Cross-field Current Instability Research Articles

Dynamics of the Magnetotail Plasma Sheet Current

Magnetospheric plasma can be investigated as a continuum by adopting magnetic field B and plasma flow u as primary parameters in the Bu paradigm or as a collection of individual particles by adopting electric field E and electrical current j as primary parameters in the Ej paradigm. It is pointed out that each paradigm has its merits and limitations. This viewpoint is illustrated further by examining several topics in magnetospheric research. The magnetic flux transport in substorm dipolarization is examined with the Ej paradigm to show why the Bu paradigm may be inappropriate in some cases due to the violation of the frozen-in condition for the validity of the Bu paradigm. There is no guarantee that large-scale plasma dynamics can always be treated accurately by the Bu paradigm. The disturbance revealed in the current disruption (CD) phenomenon has unique characteristics that can be more readily understood with the Ej paradigm. In a case study, the power dissipation in CD is evaluated to about an order of magnitude higher than that in the electron diffusion region associated with magnetic reconnection (MR). Two prominent plasma instabilities, namely tearing instability (TI) and cross-field current instability (CCI), are discussed and their relevance to substorm onset is evaluated. The mating instability developed conceptually is also briefly discussed. The development of azimuthal auroral beads (ABs) on auroral arcs formed prior to substorm onset is analyzed to show that CCI can predict well their wavelength, growth rate, and period simultaneously. In contrast, the observed azimuthal structures in ABs are inconsistent with TI that produces only meridian structures in the ionosphere. Overall, the physical insights gained in the Ej paradigm are helpful in achieving a deep understanding of several magnetospheric phenomena.

Evaluation of the Cross‐Field Current Instability as a Substorm Onset Process With Auroral Bead Properties

AbstractDetailed properties of the spatially periodic auroral beads (ABs) that develop immediately prior to auroral substorm expansion onset on breakup auroral arcs pose stringent observational constraints on the substorm onset process. Theoretical predictions of the cross‐field current instability (CCI) are evaluated in terms of these constraints. It is found that CCI satisfies these constraints associated with ABs, that is, their growth rates, wavelengths, and periods, by matching these characteristics well with the theoretical predictions of CCI. Several subtle trends in AB properties are revealed with CCI solutions applied to them. For a given observed wavelength in the ionosphere, waves with longer periods are generated at onset sites at longer downstream distances for both westward and eastward drifting ABs. Waves with higher growth rates are generated by higher drift speeds for a given observed period and for a given observed wavelength by onset sites at closer downstream distances. A distinct group of eastward drifting ABs having long periods and small wavelengths are related to onset sites with relatively high drift speeds and large downstream distances. These revelations indicate that the onset site can occur over a range of downstream distances. ABs possessing two different propagating directions may be accommodated. A two‐dimensional equilibrium current sheet can be constructed from the adopted parameters for the current sheet used in CCI solution. Some misunderstandings of CCI are clarified as well. All these findings provide strong evidence for CCI as a viable substorm onset process.

Two-point observations of low-frequency waves at 67P/Churyumov-Gerasimenko during the descent of PHILAE: comparison of RPCMAG and ROMAP

Abstract. The European Space Agency's spacecraft ROSETTA has reached its final destination, comet 67P/Churyumov-Gerasimenko. Whilst orbiting in the close vicinity of the nucleus the ROSETTA magnetometers detected a new type of low-frequency wave possibly generated by a cross-field current instability due to freshly ionized cometary water group particles. During separation, descent and landing of the lander PHILAE on comet 67P/Churyumov-Gerasimenko, we used the unique opportunity to perform combined measurements with the magnetometers onboard ROSETTA (RPCMAG) and its lander PHILAE (ROMAP). New details about the spatial distribution of wave properties along the connection line of the ROSETTA orbiter and the lander PHILAE are revealed. An estimation of the observed amplitude, phase and wavelength distribution will be presented as well as the measured dispersion relation, characterizing the new type of low-frequency waves. The propagation direction and polarization features will be discussed using the results of a minimum variance analysis. Thoughts about the size of the wave source will complete our study.

Statistical characterization of the growth and spatial scales of the substorm onset arc.

