The relationship between field-aligned currents and the auroral electrojets: A review

The recent development of several new observational techniques as well as of advanced computer simulation codes has contributed significantly to our understanding of dynamics of the three-dimensional current system during magnetospheric substorms. This paper attempts to review the main results of the last decade of research in such diverse fields as electric fields and currents in the high-latitude ionosphere and field-aligned currents and their relationship to the large-scale distribution of auroras and auroral precipitation. It also contains discussions on some efforts in synthesizing the vast amount of the observations to construct an empirical model which connects the ionospheric currents with field-aligned currents. While our understanding has been greatly improved during the last decade, there is much that is as yet unsettled. For example, we have reached only a first approximation model of the three-dimensional current system which is not inconsistent with integrated, ground-based and space observations of electric and magnetic fields. We have just begun to unfold the cause of the field-aligned currents both in the magnetosphere and ionosphere. Dynamical behaviour of the magnetosphere-ionosphere coupling relating to substorm variability can be an important topic during the coming years.On leave of absence from Kyoto Sangyo University, Kyoto 603, Japan.     

Substorm aspects of magnetic pulsations

The different types of magnetic pulsations occurring during magnetospheric substorms are analysed into the concept of polar substorms recently described in detail by Akasofu (1968). Special attention is thus paid, to the simultaneous occurrence of different types of micropulsations at different places around the earth, during the development of a substorm. Time lags between the appearance of micropulsations and other geophysical effects of the substorm are of fundamental importance in this respect. Relationships between the occurrence or spectral shape of micropulsations and the state of the magnetosphere, as determined by satellite measurements are also of interest. Recent theoretical studies about the origin of these micropulsations are reviewed: natural h.m. emissions are directly linked to the thermal plasma density, the high energetic particle fluxes and pitch angle distributions in the far magnetosphere (L 5–8). We can thus expect to be able to deduce some information about the changes of these quantities during substorms. New semi-quantitative work is reported, which tries to interpret the repetitive structure of SIP events in terms of thickness of the magnetospheric tail, and the frequency drift of IPDP's in terms of magnetospheric electric fields. The present knowledge about absorption and dispersion of hydromagnetic waves through the ionosphere or inside the submagneto-ionospheric guide is also stated, because not taking these effects into account could lead to misinterpretation of the data.     

Energy flux in the Earth's magnetosphere: Storm – substorm relationship

Three ways of the energy transfer in the Earth's magnetosphere are studied. The solar wind MHD generator is an unique energy source for all magnetospheric processes. Field-aligned currents directly transport the energy and momentum of the solar wind plasma to the Earth's ionosphere. The magnetospheric lobe and plasma sheet convection generated by the solar wind is another magnetospheric energy source. Plasma sheet particles and cold ionospheric polar wind ions are accelerated by convection electric field. After energetic particle precipitation into the upper atmosphere the solar wind energy is transferred into the ionosphere and atmosphere. This way of the energy transfer can include the tail lobe magnetic field energy storage connected with the increase of the tail current during the southward IMF. After that the magnetospheric substorm occurs. The model calculations of the magnetospheric energy give possibility to determine the ground state of the magnetosphere, and to calculate relative contributions of the tail current, ring current and field-aligned currents to the magnetospheric energy. The magnetospheric substorms and storms manifest that the permanent solar wind energy transfer ways are not enough for the covering of the solar wind energy input into the magnetosphere. Nonlinear explosive processes are necessary for the energy transmission into the ionosphere and atmosphere. For understanding a relation between substorm and storm it is necessary to take into account that they are the concurrent energy transferring ways. This revised version was published online in August 2006 with corrections to the Cover Date.     

