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.     

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.     

Flow Bursts in the Plasma Sheet and Auroral Substorm Onset: Observational Constraints on Connection Between Midtail and Near-earth Substorm Processes

Causality between near-Earth and midtail substorm processes is one of the most controversial issues about the substorm trigger mechanism. The currently most popular model, the outside-in model, assumes that near-Earth reconnection is initiated in the midtail region before substorm onset and that the associated flow burst causes tail current disruption in the near-Earth region. However, there remain some outstanding issues that may serve as critical tests of this model. The present article reviews recent satellite and ground observations addressing three such critical issues with a focus on substorm-related auroral features. First, near-Earth reconnection, even if it reaches the lobe magnetic field, does not necessarily trigger a global substorm, but it is often related to a pseudobreakup. This fact suggests that there is an additional or alternative condition for substorm development. Secondly, although there appears to be one-to-one correspondence between flow bursts in the plasma sheet and equatorward-moving auroral structures (auroral streamers), no such auroral feature that can be associated with the fast plasma flow can be identified prior to auroral breakups. On the other hand, the flow burst is widely regarded as a manifestation of reconnection and therefore, according to the outside-in model, should be created in the near-Earth plasma sheet before substorm onset. Finally, auroral arcs poleward of a breakup arc are not affected until the front of auroral intensification reaches those arcs. The last two points suggest that if substorm is triggered as the outside-in model describes, the ionosphere is electromagnetically detached from the magnetosphere, which, however, has not been addressed theoretically. Thus, it should be crucial for a better understanding of the substorm trigger process to implement the magnetosphere-ionosphere coupling in future modeling efforts and to address those basic issues as a guide for critically evaluating each model.     

Dynamic morphology of auroras

Simultaneous changes of auroral forms, brightness, and motions over the whole polar region are studied, using IGY all-sky camera records from widely distributed stations in eastern Siberia, Alaska, Canada and the northern United States. It is found that the auroral system centered in the midnight sector in the auroral zone repeatedly undergoes an expansion and subsequent contraction; during the maximum stage of the activity, the whole auroral system extends over a substantial portion of the darkened polar region. Such extensive auroral activity as a whole may be regarded as a single event, and is described in terms of the auroral substorm. The substorm has two characteristic phases, an expansive phase and a recovery phase. Characteristic auroral displays over the entire polar region during the substorm are described in detail. The basic physical processes involved for the auroral substorm are also discussed.Geomagnetic disturbances associated with the auroral substorm are also described in detail in terms of the polar magnetic substorm, and it is shown that both the auroral substorm and the polar magnetic substorm are different aspects of the manifestation of a large-scale plasma motion in the magnetosphere.The distribution of the aurora for different degrees of the geomagnetic activity is also discussed in terms of the auroral belt. It is shown that the center line of the auroral belt moves greatly with respect to its average location (namely the auroral zone), depending on the degree of the magnetic activity.     

Relationship Between Substorm Auroras and Processes in the Near-Earth Magnetotail

Although the auroral substorm has been long regarded as a manifestation of the magnetospheric substorm, a direct relation of active auroras to certain magnetospheric processes is still debatable. To investigate the relationship, we combine the data of the UV imager onboard the Polar satellite with plasma and magnetic field measurements by the Geotail spacecraft. The poleward edge of the auroral bulge, as determined from the images obtained at the LHBL passband, is found to be conjugated with the region where the oppositely directed fast plasma flows observed in the near-Earth plasma sheet during substorms are generated. We conclude that the auroras forming the bulge are due to the near-Earth reconnection process. This implies that the magnetic flux through the auroral bulge is equal to the flux dissipated in the magnetotail during the substorm. Comparison of the magnetic flux through the auroral bulge with the magnetic flux accumulated in the tail lobe during the growth phase shows that these parameters have the comparable values. This is a clear evidence of the loading–unloading scheme of substorm development. It is shown that the area of the auroral bulge developing during substorm is proportional to the total (magnetic plus plasma) pressure decrease in the magnetotail. These findings stress the importance of auroral bulge observations for monitoring of substorm intensity in terms of the magnetic flux and energy dissipation.     

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.     

Potential Plasma Instabilities For Substorm Expansion Onsets

Space plasmas present intriguing and challenging puzzles to the space community. Energy accessible to excite instabilities exists in a variety of forms, particularly for the magnetospheric environment prior to substorm expansion onsets. A general consensus of the pre-expansion magnetosphere is the development of a thin current sheet in the near-Earth magnetosphere. This review starts with a short account of the two major substorm paradigms. Highlights of some observations pertaining to the consideration of potential plasma instabilities for substorm expansion are given. Since a common thread of these paradigms is the development of a thin current sheet, several efforts to model analytically a thin current sheet configuration are described. This leads to a review on the instability analyses of several prominent candidates for the physical process responsible for substorm expansion onset. The potential instabilities expounded in this review include the cross-field current, lower-hybrid-drift, drift kink/sausage, current driven Alfvénic, Kelvin-Helmholtz, tearing, and entropy anti-diffusion instabilities. Some recent results from plasma simulations relevant to the investigation of these plasma instabilities are shown. Although some of these instabilities are generally conceived to be excited in spatially localized regions in the magnetosphere, their potentials in yielding global consequences are also explored.     