We present the first multievent study of the spatial and temporal structuring of the aurora to provide statistical evidence of the near‐Earth plasma instability which causes the substorm onset arc. Using data from ground‐based auroral imagers, we study repeatable signatures of along‐arc auroral beads, which are thought to represent the ionospheric projection of magnetospheric instability in the near‐Earth plasma sheet. We show that the growth and spatial scales of these wave‐like fluctuations are similar across multiple events, indicating that each sudden auroral brightening has a common explanation. We find statistically that growth rates for auroral beads peak at low wave number with the most unstable spatial scales mapping to an azimuthal wavelength λ≈ 1700–2500 km in the equatorial magnetosphere at around 9–12 R E. We compare growth rates and spatial scales with a range of theoretical predictions of magnetotail instabilities, including the Cross‐Field Current Instability and the Shear Flow Ballooning Instability. We conclude that, although the Cross‐Field Current instability can generate similar magnitude of growth rates, the range of unstable wave numbers indicates that the Shear Flow Ballooning Instability is the most likely explanation for our observations.

Observation of a new type of low-frequency waves at comet 67P/Churyumov-Gerasimenko

Abstract. We report on magnetic field measurements made in the innermost coma of 67P/Churyumov-Gerasimenko in its low-activity state. Quasi-coherent, large-amplitude (δ B/B ~ 1), compressional magnetic field oscillations at ~ 40 mHz dominate the immediate plasma environment of the nucleus. This differs from previously studied cometary interaction regions where waves at the cometary ion gyro-frequencies are the main feature. Thus classical pickup-ion-driven instabilities are unable to explain the observations. We propose a cross-field current instability associated with newborn cometary ion currents as a possible source mechanism.

Revisiting Time History of Events and Macroscale Interactions during Substorms (THEMIS) substorm events implying magnetic reconnection as the substorm trigger

[1] A recent trend in substorm research is the time history approach, motivated by the NASA mission Time History of Events and Macroscale Interactions during Substorms (THEMIS). We revisit three THEMIS substorm events recently reported to indicate magnetic reconnection as the substorm trigger. The evidence for the occurrence of magnetic reconnection in these events is critically examined. It is found that they are not compelling cases for magnetic reconnection as the substorm trigger. Instead, equally plausible alternative interpretations, such as current disruption by ballooning and/or cross-field current instabilities, global Alfvenic interaction, and new plasma intrusion, that do not necessarily require a change of magnetic field line topology, could be the substorm trigger. The limitations of using the time history approach to ascertain the physical process for substorm expansion onsets are also discussed.

A joint Cluster and ground-based instruments study of two magnetospheric substorm events on 1 September 2002

Abstract. We present a coordinated ground- and space-based multi-instrument study of two magnetospheric substorm events that occurred on 1 September 2002, during the interval from 18:00 UT to 24:00 UT. Data from the Cluster and Polar spacecraft are considered in combination with ground-based magnetometer and HF radar data. During the first substorm event the Cluster spacecraft, which were in the Northern Hemisphere lobe, are to the west of the main region affected by the expansion phase. Nevertheless, substorm signatures are seen by Cluster at 18:25 UT (just after the expansion phase onset as seen on the ground at 18:23 UT), despite the ~5 RE} distance of the spacecraft from the plasma sheet. The Cluster spacecraft then encounter an earthward-moving diamagnetic cavity at 19:10 UT, having just entered the plasma sheet boundary layer. The second substorm expansion phase is preceded by pseudobreakups at 22:40 and 22:56 UT, at which time thinning of the near-Earth, L=6.6, plasma sheet occurs. The expansion phase onset at 23:05 UT is seen simultaneously in the ground magnetic field, in the magnetotail and at Polar's near-Earth position. The response in the ionospheric flows occurs one minute later. The second substorm better fits the near-Earth neutral line model for substorm onset than the cross-field current instability model. Key words. Magnetospheric physics (Magnetosphereionosphere interactions; Magnetic reconnection; Auroral phenomenon)

Cause of magnetospheric substorms

Understanding the cause of magnetospheric substorms is a challenging problem in space physics but is also very rewarding since it relates to many energetic phenomena in the Earth's magnetosphere and to other impulsive energetic events in space plasmas. Substorm models can be classified into four different categories, namely: (1) internally triggered or externally driven processes in the near-Earth magnetosphere region, (2) magnetic reconnection in the mid-tail region, (3) coupling between the ionosphere and magnetosphere, and (4) simplified or abstract description of the magnetospheric system. The first category includes a rich variety of macro- and micro-instabilities (ballooning, cross-field current, lower hybrid drift, entropy anti-diffusion, and parallel current driven instabilities). This category is the most promising one since it can naturally account for several observational constraints on the substorm onset process. In particular, the model invoking a combination of the kinetic ballooning and the cross-field current instabilities is described, together with results from particle simulation on this mechanism. Crucial elements of substorm models in the other three categories are synthesized into this model as well.This article was scheduled to appear in issue 5 of Plasma Phys. Control. Fusion. To access this Special issue please follow this link: http://www.iop.org/EJ/toc/0741-3335/45/5