Long-period irregular magnetic pulsation,Pi3

In the 1973 Scientific Assembly, the International Association of Geomagnetism and Aeronomy proposed with Resolution No. 11 to settle two new classes of magnetic pulsations; Pc6 having a sinusoidal waveform with periods longer than 600 s and Pi3 having an irregular waveform with periods longer than 150 s. The present paper reviews the studies on these pulsations putting a stress on Pi3. the Pi3-type pulsations are further classified into Psc5, Psc6, Pip, Ps6 and another type. The pulsations Psc5 and Psc6 mean the damped-type pulsations associating with storm sudden commencement in the approximate period range from 150 to 600 s and longer than 600 s, respectively, while Pip and Ps6 are the quasi-sinusoidal pulsations associating with magnetospheric substorm in the appropriate period range from 100 to 400 s and from 5 to 40 min, respectively. In the present review paper a stress is further layed on morphology and theory of the Ps6-type Pi3 pulsation. The following two-snake model is concluded to be plausible. A current system with in-flowing field-aligned current, westward ionospheric current, and out-flowing field-aligned current expands toward both the dawn- and the dusk-sectors with the progress of magnetospheric substorm. Thus Ps6 is regarded to be due to a magnetic effect of a meandering of the current system during the expansion, which is compared with the meandering of two snakes along the auroral oval from the midnight point toward both the east and the west, respectively.     

Ionosphere-magnetosphere coupling

The large-scale electrical coupling between the ionosphere and magnetosphere is reviewed, particularly with respect to behavior on time scales of hours or more. The following circuit elements are included: (1) the magnetopause boundary layer, which serves as the generator for the magnetospheric-convection circuit; (2) magnetic field lines, usually good conductors but sometimes subject to anomalous resistivity; (3) the ionosphere, which can conduct current across magnetic field lines; (4) the magnetospheric particle distributions, including tail current and partial-ring currents. Magnetic merging and a viscous interaction are considered as possible generating mechanisms, but merging seems the most likely alternative. Several mechanisms have been proposed for causing large potential drops along magnetic field lines in the upper ionosphere, and many isolated measurements of parallel electric fields have been reported, but the global pattern and significance of these electric fields are unknown. Ionospheric conductivities are now thoroughly measured, but are highly variable. Simple self-consistent theoretical models of the magnetospheric-convection system imply that the magnetospheric particles should shield the inner magnetosphere and low-latitude ionosphere from most of the time-average convection electric field.     

The magnetotail and substorms

The tail plays a very active and important role in substorms. Magnetic flux eroded from the dayside magnetosphere is stored here. As more and more flux is transported to the magnetotail and stored, the boundary of the tail flares more, the field strength in the tail increases, and the currents strengthen and move closer to the Earth. Further, the plasma sheet thins and the magnetic flux crossing the neutral sheet lessens. At the onset of the expansion phase, the stored magnetic flux is returned from the tail and energy is deposited in the magnetosphere and ionosphere. During the expansion phase of isolated substorms, the flaring angle and the lobe field strength decrease, the plasma sheet thickens and more magnetic flux crosses the neutral sheet.In this review, we discuss the experimental evidence for these processes and present a phenomenological or qualitative model of the substorm sequence. In this model, the flux transport is driven by the merging of the magnetospheric and interplanetary magnetic fields. During the growth phase of substorms the merging rate on the dayside magnetosphere exceeds the reconnection rate in the neutral sheet. In order to remove the oversupply of magnetic flux in the tail, a neutral point forms in the near earth portion of the tail. If the new reconnection rate exceeds the dayside merging rate, then an isolated substorm results. However, a situation can occur in which dayside merging and tail reconnection are in equilibrium. The observed polar cap electric field and its correlation with the interplanetary magnetic field is found to be in accord with open magnetospheric models.     

Steady magnetospheric convection: A review of recent results

Theoretical pressure balance arguments have implied that steady convection is hardly possible in the terrestrial magnetotail and that steady energy input necessarily generates a cyclic loading-unloading sequence, i.e., repetitive substorms. However, observations have revealed that enhanced solar wind energy input to the magnetospheric system may either lead to substorm activity or enhanced but steady convection. This topic is reviewed with emphasis on several recent case studies of the Steady Magnetospheric Convection (SMC) events. In these cases extensive data sets from both satellite and ground-based instruments from various magnetospheric and ionospheric regions were available.Accurate distinction of the spatial and temporal scales of the magnetospheric processes is vital for correct interpretation of the observations during SMC periods. We show that on the large scale, the magnetospheric configuration and plasma convection are stable during SMC events, but that both reveal considerable differences from their quiet-time assemblies. On a shorter time scale, there are numerous transient activations which are similar to those found during substorms, but which presumably originate from a more distant tail reconnection process, and map to the poleward boundary of the auroral oval. The available observations and the unresolved questions are summarized here.The tail magnetic field during SMC events resembles both substorm growth and recovery phases in the neartail and midtail, respectively, but this configuration may remain stable for up to ten hours. Based on observations and model results we discuss how the magnetospheric system avoids pressure balance problems when the plasma convects earthward.Finally, the importance of further coordinated studies of SMC events is emphasized. Such studies may shed more light on the substorm dynamics and help to verify quantitatively the theoretical models of the convecting magnetosphere.     

