instabil

ISSN:0148-0227    

DOI:10.1029/95JA01987

年代:1996

数据来源: WILEY

摘要:

This paper attempts to synthesize the diverse number of observations of electric fields and currents in the high‐latitude ionosphere during substorms. By demonstrating that there are often spatial shifts among regions of high ionospheric conductivity, large electric fields and intense currents in the auroral electrojet, it is shown that substorm time variations of the current patterns over the entire polar region consist of two basic components. The first is related to the two‐cell convection pattern and the second to the westward electrojet in the dark sector, which is in turn related to the three‐dimensional wedge current system. These two components result from the relative strength of electric fields and conductivities in the intensification of the auroral electrojet and are identified as the signatures for directly driven and the unloading components in solar wind‐magnetosphere interactions. We contend that disturbed intervals do not necessitate the presence of substorm expansion‐phase activity and that the vast number of earlier complex results concerning the auroral electrojet can be ascertained from the high degree of variability of the two components, depending on substorm events, substorm phases, and their own spatial/temporal scale sizes. It is demonstrated that several major issues that have remained controversial are now accounted for reasonably well in terms of this two‐component electrojet model. We also predict specific features of the substorm auroral electrojet that have not yet be

ISSN:0148-0227    

DOI:10.1029/96JA00142

年代:1996

数据来源: WILEY

摘要:

Substorm phenomena are reviewed with emphasis on the magnetospheric source region of the onset, on the morphology of the initial breakup and subsequent activations, and on the variable character of individual substorms. We provide evidence that before the substorm onset and during the following activations an intense, thin current sheet is formed at the interface between the quasi‐dipolar and taillike magnetic field regions. We infer that the initial breakup, the following multiple activations, pseudobreakups, and other short‐term activations during nonsubstorm times are all similar in morphology and have the same formation mechanism. We postulate that the elementary units of energy dissipation, impulsive dissipation events, which are localized in space and have a short lifetime of ∼1 min, are the manifestations of tail reconnection. We also emphasize the evidence that previous authors have presented in favor of this time dependence and localization. On the basis of the above, we suggest that there are two basic magnetospheric processes responsible for energy storage and dissipation during both substorm and nonsubstorm times: A global and slow quasi‐static tail reconfiguration responsible for the energy storage, and a sequence of local, sporadic, short‐term energy dissipation events. These competitive processes can be observed most the time in some part of the plasma sheet; their relative intensity determines the type of large‐scale dynamic evolution. In this scenario, the various dynamical situations are interpreted as variations in the balance between the two competin

ISSN:0148-0227    

DOI:10.1029/95JA03192

年代:1996

数据来源: WILEY

摘要:

Observations and models of current disruption in the Earth's magnetosphere are briefly reviewed. At the approach of current disruption onset, the cross‐tail current sheet shows a rapid growth in the current density, a large upsurge in the duskward ion bulk speed to nearly the ion thermal speed, an increase in the plasma pressure and its isotropy, a rise in the plasma beta, and a decrease in the current sheet thickness to a length scale comparable to the thermal ion gyroradius. During current disruption, there are (1) large changes in the local magnetic and electric fields, (2) significant magnetic and electric fluctuations over a broad frequency range, (3) magnetic field‐aligned counterstreaming electron beams, (4) ion energization perpendicular to the magnetic field, and (5) reduction in the cross‐tail current by an amount similar to that built up during the growth phase. Observations further indicate that regions of local reversal of the north‐south magnetic field component are not necessarily sites of intense particle energization. Remote sensing of disruption activities shows that at least some current disruptions are not caused by a disturbance propagating earthward from the tail beyond 10 REdownstream. The timescale involved is comparable to or shorter than the ion gyroperiod. Current disruption thus has spatial and temporal scales outside the MHD regime. Several models for current disruption are briefly discussed. Two roles are considered for the cross‐field current instability proposed for current disruption. It can provide anomalous resistivity for magnetic reconnection as advocated by the traditional viewpoint or act singly to instigate global changes of the magnetosphere during the initial substorrn expansion phase. The latter role is elaborated by showing that the instability may modify significantly the local current density and any such process will alter the force equilibrium in the current sheet and give rise to an efficient plasma and energy transport on a global scale. Furthermore, such a process can generate field‐aligned current with intensity comparable to those associated with an auroral breakup arc at substorrn expansion onset. This scenario leads to a new emphasis that in addition to magnetic reconnection, rapid conversion of magnetic energy into particle energy in magnetotail systems may take place without a magnetic X line or separatrix playing the key role in energy

ISSN:0148-0227    

DOI:10.1029/96JA00079

年代:1996

数据来源: WILEY

摘要:

The linear prediction filters computed byBargatze et al.[1985] have resulted in a turning point in the study of solar wind‐magnetosphere coupling. The evolution of the filters with varying activity provides a clear demonstration that the coupling is nonlinear. The filters have thus brought about the end of one era of linear correlative studies and the beginning of a new era of nonlinear dynamical studies. Two separate, but complementary, approaches have emerged in these dynamical studies, analogue modeling and data‐based phase space reconstruction. The reconstruction research has evolved from the original autonomous method studies, which were not conclusive, to the more recent input‐output studies that are more appropriate for the solar wind‐driven magnetosphere and have produced more reliable results. At present it appears that the modeling and reconstruction approaches may be merged in future attempts to produce analogue models directly from the results of the input‐output data‐based methods. If this can be accomplished, it will constitute a major step forward toward the goal of a low‐dimensional analogue model of the magnetospheric dynamics derived directly from data and interpreted in terms of magnetospheric physics. These developments are reviewed in three sections: autonomous data analysis methods, analogue models, and input‐output data analysis methods. The introduction provides sufficient information to read each of these section

ISSN:0148-0227    

DOI:10.1029/96JA00563

年代:1996

数据来源: WILEY

摘要:

Ion anisotropies in the sub‐MeV/nucleon energy region have been measured during the inbound pass of Ulysses through the Jovian magnetosphere. Azimuthal flows in the direction opposite to corotation were detected at several different times, each lasting approximately hours, in the boundary layer and the outer magnetosphere. Similar flows were also observed in parts of the middle magnetosphere whenever Ulysses was far away from the plasma sheet. Such flows were not detected when the Voyager spacecraft traversed the dayside magnetosphere. This could be explained by the fact that Ulysses found the dayside outer magnetosphere in a greatly extended state, compared with the Voyager encounters. In addition, Ulysses also traversed the dayside middle magnetosphere at higher magnetic latitudes than the Voyager spacecraft. The plasma composition during periods of anticorotational flow was more like that measured during solar energetic particle events rather than that measured during the plasma sheet crossings, implying an external source, i.e., the solar wind. From the ion composition and energy spectra we show that solar wind interaction may be an important factor in determining the plasma flow in many regions of the dayside magnetosphere. Mechanisms such as large‐scale magnetic reconnection, “viscous‐like” interactions, and impulsive penetration of plasmoids were ruled out on the basis of magnetic field measurements and charged particle distribution functions around the time of the outermost magnetopause crossing. Adapting recently formulated models of the situation in the terrestrial magnetosphere to Jupiter, we suggest that the anticorotational flows and solar wind‐like composition are caused by “patchy” reconnection at high latitudes. Plasma from the reconnected flux tube forms a low‐latitude boundary layer, from which the solar wind plasma enters the outer magnetosphere. In this model, anticorotational flows in the middle magnetosphere could also be caused by solar wind plasma entering the high‐latitude regions directly from th

ISSN:0148-0227    

DOI:10.1029/96JA00757

年代:1996

数据来源: WILEY