Particle acceleration in the magnetotail

Very prominent particle acceleration occurs during current disruption in the magnetotail. Although particle acceleration is frequently attributed to magnetic reconnection alone, particle heating and acceleration can equally well be associated with plasma instabilities which can be unrelated to magnetic reconnection. In this paper, the characteristics of particle acceleration for a well observed current disruption event is examined in detail. It is found that particle acceleration during this current disruption event is consistent with that expected from the development of the cross-field current instability. Furthermore, observational evidence of magnetic reconnection not causing particle acceleration during this current disruption event is also found.

Rapid large-scale magnetic-field dissipation in a collisionless current sheet via coupling between Kelvin-Helmholtz and lower-hybrid drift instabilities.

Rapid large-scale magnetic-field dissipation is observed in a full kinetic simulation of cross-field current instabilities in a current sheet even when the thickness of the current sheet is at ion scale. The Kelvin-Helmholtz instability caused by the velocity shear between the current-carrying ions and the cold background ions excites the lower-hybrid drift instability at the edges of the undulated current sheet. We show that the nonlinear coupling between these two instabilities is responsible for the observed rapid dissipation. The simulation result presents a new route for magnetic-field dissipation in an ion-scale current sheet and demonstrates the general significance of nonlinear cross-scale coupling in collisionless plasmas.

A Multiscale Model for Substorms

Most substorm researchers assume substorms to be caused by a unique large-scale process. However, a critical evaluation of substorm observations indicates that a new paradigm is needed to understand the substorm phenomenon and the magnetospheric dynamics in general. It is proposed here that substorms involve a number of physical processes covering over a wide range of spatial and temporal scales. Potential candidates include the kinetic or shear ballooning instability, the Kelvin-Helmholtz instability, the cross-field current instability, the tearing instability, and magnetic reconnection. An observational constraint on the qualified process for substorm onset is that it must be associated with magnetic field lines of auroral arcs since substorm onsets start with brightening of a pre-existing auroral arc. Which particular process dominates in a given substorm depends on the present and past states of the magnetosphere as well as the external solar wind. The magnetosphere is almost perpetually driven by the solar wind to be near a critical point and in a metastable state. Magnetospheric disturbances occur sporadically in multiple localized sites. A substorm is realized when the combined effect of these localized disturbances become global in extent, much like the system-wide activity in a sandpile or avalanche model.

Role of ionospheric effects and plasma sheet dynamics in the formation of auroral arcs

At the ionospheric level, the substorm onset (expansion phase) is marked by the initial brightening and subsequent breakup of a pre-existing auroral arc. According to the field line resonance (FLR) wave model, the substorm-related auroral arc is caused by the field-aligned current carried by FLRs. The FLRs are standing shear Alfven wave structures that are excited along the dipole/quasi-dipole lines of the geomagnetic field. The FLRs (that can cause auroral arc) thread from the Earthward edge of the plasma sheet and link the auroral arc to the plasma sheet region of 6–15 RE. The region is associated with magnetic fluctuations that result from the nonlinear wave-wave interactions of the cross-field current-instability. The instability (excited at the substorm onset) disrupts the cross-tail current which is built up during the growth phase of the substorms and results in magnetic fluctuations. The diversion of the current to polar regions can lead to auroral arc intensification. The current FLR model is based on the amplitude equations that describe the nonlinear space-time evolution of FLRs in the presence of ponderomotive forces exerted by large amplitude FLRs (excited during substorms). The present work will modify the FLR wave model to include the effects arising from magnetic fluctuations that result from current disruption near the plasma sheet (6–15 RE). The nonlinear evolution of FLRs is coupled with the dynamics of plasma sheet through a momentum exchange term (resulting from magnetic fluctuations due to current disruption) in the generalized Ohm's law. The resulting amplitude equations including the effects arising from magnetic fluctuations can be used to study the structure of the auroral arcs formed during substorms. We have also studied the role of feedback mechanism (in a dipole geometry of the geomagnetic field) in the formation of the discrete auroral arc observed on the nightside magnetosphere. The present nonlinear dispersive model (NDM) is extended to include effects arising from the low energy electrons originating from the plasma sheet boundary layer. These electrons increase the ionospheric conductivity in a localized patch and enhance the field-aligned current through a feedback mechanism. The feedback effects were studied numerically in a dipole geometry using the the NDM. The numerical studies yield the magnitude of the field-aligned current that is large enough to form a discrete auroral arc. Our studies provide theoretical support to the observational work of Newell et al. that the feedback instability plays a major role in the formation of the discrete auroral arcs observed on the nightside magnetosphere.