The Role and Contributions of Energetic Neutral Atom (ENA) Imaging in Magnetospheric Substorm Research

Energetic Neutral Atom (ENA) imaging has contributed substantially to substorm research. This technique has allowed significant advances in areas such as observation and quantification of injected particle drift as a function of energy, observation of dynamics in the tail that are directly related to the effects of imposed (growth phase) and induced (expansion phase) electric fields on the plasma, the prompt extraction of oxygen from the ionosphere during substorms, the relationship between storms and substorms, and the timing of substorm ENA signatures. We present discussion of the advantages and shortcomings of the ENA technique for studying space plasmas. Although the technique is in its infancy, it is yielding results that enrich our understanding of the substorm process and its effects.     

The roles of direct input of energy from the solar wind and unloading of stored magnetotail energy in driving magnetospheric substorms

This paper presents the consensus arrived at by the authors with respect to the contributions to the substorm expansive phase of direct energy input from the solar wind and from energy stored in the magnetotail which is released in a sometimes unpredictable manner. Two physical processes, neither of which can be ignored, are considered to be of importance in the dispensation of the energy input from the solar wind. One of these is the driven process in which energy, supplied from the solar wind, is directly dissipated in the ionosphere with the only clearly definable delay being due to the inductance of the magnetosphere-ionosphere system. The other is the loading-unloading process in which energy from the solar wind is first stored in the magnetotail and then is suddenly released to be deposited in the ionosphere as a consequence of external changes in the interplanetary medium or internal triggering processes. Although the driven process appears to be more dominant on a statistical basis in terms of solar wind-geomagnetic activity relationships, one or the other of the two above processes may dominate for any individual cases. Moreover, the two processes may operate simultaneously during a given phase of the substorm, e.g., the magnetotail may experience loading as the driven system increases in strength. Thus, in our approach, substorms are described in terms of physical processes which we infer to be operative in the magnetosphere and the terminology of the past (e.g., phases) is related to those inferred physical processes. The pattern of substorm development in response to changes in the interplanetary medium is presented for a canonical isolated substorm.Now at Max-Planck-Institut für Physik und Astrophysik, Institut für Extraterrestrische Physik, D-8046 Garching, F.R.G.     

Ballooning Instability as a Mechanism of the Near-Earth Onset of Substorms

After introducing a mathematical definition of the tail-like equilibrium and the dipole-like equilibrium in the magnetosphere, it is shown by using physical intuition based on the Energy Principle that the incompressible assumption for the ballooning instability is more valid for the tail-like configuration when the unstable ballooning mode is strongly localized near the equator. Therefore, before the substorm onset, the near-Earth plasma sheet becomes more tail-like and more likely to be subject to the ballooning instability without the stabilizing influence of the compressibility, when the critical plasma due to the stabilizing tension force is exceeded. The onset of the ballooning instability in the near-Earth plasma sheet seems promisingly relevant to the substorm onset phenomena. Also, the effect of the stochastic plasma dynamics on the ballooning and interchange instabilities is clearly shown.     

The THEMIS Array of Ground-based Observatories for the Study of Auroral Substorms

The NASA Time History of Events and Macroscale Interactions during Substorms (THEMIS) project is intended to investigate magnetospheric substorm phenomena, which are the manifestations of a basic instability of the magnetosphere and a dominant mechanism of plasma transport and explosive energy release. The major controversy in substorm science is the uncertainty as to whether the instability is initiated near the Earth, or in the more distant >20 Re magnetic tail. THEMIS will discriminate between the two possibilities by using five in-situ satellites and ground-based all-sky imagers and magnetometers, and inferring the propagation direction by timing the observation of the substorm initiation at multiple locations in the magnetosphere. An array of stations, consisting of 20 all-sky imagers (ASIs) and 30-plus magnetometers, has been developed and deployed in the North American continent, from Alaska to Labrador, for the broad coverage of the nightside magnetosphere. Each ground-based observatory (GBO) contains a white light imager that takes auroral images at a 3-second repetition rate (“cadence”) and a magnetometer that records the 3 axis variation of the magnetic field at 2 Hz frequency. The stations return compressed images, “thumbnails,” to two central databases: one located at UC Berkeley and the other at the University of Calgary, Canada. The full images are recorded at each station on hard drives, and these devices are physically returned to the two data centers for data copying. All data are made available for public use by scientists in “browse products,” accessible by using internet browsers or in the form of downloadable CDF data files (the “browse products” are described in detail in a later section). Twenty all-sky imager stations are installed and running at the time of this publication. An example of a substorm was observed on the 23rd of December 2006, and from the THEMIS GBO data, we found that the substorm onset brightening of the equatorward arc was a gradual process (>27 seconds), with minimal morphology changes until the arc breaks up. The breakup was timed to the nearest frame (<3 s) and located to the nearest latitude degree at about ±3^oE in longitude. The data also showed that a similar breakup occurred in Alaska ～10 minutes later, highlighting the need for an array to distinguish prime onset.     