Coupling between the outer magnetosphere and the high-latitude ionosphere

The coupling between the ionosphere and the outer magnetosphere depends on the topology of the geomagnetic field. Some aspects of the closed and open magnetospheric models are briefly discussed.The assumption that the geomagnetic field lines are equipotentials is critisized both on observational and on theoretical grounds. Measurements of H Doppler profiles, of precipitating particles above the ionosphere, and of charged particle densities in the magnetosphere indicate the existence of electric fields, E_\\, parallel with the magnetic field.Two different models of E_\\ are considered. Both models violate the condition of frozen-in magnetic fields. In one of them there are occasional transient electric field impulses along the field lines which cause precipitation splashes. The other model invokes electrostatic fields which vanish occasionally due to instabilities. This gives rise to precipitation splashes of about equal numbers of ions and electrons.The latter model seems to be favoured by known satellite data concerning the pitch angle distributions of electrons above the ionosphere.It is suggested that electric fields in space should be measured by satellites and rockets. Expected values of the fields in different regions of space are given.     

Fluctuating magnetic fields in the magnetosphere

The study of ULF waves in space has been in progress for about 12 years. However, because of numerous observational difficulties the properties of the waves in this frequency band (10^-3 to 1 Hz) are poorly known. These difficulties include the nature of satellite orbits, telemetry limitations on magnetometer frequency response and compromises between dynamic range and resolution. Despite the paucity of information, there is increasing recognition of the importance of these measurements in magnetospheric processes. A number of recent theoretical papers point out the roles such waves play in the dynamic behavior of radiation belt particles.At the present time the existing satellite observations of ULF waves suggest that the level of geomagnetic activity controls the types of waves which occur within the magnetosphere. Consequently, we consider separately quiet times, times of magnetospheric substorms and times of magnetic storms. Within each of these categories there are distinctly different wave modes distinguished by their polarization: either transverse or parallel to the ambient field. In addition, these wave phenomena occur in distinct frequency bands. In terms of the standard nomenclature of ground micropulsation studies ULF wave types observed in the magnetosphere include quiet time transverse — Pc 1, Pc 3, Pc 4, Pc 5 quiet time compressional — Pc 1 and Pi 1; substorm compressional Pi 1 and Pi 2; storm transverse — Pc 1; storm compressional Pc 4, 5. The satellite observations are not yet sufficient to determine whether the various bands identified in the ground data are equally appropriate in space.Publication No. 982. Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Calif. 90024.     

Substorms and Their Solar Wind Causes

Consequences of the solar wind input observed as large scale magnetotail dynamics during substorms are reviewed, highlighting results from statistical studies as well as global magnetosphere/ionosphere observations. Among the different solar wind input parameters, the most essential one to initiate reconnection relatively close to the Earth is a southward IMF or a solar wind dawn-to-dusk electric field. Larger substorms are associated with such reconnection events closer to the Earth and the magnetotail can accumulate larger amounts of energy before its onset. Yet, how and to what extent the magnetotail configuration before substorm onset differs for different solar wind driver is still to be understood. A strong solar wind dawn-to-dusk electric field is, however, only a necessary condition for a strong substorm, but not a sufficient one. That is, there are intervals when the solar wind input is processed in the magnetotail without the usual substorm cycle, suggesting different modes of flux transport. Furthermore, recent global observations suggest that the magnetotail response during the substorm expansion phase can be also controlled by plasma sheet density, which is coupled to the solar wind on larger time-scales than the substorm cycle. To explain the substorm dynamics it is therefore important to understand the different modes of energy, momentum, and mass transport within the magnetosphere as a consequence of different types of solar wind-magnetosphere interaction with different time-scales that control the overall magnetotail configuration, in addition to the internal current sheet instabilities leading to large scale tail current sheet dissipation.     