Cross‐field current instability as a catalyst of the explosive reconnection in the geomagnetotail

Consequences of the turbulence excited by the cross‐field current instability (CCI) for explosive reconnection and the tapping of lobe energy into the current disruption region are considered. It is shown that choking of the electron compressibility effect to restore the appropriate reconnection (tearing) mode may be considerably facilitated by the combined action of the pitch angle diffusion induced by CCI and the shielding effect of transient electrons. Another promising way of exciting the tearing mode is by coordinating it with the particle exchange between trapped and transient electron populations induced by the CCI turbulence, which is modulated in turn by the tearing mode because of local thinning of the current sheet.

Nonlinear instability of the geomagnetotail current sheet combining the features of tearing and cross‐field current instabilities

The substorm onset instability suggested in this paper combines the features of “macroscale” electromagnetic disturbance of the tearing mode type and the “microscale” plasma turbulence, which is generated by the cross‐field current instability and modulated in coherence with the large‐scale disturbance. Both components are involved in a feedback loop. It is assumed that the current sheet (CS) is thin enough for cross‐field current instability (CCI) turbulence to already exist. Current density variations of the large‐scale disturbance result in additional CCI generation; on the other hand, that additional plasma turbulence provides in quasi‐linear approximation a flux of electrons which violates the frozen‐in condition for magnetized electrons and thus destabilizes the tearing mode. The process is nonlinear. However, the threshold amplitude diminishes as the CS thins, so that the instability sets in when the threshold gets smaller than the background fluctuations.

Particle acceleration in disruption of the tail current sheet

The current sheet in the Earth's magnetotail is a favorite site for consideration of particle acceleration. One of the major findings from the AMPTE/CCE program is the occurrence of highly fluctuating magnetic field and particle acceleration in the near-Earth tail current sheet at substorm onsets. This phenomenon, called current disruption, signals a substantial reduction of the cross-tail current locally in association with the formation of the substorm current wedge. This high level of field fluctuation subsides with the magnetic field returning to a more dipolar configuration. The study of current disruption has rekindled interest in the near-Earth tail (|X| < ∼ 12 RE) current sheet as a substorm initiation site rather than the mid-tail (|X| ∼ 15–20 RE) as advocated by the traditional near-Earth neutral line model. Furthermore, the study of this phenomenon has led to the development of a substorm onset model based on a current-driven plasma instability termed the cross-field current instability. We review here observations of particle acceleration during current disruption. We show that at least some current disruptions are initiated locally at |X| < 12 RE and not by a disturbance travelling from the mid-tail region. Furthermore, we show observations indicating that local reversals in the north-south component of the magnetic field in the neutral sheet are not necessarily sites of intense particle energization, contrary to the expectation from magnetic reconnection theories. Comparison between observations and preliminary theoretical predictions of the cross-field current instability model gives reasonably good agreement. Local current reduction can lead to loss of equilibrium and a global field reconfiguration from a stretched state before current disruption to a relaxed state afterwards. This reconfiguration can lead to additional particle energization through the induced electric fields arising from this global field reconfiguration.

Kinetic ballooning instability for substorm onset and current disruption observed by AMPTE/CCE

A new interpretation of AMPTE/CCE observation of substorm onset and current disruption and the corresponding physical processes are presented. Toward the end of late growth phase the plasma β increases to ≥50 and a low frequency instability with a wave period of 50–75 sec is excited and grows to a large amplitude at the current disruption onset. At the onset, higher frequency instabilities are excited so that the plasma and electromagnetic field form a turbulent state. Plasma transport takes place to modify the ambient pressure profile so that the ambient magnetic field recovers from a tail‐like geometry to a more dipole‐like geometry. A new theory of kinetic ballooning instability (KBI) is proposed to explain the low frequency instability and the high β threshold (βc≥50) observed by AMPTE/CCE. The stabilizing effect is mainly due to kinetic effects of trapped electrons and finite ion Larmor radii which give rise to a large parallel electric field and hence a parallel current that greatly enhances the stabilizing effect of field line tension. As a result βc is greatly increased over the ideal MHD ballooning instability threshold by ≥ O(10² ‐ 10³). The wave‐ion magnetic drift resonance effect produces a perturbed resonant ion velocity distribution centered at a duskward velocity roughly equal to the average ion magnetic drift velocity. This perturbed ion distribution explains the enhanced duskward ion flux during the explosive growth phase and can excite higher frequency instabilities (such as the cross‐field current instability).