Transient phenomena in the magnetotail and their relation to substorms

Recent observations of magnetic field, plasma flow and energetic electron anisotropies in the magnetotail plasma sheet during substorms have provided strong support for the idea that a magnetospheric substorm involves the formation of a magnetic neutral line (the substorm neutral line) within the plasma sheet at X _SM — 10R _E to -25R _E. An initial effect, in the tail, of the neutral line's formation is the severance of plasma sheet field lines to form a plasmoid, i.e., a closed magnetic loop structure, that is quickly (within 5–10 min) ejected from the tail into the downstream solar wind. The plasmoid's escape leaves a thin downstream plasma sheet through which plasma and energetic particles stream continuously into the solar wind, often throughout the duration of the substorm's expansive phase. Southward oriented magnetic field threads this tailward-flowing plasma but its detection, as an identifier of the occurrence of magnetic reconnection, is made difficult by the thinness and turbulence of the downstream plasma sheet. The thinning of the plasma sheet downstream of the neutral line is observed, by satellites located anywhere but very close to the tail's midplane, as a plasma dropout. Multiple satellite observations of plasma droputs suggest that the substorm neutral line often extends across a large fraction (> ) of the tail's breadth. Near the time of substorm recovery the substorm neutral line moves quickly tailward to a more distant location, progressively inflating the closed field lines earthward of it, to reform the plasma sheet.Proceedings of the Symposium on Solar Terrestrial Physics held in Innsbruck, May–June 1978.     

Global Particle Simulation Study of Substorm Onset and Particle Acceleration

This paper reports the spatial and temporal development of Bursty Bulk Flows (BBFs) created by the reconnection as well as current disruptions (CDs) in the near-Earth tail using our 3D global EM particle simulation with a southward turning IMF in the context of the substorm onset. Recently, observations show that BBFs are often accompanied by current disruptions for triggering substorms. We haver examined the dynamics of BBFs and CDs in order to understand the timing and triggering mechanism of substorms. As the solar wind with the southward IMF advances over the Earth, the near-Earth tail thins and the sheet current intensifies. Before the peak of the current density becomes maximum, the reconnection takes place, which ejects particles from the reconnection region. Because of the earthward flows the peak of the current density moves toward the Earth. The characteristics of the earthward flows depend on the ions and electrons. Electrons flow back into the inflow region (the center of reconnection region), which provides current closure. Therefore the structure of electron flows near the reconnection region is rather complicated. In contrast, the ion earthward flows are generated far from the reconnection region. These earthward flows pile up near the Earth. The ions mainly drift toward the duskside. The electrons are diverted toward the duskside. Due to the pile-up, dawnward current is generated near the Earth. This dawnward current dissipates rapidly with the sheet current because of the opposite current direction, which coincides with the dipolarization in the near-Earth tail. At this time the wedge current may be created in our simulation model. This simulation study shows the sequence of the substorm dynamics in the near-Earth tail, which is similar to the features obtained by the multisatellite observations. The identification of the timing and mechanism of triggering substorm onset requires further studies in conjunction with observations.     