High-Latitude Particle Precipitation and its Relationship to Magnetospheric Source Regions

Two central issues in magnetospheric research are understanding the mapping of the low-altitude ionosphere to the distant regions of the magnetsphere, and understanding the relationship between the small-scale features detected in the various regions of the ionosphere and the global properties of the magnetosphere. The high-latitude ionosphere, through its magnetic connection to the outer magnetosphere, provides an important view of magnetospheric boundaries and the physical processes occurring there. All physical manifestations of this magnetic connectivity (waves, particle precipitation, etc.), however, have non-zero propagation times during which they are convected by the large-scale magnetospheric electric field, with phenomena undergoing different convection distances depending on their propagation times. Identification of the ionospheric signatures of magnetospheric regions and phenomena, therefore, can be difficult. Considerable progress has recently been made in identifying these convection signatures in data from low- and high-altitude satellites. This work has allowed us to learn much about issues such as: the rates of magnetic reconnection, both at the dayside magnetopause and in the magnetotail; particle transport across the open magnetopause; and particle acceleration at the magnetopause and the magnetotail current sheets.     

Large-scale electric field in the magnetosphere

A review is given on the distribution and origin of the large-scale electric field in the magnetosphere and its influence on the dynamical behavior of the magnetospheric plasma. Following a general discussion on the gross structure of the magnetosphere and its tail, two principal electric field systems are deduced from ground-based geomagnetic variations. One is responsible for the polar substorm, the DP 1 field, which is closely associated with the activation of the auroral electrojet. The other is responsible for the twin current vortices, the DP 2 field, and this represents the general convective system set up in the magnetospheric plasma.The origin of these magnetospheric electric fields is possibly resided in the domain of the solar wind interacting with the outer geomagnetic field. However, the mechanism, in which the energy is transferred, is still quite controversial. Several theories so far proposed are re-examined, and some modification of them are suggested to have a consistent understanding of these two types of electric fields. The effects of electric fields on magnetospheric plasma dynamics are described, such as the formation of the plasmapause, the acceleration and diffusion of energetic particles in the radiation belt.     

Coupling at the Atmosphere-Ionosphere-Magnetosphere Interface and Resonant Phenomena in the ULF Range

We discuss the electromagnetic processes in the ULF range which are important for the coupling between the atmosphere, ionosphere, and magnetosphere (AIM). The main attention is given to the Pc1–2 frequency ranges (f≈0.1–10 Hz) where some natural resonances in the AIM system are located. In particular, we consider the resonant structures in the spectra of the magnetic background noise related to the Alfvén resonances in the ionosphere as a possible diagnostic tool for studies of the ionospheric parameters. We also discuss the self-excitation of Alfvén waves in the ionosphere due to the AIM coupling and the role of such waves in the acceleration of electrons in the upper ionosphere and magnetosphere. Precipitation of magnetospheric ions due to their interaction with the ion-cyclotron waves is analyzed in relation to the ionospheric current systems, formation of partial ring current, and the influence of the ionosphere-magnetosphere feedback on the generation of such waves.     

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.     

Substorm timings and timescales: A new aspect

The magnetospheric substorm is a fundamental element of magnetospheric disturbances. After more than 40 years of intensive studies, various aspects of substorm morphology have been qualitatively established. Observations from the International Solar-Terrestrial Physics (ISTP) mission during the last decade have provided more detailed and complete pictures of substorms than before and, consequently, have provided new insights into substorm mechanisms. From the global auroral imaging it is shown that substorm onsets are locally confined; however, the effects of substorms involve a very large space at different times. Observations relying on in situ techniques can be misleading and can introduce confusion if not properly interpreted. On the other hand, remote sensing techniques such as global auroral imaging not only provide a robust means for studying substorm phenomenology but also yield relatively consistent results. This article reviews and summarizes a number of substorm studies conducted based primarily on global auroral images from NASA's Polar satellite, with a main focus on “quantitative” substorm morphology (i.e., onset timing, locations, energy input, and substorm timescales). These studies conclude that (1) auroral breakups are the most reliable substorm indicator, whereas other commonly used onset proxies may not always be associated with substorms and are subject to a propagation delay; (2) after breakup, the expanded auroral bulge can move either westward (60%) or eastward (40%); and (3) a typical substorm expansion phase lasts ～10 minutes and increases with increasing distances from the onset. A key conclusion from some recent studies seems to suggest that magnetotail reconnection, if it ever exists, is a consequence of substorm expansion onset. These findings provide constraints for substorm models and theories.     

