Observational evidence for an inside-out substorm onset scenario
TL;DR: In this paper, the authors present observations which provide strong support for a substorm expansion phase onset scenario in which a localized inner magnetospheric instability developed first and was later followed by the development of a Near Earth Neutral Line (NENL) farther down-tail.
Abstract: . We present observations which provide strong support for a substorm expansion phase onset scenario in which a localized inner magnetospheric instability developed first and was later followed by the development of a Near Earth Neutral Line (NENL) farther down-tail. Specifically, we find that the onset began as a localized brightening of an intensified growth phase arc which developed as a periodic series of arc-aligned (i.e. azimuthally arrayed) bright spots. As the disturbance grew, it evolved into vortical structures that propagated poleward and eventually morphed into an east-west aligned arc system at the poleward edge of the auroral substorm bulge. The evolution of the auroral intensity is consistent with an exponential growth with an e-folding time of around 188 s (corresponding to a linear growth rate, γ of 5.33×10−3 s−1). During the initial breakup, no obvious distortions of auroral forms to the north were observed. However, during the expansion phase, intensifications of the poleward boundary of the expanding bulge were observed together with the equatorward ejection of auroral streamers into the bulge. A strong particle injection was observed at geosynchronous orbit, but was delayed by several minutes relative to onset. Ground magnetometer data also shows a two phase development of mid-latitude positive H-bays, with a quasi-linear increase in H between the onset and the injection. We conclude that this event provides strong evidence in favor of the so-called "inside-out" substorm onset scenario in which the near Earth region activates first followed at a later time by the formation of a near-to-mid tail substorm X-line. The ballooning instability is discussed as a likely mechanism for the initial onset.
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TL;DR: In this paper, the authors used simultaneous ground-based, all-sky camera observations from a geomagnetically conjugate Iceland-Syowa Station pair to demonstrate that the auroral beads, whose wavelength is ∼30-50 km, evolve synchronously in the northern and southern hemispheres and have remarkable interhemispheric similarities.
Abstract: [1] Auroral beads, i.e., azimuthally arrayed bright spots resembling a pearl necklace, have recently drawn attention as a possible precursor of auroral substorms. We used simultaneous, ground-based, all-sky camera observations from a geomagnetically conjugate Iceland-Syowa Station pair to demonstrate that the auroral beads, whose wavelength is ∼30–50 km, evolve synchronously in the northern and southern hemispheres and have remarkable interhemispheric similarities. In both hemispheres: 1) they appeared almost at the same time; 2) their longitudinal wave number was similar ∼300–400, corresponding bead separation being ∼1° in longitude; 3) they started developing into a larger scale spiral form at the same time; 4) their propagation speeds and their temporal evolution were almost identical. These interhemispheric similarities provide strong evidence that there is a common driver in the magnetotail equatorial region that controls the major temporal evolution of the auroral beads; thus, the magnetosphere plays a primary role in structuring the initial brightening arc in this scale size.
63 citations
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TL;DR: Evidence is provided that near-Earth current disruption, occurring before the conventional tail reconnection signatures, triggered the onset of a magnetospheric substorm, and the observed auroral intensification and tail reconnections are not causally linked.
Abstract: Angelopoulos et al. (Research Articles, 15 August 2008, p. 931) reported that magnetic reconnection in Earth’s magnetotail triggered the onset of a magnetospheric substorm. We provide evidence that (i) near-Earth current disruption, occurring before the conventional tail reconnection signatures, triggered the onset; (ii) the observed auroral intensification and tail reconnection are not causally linked; and (iii) the onset they identified is a continuation of earlier substorm activities.
63 citations
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Johns Hopkins University Applied Physics Laboratory1, Space Science Institute2, University of California, Los Angeles3, Saint Petersburg State University4, Swedish Institute of Space Physics5, University of Alaska Fairbanks6, Austrian Academy of Sciences7, National Institutes of Natural Sciences, Japan8, Princeton Plasma Physics Laboratory9, University of New Hampshire10, Peking University11
TL;DR: Two mechanisms for the generation of the pre-onset current sheet are discussed, namely magnetic flux addition to the tail lobes, or other high-latitude perturbations, and magnetic flux evacuation from the near-Earth tail associated with dayside reconnection.