A substorm model with onset location tied to an auroral arc

Two important observational constraints on substorm onset theories are that (1) substorm onset occurs on field lines of a pre‐existing auroral arc, and (2) reduction of the cross‐tail current occurs in the near‐Earth magnetotail. Recent progress in linking generation of at least some auroral arcs to field line resonances (FLRs) and current disruption to possibly the cross‐field current instability (CCI) suggests a synergistic combination of these two theories. We suggest that as the cross‐tail current sheet thins to the extent that ions become unmagnetized at the end of substorm growth phase, the equatorial electric field associated with FLRs can amplify significantly the cross‐tail current to exceed the instability threshold. This combined theory naturally accounts for the observational constraints of substorm onset starting on a pre‐existing auroral arc and linking closely with current reduction in the near‐Earth magnetotail. It is compatible with the observation of the most equatorward auroral arc being the most likely one for initial brightening because the cross‐tail current is most intense in its near‐Earth end and is therefore most likely to exceed the unstable threshold for CCI. It is also consistent with auroral precipitation fading to be a common feature before an auroral breakup. Other predictions yet to be tested are that (1) the onset is more favorable for the phase of the resonance in which the resonance‐associated field‐aligned current is directed from the ionosphere to the magnetosphere, and (2) the onset is more favorable poleward of the resonance shell or auroral arc.

Collisionless reconnection in the magnetotail

Recent developments regarding collisionless reconnection in current sheets with a finite normal magnetic field component ( B z ) are reviewed. In 2-D x, z configurations the ion tearing mode is stabilized by the electron compressibility. When the y dependence is included, cross-field current instabilities can be excited. Of these, the drift kink mode appears to be particularly important. 3-D electromagnetic particle simulations indicate that this mode can act as the precursor to the growth of tearing modes and subsequent reconnection.

Nonlinear analysis of generalized cross-field current instability

Analysis of the generalized cross-field current instability is carried out in which cross-field drift of both the ions and electrons and their temperatures are permitted to vary in time. The unstable mode under consideration is the electromagnetic generalization of the classical modified-two-stream instability. The generalized instability is made of the modified-two-stream and ion-Weibel modes. The relative importance of the features associated with the ion-Weibel mode and those of the modified-two-stream mode is assessed. Specific applications are made to the Earth’s neutral sheet prior to substorm onset and to the Earth’s bow shock. The numerical solution indicates that the ion-Weibel mode dominates in the Earth’s neutral sheet environment. In contrast, the situation for the bow shock is dominated by the modified-two-stream mode. Notable differences are found between the present calculation and previous results on ion-Weibel mode which restrict the analysis to only parallel propagating waves. However, in the case of Earth’s bow shock for which the ion-Weibel mode plays no important role, the inclusion of the electromagnetic ion response is found to differ little from the previous results which treats ions responding only to the electrostatic component of the excited waves.

A cross‐field current instability for substorm expansions

We investigate a cross‐field current instability (CFCI) as a candidate for current disruption during substorm expansions. The numerical solution of the linear dispersion equation indicates that (1) the proposed instability can occur at the inner edge or the midsection of the neutral sheet just prior to the substorm expansion onset although the former environment is found more favorable at the same drift speed scaled to the ion thermal speed, (2) the computed growth time is comparable to the substorm onset time, and (3) the excited waves have a mixed polarization with frequencies near the ion gyrofrequency at the inner edge and near the lower hybrid frequency in the midtail region. On the basis of this analysis we propose a substorm development scenario in which plasma sheet thinning during the substorm growth phase leads to an enhancement in the relative drift between ions and electrons. This results in the neutral sheet being susceptible to the CFCI and initiates the diversion of the cross‐tail current through the ionosphere. Whether or not a substorm current wedge is ultimately formed is regulated by the ionospheric condition. A large number of substorm features can be readily understood with the proposed scheme. These include (1) precursory activities (pseudobreakups) prior to substorm onset, (2) substorm initiation region to be spatially localized, (3) three different solar wind conditions for substorm occurrence, (4) skew towards evening local times for substorm onset locations, (5) different acceleration characteristics between ions and electrons, (6) tailward spreading of current disruption region after substorm onset, and (7) local time expansion of substorm current wedge with possible discrete westward jump for the evening expansion.