Global Imaging of Proton and Electron Aurorae in the far Ultraviolet

The IMAGE spacecraft carries three FUV photon imagers, the Wideband Imaging Camera (WIC) and two channels, SI-12 and SI-13, of the Spectrographic Imager. These provide simultaneous global images, which can be interpreted in terms of the precipitating particle types (protons and electrons) and their energies. IMAGE FUV is the first space-borne global imager that can provide instantaneous global images of the proton precipitation. At times a bright auroral spot, rich in proton precipitation, is observed on the dayside, several degrees poleward of the auroral zone. The spot was identified as the footprint of the merging region of the cusp that is located on lobe field lines when IMF B_z was northward. This identification was based on compelling statistical evidence showing that the appearance and location of the spot is consistent with the IMF B_z and B_y directions. The intensity of the spot is well correlated with the solar wind dynamic pressure and it was found that the direct entry of solar wind particles could account for the intensity of the observed spot without the need for any additional acceleration. Another discovery was the observation of dayside sub-auroral proton arcs. These arcs were observed in the midday to afternoon MLT sector. Conjugate satellite observations showed that these arcs were generated by pure proton precipitation. Nightside auroras and their relationship to substorm phases were studied through single case studies and in a superimposed epoch analysis. It was found that generally there is substantial proton precipitation prior to substorms and the proton intensity only doubles at substorm onset while the electron auroral brightness increases on average by a factor of 5 and sometimes by as much as a factor of 10. Substorm onset occurs in the central region of the pre-existing proton precipitation. Assuming that nightside protons are precipitating from a quasi-stable ring current at its outer regions where the field lines are distorted by neutral sheet currents we can associate the onset location with this region of closed but distorted field lines relatively close to the earth. Our results also show that protons are present in the initial poleward substorm expansion however later they are over taken by the electrons. We also find that the intensity of the substorms as quantified by the intensity of the post onset electron precipitation is correlated with the intensity of the proton precipitation prior to the substorms, highlighting the role of the pre-existing near earth plasma in the production of the next substorm.     

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.     

Wave-particle interactions and their relevance to substorms

Wave-particle effects are implicit in most models of radial diffusion and energization of Van Allen belt particles; they were explicitly used in the wave turbulence model for trapped particle precipitation and trapped flux limitations by Kennel and Petschek, Cornwall and by many others. Liemohn used wave-particle interactions to work out a theory of path-integrated whistler amplification process to explain the lack of large per-hop attenuation of multiple-hop LF whistlers.Others have now used wave-particle interactions to construct theories of ELF and VLF chorus. In the present paper we shall review the observations and some of the pertinent theoretical interpretations of wave-particle effects as they relate to substorm and storm-time phenomena. If substorms develop as a result of magnetic merging, then it seems clear that wave-particle interactions in the dissipative or so-called diffusion region of the reconnection zone may be of great importance. The plasma sheet thinning and flow towards the Earth lead inevitably to the development of particle distribution functions that contain free energy in a pitch-angle anisotropy. Such free energy can be released via plasma wave instabilities. The subsequent wave-particle interactions can result in both strong and weak diffusion of particles into loss cones with consequent precipitation fluxes into the auroral zone. Ring current proton spectra also should be unstable against various plasma instabilities with consequent ring current decay and precipitations. Wave-particle interactions must play some important roles in auroral arcs, electrojets and other phenomena related to substorms. These aspects of wave-Paticle interaction will be covered     

Hysteresis Provides Self-Organization in a Plasma Model

The magnetosphere is a multi-scale spatio-temporal complex dynamical system. Self-organization is a possible solution to the seemingly contradicting observation of the repeatable and coherent substorm phenomena with underlying complex behavior in the plasma sheet. Self-organization, through spatio-temporal chaos, emerges naturally in a plasma physics model with sporadic dissipation.     

OpenGGCM Simulations for the THEMIS Mission

The THEMIS mission provides unprecedented multi-point observations of the magnetosphere in conjunction with an equally unprecedented dense network of ground measurements. However, coverage of the magnetosphere is still sparse. In order to tie together the THEMIS observations and to understand the data better, we will use the Open Geospace General Circulation Model (OpenGGCM), a global model of the magnetosphere-ionosphere system. OpenGGCM solves the magnetohydrodynamic (MHD) equations in the outer magnetosphere and couples via field aligned current (FAC), electric potential, and electron precipitation to a ionosphere potential solver and the Coupled Thermosphere Ionosphere Model (CTIM). The OpenGGCM thus provides a global comprehensive view of the magnetosphere-ionosphere system. An OpenGGCM simulation of one of the first substorms observed by THEMIS on 23 March 2007 shows that the OpenGGCM reproduces the observed substorm signatures very well, thus laying the groundwork for future use of the OpenGGCM to aid in understanding THEMIS data and ultimately contributing to a comprehensive model of the substorm process.     

Chromospheric flares or chromospheric aurorae?

This paper discusses some of the well-documented flare phenomena and possible analogies with magnetospheric substorm phenomena. Such analogies do exist, but also important differences. The combination of forces from magnetospheric and solar physicists will bring us closer to the understanding of these nearby examples of unstable cosmic plasmas.     

Self-organization in a current sheet model

Self-organization is a possible solution to the seemingly contradicting observation of the repeatable and coherent substorm phenomena with underlying complex behavior in the plasma sheet. Self-organization, through spatio-temporal chaos, emerges naturally in a plasma physics model with sporadic dissipation. This revised version was published online in August 2006 with corrections to the Cover Date.