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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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.
96 citations
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.
82 citations
Cites background from "Observational evidence for an insid..."
...…observations from the ground [Donovan et al., 2006; Liang et al., 2008; Sakaguchi et al., 2009] and from spacecraft [Elphinstone et al., 1995; Henderson, 2009] have shown that in the initial brightening arc, there exists a characteristic small-scale auroral structure, consisting of…...
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TL;DR: In a global magnetohydrodynamic (MHD) simulation of the growth phase of a synthetic substorm, it is found that the self‐consistent formation and destabilization of localized magnetic field minima in the near‐Earth magnetotail are found.
Abstract: Explosive magnetotail activity has long been understood in the context of its auroral manifestations. While global models have been used to interpret and understand many magnetospheric processes, the temporal and spatial scales of some auroral forms have been inaccessible to global modeling creating a gulf between observational and theoretical studies of these phenomena. We present here an important step toward bridging this gulf using a newly developed global magnetosphere-ionosphere model with resolution capturing ≲ 30 km azimuthal scales in the auroral zone. In a global magnetohydrodynamic (MHD) simulation of the growth phase of a synthetic substorm, we find the self-consistent formation and destabilization of localized magnetic field minima in the near-Earth magnetotail. We demonstrate that this destabilization is due to ballooning-interchange instability which drives earthward entropy bubbles with embedded magnetic fronts. Finally, we show that these bubbles create localized field-aligned current structures that manifest in the ionosphere with properties matching observed auroral beads.
66 citations
Cites background from "Observational evidence for an insid..."
...These dynamics result in the generation of localized perturbations in ionospheric field‐aligned currents and corresponding auroral signatures commonly referred to as beads (e.g., Henderson, 2009; Motoba et al., 2012; Nishimura et al., 2016)....
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...In observations, auroral beads have also been reported both as onset arc structures (Henderson, 2009; Kalmoni et al., 2015, 2017; Nishimura et al., 2016) and not (Panov et al....
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...In observations, auroral beads have also been reported both as onset arc structures (Henderson, 2009; Kalmoni et al., 2015, 2017; Nishimura et al., 2016) and not (Panov et al., 2019; Uritsky et al., 2009; Xing et al., 2020)....
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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.
65 citations
TL;DR: In this article, the authors used the Rice Convection Model-Equilibrium (RCM-E) and kinetic instability properties of substorm onsets to characterize the occurrence probabilities and properties of substorm auroral onset waves.
Abstract: Auroral substorms are often associated with optical ray or bead structures during initial brightening (substorm auroral onset waves). Occurrence probabilities and properties of substorm onset waves have been characterized using 112 substorm events identified in THEMIS all-sky imager data, and compared to Rice Convection Model-Equilibrium (RCM-E) and kinetic instability properties. All substorm onsets were found to be associated with optical waves, and thus optical waves are a common feature of substorm onset. Eastward-propagating wave events are more frequent than westward-propagating wave events, and tend to occur during lower-latitude substorms (stronger solar wind driving). The wave propagation directions are organized by orientation of initial brightening arcs. We also identified notable differences in wave propagation speed, wavelength (wavenumber), period and duration between westward and eastward propagating waves. In contrast, the wave growth rate does not depend on the propagation direction or substorm strength but is inversely proportional to the wave duration. This suggests that the waves evolve to poleward expansion at a certain intensity threshold, and that the wave properties do not directly relate to substorm strengths. However, waves are still important for mediating the transition between the substorm growth phase and poleward expansion. The relation to arc orientation can be explained by magnetotail structures in the RCM-E, indicating that substorm onset location relative to the pressure peak determines the wave propagation direction. The measured wave properties agree well with kinetic ballooning interchange instability, while cross-field current instability and electromagnetic ion cyclotron instability give much larger propagation speed and smaller wave period.
64 citations
Cites background from "Observational evidence for an insid..."
...Such ray structures are also called beads and can be characterized as an optical wave-like structure along an initial brightening arc [Donovan et al., 2006; Henderson, 2009; Rae et al., 2010] (hereinafter called substorm auroral onset waves or onset waves for convenience)....
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References
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TL;DR: In this paper, a nonlinear MHD model of the line tied Rayleigh-Taylor instability is proposed to deal with large-scale explosive events, where the nonlinearity is destabilizing and broadening causing the linear instability to develop fingers and broaden into the linearly stable region.
62 citations
"Observational evidence for an insid..." refers background in this paper
...As described by Cowley and Artun(1997), the growth could be have been associated with an even faster “explosive” instability that leads to a “detonation”....
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TL;DR: In this paper, the authors used magnetometer data from the CANOPUS Churchill line to study the dynamics of substorm electrojets during the growth and early expansive phases of the substorm.
Abstract: Meridian-scanning photometer (MSP) and magnetometer data from the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS) ground-based array have been used to study the dynamics of the substorm expansive phase. The relative latitudinal motions of the MSP 630.0, 557.7, and 486.1 nm emissions for eight isolated events have been studied. The data show the expansive phase to comprise three stages: an explosive (tens of seconds) onset; rapid poleward motion of 557.7 nm emissions of the order of a few minutes; and a period of slower (tens of minutes) poleward moving 630.0 nm emissions. We interpret the rapid poleward motion of the 557.7 nm data in terms of a region of instability, expanding rapidly down the magnetotail, possibly accelerating plasma sheet electrons as it proceeds. All events show that lobe flux reconnection occurs after near-Earth onset, with typical time delays between 1 and 5 min. The extremely short time interval between near-Earth onset and the beginning of lobe flux reconnection presents a severe observational constraint on any substorm model which attempts to explain the substorm expansive phase. Magnetometer data from the CANOPUS Churchill line have been used to study the dynamics of the substorm electrojets during the growth and early expansive phases of the substorm. The borders of these electrojets closely follow the motion of the MSP data during the growth and expansive phases. At the instant of expansive phase onset the growth phase electrojets (as observed along the Churchill meridian line) disappear, being replaced by the substorm westward electrojet associated with the current wedge. The equatorward border of the substorm westward electrojet is embedded within the proton aurora emissions and appears at expansive phase onset ∼2° equatorward of the growth phase electrojets, further showing the near-Earth proximity of expansive phase onset. No indication of reconnection on closed field lines prior to expansive phase onset was observed in either the magnetometer or the photometer data.