Abstract: Modes and manifestations of the explosive activity in the Earth’s magnetotail, as well as its onset mechanisms and key pre-onset conditions are reviewed. Two mechanisms for the generation of the pre-onset current sheet are discussed, namely magnetic flux addition to the tail lobes, or other high-latitude perturbations, and magnetic flux evacuation from the near-Earth tail associated with dayside reconnection. Reconnection onset may require stretching and thinning of the sheet down to electron scales. It may also start in thicker sheets in regions with a tailward gradient of the equatorial magnetic field $B_{z}$
; in this case it begins as an ideal-MHD instability followed by the generation of bursty bulk flows and dipolarization fronts. Indeed, remote sensing and global MHD modeling show the formation of tail regions with increased $B_{z}$
, prone to magnetic reconnection, ballooning/interchange and flapping instabilities. While interchange instability may also develop in such thicker sheets, it may grow more slowly compared to tearing and cause secondary reconnection locally in the dawn-dusk direction. Post-onset transients include bursty flows and dipolarization fronts, micro-instabilities of lower-hybrid-drift and whistler waves, as well as damped global flux tube oscillations in the near-Earth region. They convert the stretched tail magnetic field energy into bulk plasma acceleration and collisionless heating, excitation of a broad spectrum of plasma waves, and collisional dissipation in the ionosphere. Collisionless heating involves ion reflection from fronts, Fermi, betatron as well as other, non-adiabatic, mechanisms. Ionospheric manifestations of some of these magnetotail phenomena are discussed. Explosive plasma phenomena observed in the laboratory, the solar corona and solar wind are also discussed.
58 citations
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TL;DR: In this article, the authors used simultaneous ground-based, all-sky camera observations from a geomagnetically conjugate Iceland-Syowa Station pair to demonstrate that the auroral beads, whose wavelength is ∼30-50 km, evolve synchronously in the northern and southern hemispheres and have remarkable interhemispheric similarities.
Abstract: [1] Auroral beads, i.e., azimuthally arrayed bright spots resembling a pearl necklace, have recently drawn attention as a possible precursor of auroral substorms. We used simultaneous, ground-based, all-sky camera observations from a geomagnetically conjugate Iceland-Syowa Station pair to demonstrate that the auroral beads, whose wavelength is ∼30–50 km, evolve synchronously in the northern and southern hemispheres and have remarkable interhemispheric similarities. In both hemispheres: 1) they appeared almost at the same time; 2) their longitudinal wave number was similar ∼300–400, corresponding bead separation being ∼1° in longitude; 3) they started developing into a larger scale spiral form at the same time; 4) their propagation speeds and their temporal evolution were almost identical. These interhemispheric similarities provide strong evidence that there is a common driver in the magnetotail equatorial region that controls the major temporal evolution of the auroral beads; thus, the magnetosphere plays a primary role in structuring the initial brightening arc in this scale size.
56 citations
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01 Jan 2011
TL;DR: In this article, a statistical study of pressure changes in the near-Earth Magnetotail associated with Substorm Expansion Onsets is presented. And the authors classify the Time Sequence of Events Leading to Substorm Onset based on THEMIS All-Sky Imager Observations.
Abstract: Part I Solar Wind-Magnetosphere Interaction 1. The Magnetopause, its Boundary Layers and Pathways to the Magnetotail 2. Quantifying Energy Transfer at the Magnetopause 3. Long-period ULF Waves Driven by Periodic Solar Wind Disturbances Part II Magnetotail Dynamics 4. Magnetotail Dynamics: Survey of Recent Progress 5. Physical Processes for Magnetospheric Substorm Expansion Onsets 6. Cluster Observations of Plasma Bubbles, BBFs and Their Wakes 7. A Statistical Study of Pressure Changes in the near-Earth Magnetotail Associated With Substorm Expansion Onsets 8. Categorization of the Time Sequence of Events Leading to Substorm Onset Based on THEMIS All-Sky Imager Observations Part III Inner Magnetospheric Processes 9. Dynamic Inner Magnetosphere: A Tutorial and Recent Advances 10. Self-Consistent Simulations of Plasma Waves and their Effects on Energetic Particles 11. Topology of High-Latitude Magnetospheric Currents 12. Detection of Dynamical Complexity Changes in Dst Time Series Using Entropy Concepts and Rescaled Range Analysis Part IV Waves and Turbulence in Magnetosphere-Ionosphere Coupling 13. Magnetospheric ULF Waves: A Review 14. ULF Waves in the Topside Ionosphere: Satellite Observations and Modeling 15. Evidence for a Multi-Scale Aurora Part V Ionospheric Driver of the Magnetosphere 16. Influences of the Ionosphere, Thermosphere and Magnetosphere on Ion Outflows 17. Heavy Ion Energization, Transport, and Loss in the Earth's Magnetosphere 18. Data Assimilation Models: A 'New' Tool for Ionospheric Science and Applications Part VI Extraterrestrial Magnetospheres 19. Large-Scale Structure in the Magnetospheres of Jupiter and Saturn 20. Lunar Plasma Environment Index
53 citations
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References
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TL;DR: In this paper, a working model of simultaneous auroral activity over the entire polar region is presented in terms of the auroral substorm, which has two characteristic phases, an expansive phase and a recovery phase.