Observations of birkeland currents

Recent measurements of precipitating energetic particles and vector magnetic fields from satellites and sounding rockets have verified the existence of geomagnetically-aligned electric currents at high latitudes in the ionosphere and magnetosphere. The spatial and temporal configuration of such currents, now commonly called Birkeland currents, has delineated their role in providing ionospheric closure of magnetospheric current systems, and gross features of these current systems may be understood in terms of theoretical models of magnetospheric convection. The association of Birkeland currents with auroral features on a very small scale suggests that auroral acceleration may result from the current flow.     

Ring Currents and Internal Plasma Sources

The discovery of terrestrial O⁺ and other heavy ions in magnetospheric hot plasmas, combined with the association of energetic ionospheric outflows with geomagnetic activity, led to the conclusion that increasing geomagnetic activity is responsible for filling the magnetosphere with ionospheric plasma. Recently it has been discovered that a major source of ionospheric heavy ion plasma outflow is responsive to the earliest impact of coronal mass ejecta upon the dayside ionosphere. Thus a large increase in ionospheric outflows begins promptly during the initial phase of geomagnetic storms, and is already present during the main phase development of such storms. We hypothesize that enhancement of the internal source of plasma actually supports the transition from substorm enhancements of aurora to storm-time ring current development in the inner magnetosphere. Other planets known to have ring current-like plasmas also have substantial internal sources of plasma, notably Jupiter and Saturn. One planet having a small magnetosphere, but very little internal source of plasma, is Mercury. Observations suggest that Mercury has substorms, but are ambiguous with regard to the possibility of magnetic storms of the planet. The Messenger mission to Mercury should provide an interesting test of our hypothesis. Mercury should support at most a modest ring current if its internal plasma source is as small as is currently believed. If substantiated, this hypothesis would support a general conclusion that the magnetospheric inflationary response is a characteristic of magnetospheres with substantial internal plasma sources. We quantitatively define this hypothesis and pose it as a problem in comparative magnetospheres.     

Primary sources of large-scale Birkeland currents

We review generation mechanisms of Birkeland currents (field-aligned currents) in the magnetosphere and the ionosphere. Comparing Birkeland currents predicted theoretically with those studied observationally by spacecraft experiments, we present a model for driving mechanism, which is unified by the solar wind-magnetosphere interaction that allows the coexistence of steady viscous interaction and unsteady magnetic reconnection. The model predicts the following: (1) the Region 1 Birkeland currents (which are located at poleward part of the auroral Birkeland-current belt, and constitute quasi-permanently and stably a primary part of the overall system of Birkeland currents) would be fed by vorticity-induced space charges at the core of two-cell magnetospheric convection arisen as a result of viscous interaction between the solar wind and the magnetospheric plasma, (2) the Region 2 Birkeland currents (which are located at equatorward part of the auroral Birkeland-current belt, and exhibit more variable and localized behavior) would orginate from regions of plasma pressure inhomogeneities in the magnetosphere caused by the coupling between two-cell magnetospheric convection and the hot ring current, where the gradient-B current and/or the curvature current (presumably the hot plasma sheet-ring current) are forced to divert to the ionosphere, (3) the Cusp Birkeland currents (which are located poleward of and adjacent to the Region 1 currents and are strongly controlled by the interplanetary magnetic field (IMF)) might be a diversion of the inertia current which is newly and locally produced in the velocity-decelerated region of earthward solar wind where the magnetosphere is eroded by dayside magnetic reconnection, (4) the nightside Birkeland currents which are connected to a part of the westward auroral electrojet in the Harang discontinuity sector might be a diversion of the dusk-to-dawn tail current resulting from localized magnetic reconnection in the magnetotail plasma sheet where plasma density and pressure are reduced.