50 citations
TL;DR: In this paper, the authors examined the consequences of a dispersion relation which includes the effects of wave particle interactions and the depolarizing effects of a finitely conducting ionosphere in a dipole magnetic field.
49 citations
"Observational evidence for an insid..." refers background in this paper
...Numerous models or mechanisms have been introduced over the past 40 years in attempts to explain the observed phenomenology (e.g.Swift, 1967; Hones et al., 1973; Russell and McPherron, 1973; McPherron et al., 1973; Hones, 1977; Lui, 1978; Lui et al., 1988; Roux, 1985; Smith et al., 1986; Rostoker…...
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TL;DR: In this paper, the authors use Canadian Auroral Network for the OPEN Program Unified Study All-Sky Imager (ASI) and Meridional Scanning Photometer (MSP) data as the basis for a study of the dynamics of large-scale (hundreds of kilometers) auroral vortices.
Abstract: We use Canadian Auroral Network for the OPEN Program Unified Study All-Sky Imager (ASI) and Meridional Scanning Photometer (MSP) data as the basis for a study of the dynamics of large-scale (hundreds of kilometers) auroral vortices. We consider 28 events corresponding to a range of auroral activity levels. Three of these are presented in detail, one corresponding to growth phase, one to pseudo-breakup and one to expansive phase onset. We show that vortex formation starts from a discrete arc with half thickness δ of the order of 20 km. This arc intensifies near the poleward boundary of enhanced proton aurora, as seen in the Hydrogen β (Hβ) MSP data and becomes azimuthally structured. This structuring is in the form of vortices with wavelength of the order of ∼ 2πδ. The vortices intensify and extend radially, leading to broadening of the initial arc. While the sizes and growth rates of the vortices vary, the overall scenario of vortex evolution is similar for all of the events. Structures that develop during the growth phase saturate at latitudes matching the poleward boundary of Hβ emissions and pseudo-breakup structures saturate further poleward. Expansive phase onset vortices expand poleward in a similar fashion, but we do not observe any saturation stage, presumably due to limitations imposed by the ASI field of view. We present results of shear flow ballooning vortex modeling in which we used initial conditions and parameters consistent with our observations. On the basis of our model results, we speculate that all of these experimentally observed vortices are the result of shear flow ballooning instability in the hot proton region in the near-Earth plasma sheet.
45 citations
"Observational evidence for an insid..." refers background in this paper
...Such spatially periodic auroral spots have been seen prior to the onset of substorm expansion phase in other studies as well (Henderson, 1994; Elphinstone et al., 1995; Samson et al., 1996; Voronkov et al., 2000, 2003)....
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TL;DR: The Gillam All-Sky Imager (GSAI) was used to register large-scale vortices during pseudo-breakup and breakup registered by the Gillam all-sky imager as mentioned in this paper.
Abstract: [1] Following the database of large-scale vortices during pseudo-breakup and breakup registered by the Gillam All-Sky Imager, we selected one event (19 February 1996) for a detailed consideration. This event is a sequence of pseudo-breakup and local substorm, and breakup followed by the large substorm, which is isolated from the previous pseudo-breakup by the second growth phase. Commencement of these elements of auroral activity was clearly seen above the Churchill line of the Canadian Auroral Network for the OPEN Program Unified Study (CANOPUS; pseudo-breakup was completely covered by the field of view of the Gillam All-Sky Imager). Geotail was located at ∼19 RE in the equatorial plane of midnight sector, which, along with supporting observations from two geostationary satellites (GOES 8 and 9), allowed for a comparison of ground-based, geostationary orbit and midtail signatures. The pseudo-breakup consisted of two distinct stages: a near-exponential arc intensity growth and a poleward vortex expansion that started simultaneously with dipolarization in the inner magnetosphere. The latter corresponded to explosive onset of short-period (tens of millihertz) pulsations observed at geostationary orbit and on the ground in the vicinity of the arc. No significant disturbances poleward of the vortex were observed. Pseudo-breakup was followed by the second growth phase, which involved a significant thinning of the plasma sheet. Breakup was of a similar two-stage character as the pseudo-breakup. Full onset of the expansive phase that followed breakup was seen simultaneously by all instruments including Geotail, which detected strong perturbations in the midtail. The expansive phase onset launched the second postbreakup package of Pi2 pulsations that were of larger amplitude. Finally, during the substorm recovery phase, the poleward boundary intensifications (PBIs) were observed as long-period, on the order of 10 min, pulses of electron precipitation. PBI commencement coincided with bursty flows and pulses of plasma energization in the midtail. Observed features support recent ideas claiming that we are dealing with processes (breakup, full onset of the expansive phase, and PBIs) of a distinct physical nature that require different commencement thresholds, namely, the inner plasma sheet instability (pseudo-breakup and breakup), midtail reconnection (expansive phase onset), and further magnetotail dynamics during the recovery phase (PBIs).
44 citations