Abstract: A working model of simultaneous auroral activity over the entire polar region is presented in terms of the auroral substorm. The substorm has two characteristic phases, an expansive phase and a recovery phase. Each phase is divided into three stages, and characteristic auroral displays over the entire polar region during each stage are described in detail. Further, all the major features seen at a single station are combined into a consistent picture of large-scale auroral activity.
1,363 citations
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TL;DR: In this paper, the authors present a phenomenological model of the magnetospheric substorm sequence, which can be divided into three main phases: the growth phase, the expansion phase, and the recovery phase.
Abstract: In the eight preceding papers, two magnetospheric substorms on August 15, 1968, were studied with data derived from many sources. In this, the concluding paper, we attempt a synthesis of these observations, presenting a phenomenological model of the magnetospheric substorm. On the basis of our results for August 15, together with previous reports, we believe that the substorm sequence can be divided into three main phases: the growth phase, the expansion phase, and the recovery phase. Observations for each of the first three substorms on this day are organized according to this scheme. We present these observations as three distinct chronologies, which we then summarize as a phenomenological model. This model is consistent with most of our observations on August 15, as well as with most previous reports. In our interpretation we expand our phenomenological model, briefly described in several preceding papers. This model follows closely the theoretical ideas presented more quantitatively in recent papers by Coroniti and Kennel (1972a, b; 1973). A southward turning of the interplanetary magnetic field is accompanied by erosion of the dayside magnetosphere, flux transport to the geomagnetic tail, and thinning and inward motion of the plasma sheet. Our observations indicate, furthermore, that the expansionmore » phase of substorms can originate near the inner edge of thc plasm sheet as a consequence of rapid plasma sheet thinnig. At this time a portion of the inner edge of the tail current is short circuited' through the ionosphere. This process is consistent with the formation of a neutral point in the near-tail region and its subsequent propagation tailward. However, the onset of the expansion phase of substorms is found to be far from a simple process. Expansion phases can be centered at local times far from midnight, can apparently be localized to one meridian, and can have multiple onsets centered at different local times. Such behavior indicates that, in comparing observations occurring in different substorms, careful note should be made of the localization and central meridian of cach substorm. (auth)« less
1,067 citations
Journal Article•
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TL;DR: In this article, observations made during three substorms on August 15, 1968, are shown to be consistent with current theoretical ideas about the cause of substorm, and the phenomenological model described in several preceding papers is further expanded.
Abstract: Observations made during three substorms on August 15, 1968, are shown to be consistent with current theoretical ideas about the cause of substorms. The phenomenological model described in several preceding papers is further expanded. This model follows closely the theoretical ideas presented more quantitatively in recent papers by Coronti and Kennel (1972 and 1973).
951 citations
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TL;DR: In this article, a phenomenological or qualitative model of the substorm sequence is presented, where the flux transport is driven by the merging of the magnetospheric and interplanetary magnetic fields.
Abstract: The tail plays a very active and important role in substorms. Magmetic flux eroded from the dayside magnetosphere is stored here. As more and more flux is transported to the magnetotail and stored, the boundary 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. The experimental evidence for these processes is discussed and a phenomenological or qualitative model of the substorm sequence is presented. 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.
543 citations
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TL;DR: In this paper, the authors studied possible braking mechanisms of high-speed ion flows in the near Earth central plasma sheet for radial distances between 9 and 19 Earth Radii (RE) on the basis of observations made by the AMPTE/IRM satellite.
Abstract: We have studied possible braking mechanisms of high-speed ion flows in the near-Earth central plasma sheet for radial distances between 9 and 19 Earth Radii (RE) on the basis of observations made by the AMPTE/IRM satellite. Flows with velocities in excess of 400 km/s are almost always Earthward for this range, indicating that the source of the flows is beyond 19 RE. Though the occurrence rate of the high-speed flows substantially decreases when the satellite comes closer to the Earth, high-speed flows with velocities higher than 600 km/s are still observed. We suggest that the high-speed flows are stopped at a clear boundary between the regions of dipolar field and tail-like field in the plasma sheet. The boundary corresponds to the inner edge of the neutral sheet. The average jump of the magnetic field at the boundary, which is estimated from the observations by assuming a pressure balance, is 6.7 nT. The inertia current caused by the braking of the flow and the current caused by pileup of the magnetic flux at the stopping point are quantitatively estimated and discussed in relation to the formation of the substorm current wedge.
449 citations
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