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, Nishimura et al. revisited three substorm events reported as being triggered by a mechanism related to preonset auroras, based on THEMIS ground-based all-sky imager data.
Abstract: . Nishimura et al. (2010) proposed a new plasma intrusion or preonset aurora
scenario of substorm triggering. In this scenario, a substorm is triggered by
a fast earthward flow generated at the distant neutral line which corresponds
to a preonset auroral streamer or arc in the ionosphere propagating from the
auroral poleward boundary to the initial auroral brightening site, i.e.,
“preonset aurora”. In the present paper, we revisited three substorm events
reported as being triggered by such a mechanism related to preonset auroras,
based on THEMIS ground-based all-sky imager data. Unlike previous studies, we
examined the arrival timing of the preonset aurora relative to the three
steps of auroral onset arc development (initial brightening, enhancement of
the wave-like structure, and poleward expansion) to make the role of the
preonset aurora in the auroral steps clearer. Our detailed timing analysis
found that preonset auroral streamers reached the auroral onset arc but away
from the initial brightening site after initial brightening for two events,
while no preonset aurora reaching the initial brightening site could be
identified for the other event. This result suggests that the processes
associated with auroral streamers are unlikely to affect at least initial
brightening, even if we consider not only the presence and arrival timing and
location of the auroral streamers but also the scale of the corresponding
flow and flow vortices. We list a series of open questions for testing the
preonset aurora scenario further in future studies. Keywords. Magnetospheric physics (storms and substorms; auroral phenomena; magnetotail)
12 citations
Cites background from "Observational evidence for an insid..."
...Many studies (e.g., Henderson, 2009; Rae et al., 2009; Mende et al., 2011; Murphy et al., 2014b; Kepko, 2014), however, reported substorm events without any auroral streamers and arcs moving to the auroral onset arc before initial auroral brightening and even poleward expansion....
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...It then extends westward and eastward, and a wave-like or bead-like structure grows gradually (e.g., Davis, 1962; Elphinstone et al., 1995; Friedrich et al., 2001; Donovan et al., 2007; Liang et al., 2008; Sakaguchi et al., 2009a, b; Henderson, 2009; Rae et al., 2009, 2010; Kepko et al., 2009; Motoba et al., 2012, 2015; Chang et al., 2012; Murphy et al., 2014b; Chang and Cheng, 2015; Kalmoni et al., 2015, 2017; Motoba and Hirahara, 2016; Nishimura et al., 2016)....
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...…(e.g., Davis, 1962; Elphinstone et al., 1995; Friedrich et al., 2001; Donovan et al., 2007; Liang et al., 2008; Sakaguchi et al., 2009a, b; Henderson, 2009; Rae et al., 2009, 2010; Kepko et al., 2009; Motoba et al., 2012, 2015; Chang et al., 2012; Murphy et al., 2014b; Chang and Cheng,…...
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Journal Article•
TL;DR: The state of the magnetosphere on August 15, 1968, as defined by magnetic indices and ground magnetograms, is described in this article, where the OGO 5 satellite was inbound on the midnight meridian through the cusp region of the geomagnetic tail.
Abstract: The state of the magnetosphere on August 15, 1968, as defined by magnetic indices and ground magnetograms, is described. Onset times of various phases of two magnetospheric substorms are determined. These substorms occurred while the OGO 5 satellite was inbound on the midnight meridian through the cusp region of the geomagnetic tail. It is concluded that at least two worldwide substorm expansions were preceded by growth phases.
12 citations
TL;DR: In this article, a detailed study on the substorm expansion behavior on 3 November 1998, using the global auroral images from Polar and the conjugate central plasma sheet data from Geotail in the near-Earth tail is presented.
Abstract: [1] Previous studies of auroral behaviors have suggested that the substorm expansion is a result of a chain of processes originating in the magnetotail. We present a detailed study on the substorm expansion behavior on 3 November 1998, using the global auroral images from Polar and the conjugate central plasma sheet data from Geotail in the near-Earth tail. The closer inspection of the auroral data enables us to characterize the stepwise feature of the auroral evolution, which were found to be spatially and temporally separable into two intense auroral activities: the initial brightening of the substorm arc and the second aurora that took place eastward of the first one with the time delay of 4.5 min. Comparing the near-Earth tail observation of the fast plasma flow, we show that the observed time lag of 4.5 min can be explained by a systematic chain of plasma processes in the central plasma sheet, indicating that this time delay can be interpreted as a generic behavior. The generality of the stepwise evolution within the expected time lag of 2–6 min is also pursued, and is found for three out of six substorms. With respect to the stepwise feature, our observation of six substorms suggests that there are at least three types of substorm expansion dynamics that are attributable to different chains of tail processes.
11 citations
TL;DR: In this paper, the authors suggest that the auroral beads are related to plasma instabilities in the magnetosphere, which are often known to generate wavy auroral precipitations.
Abstract: During the Grand Finale Phase of Cassini, the Ultraviolet Imaging Spectrograph on board the spacecraft detected repeated detached small-scale auroral structures. We describe these structures as auroral beads, a term introduced in the terrestrial aurora. Those on DOY 232 2017 are observed to extend over a large range of local times, i.e., from 20 LT to 11 LT through midnight. We suggest that the auroral beads are related to plasma instabilities in the magnetosphere, which are often known to generate wavy auroral precipitations. Energetic neutral atom enhancements are observed simultaneously with auroral observations, which are indicative of a heated high pressure plasma region. During the same interval we observe conjugate periodic enhancements of energetic electrons, which are consistent with the hypothesis that a drifting interchange structure passed the spacecraft. Our study indicates that auroral bead structures are common phenomena at Earth and giant planets, which probably demonstrates the existence of similar fundamental magnetospheric processes at these planets.
11 citations
Cites methods from "Observational evidence for an insid..."
...This term has been used at Earth to describe auroral features with similar106 morphological characteristics (Liang et al. 2008; Henderson 2009; Rae et al. 2009)....
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Dissertation•
01 Jan 2010
TL;DR: Yang et al. as discussed by the authors showed that the entropy parameter PV, where P is the pressure and V is the volume of a flux tube with unit magnetic flux, plays a central role in the earthward plasma convection from the near and middle-Earth plasma sheet to the inner magnetosphere.
Abstract: Inner Magnetospheric Modeling During Geomagnetic Active Times by Jian Yang In this thesis we show that the entropy parameter PV, where P is the pressure and V is the volume of a flux tube with unit magnetic flux, plays a central role in the earthward plasma convection from the nearand middle-Earth plasma sheet to the inner magnetosphere. This work presents a series of numerical simulations, investigating the relationship between the value of PV and the different features of plasma earthward transport that occur during different types of events in geomagnetic active times. The simulations are conducted using the Rice-Convection-Model (RCM) and the Rice-Convection-Model-Equilibrium (RCM-E) that have carefully designed boundary conditions to simulate the effect of various values of PV. In Chapter 3 we present results of an RCM simulation of a sawtooth event where it is found that a dramatic reduction of PV on the boundary along a wide range of local times produces interchange convection in the inner magnetosphere and drives spatially quasi-periodic Birkeland currents that suggest an explanation for the finger-like aurora usually observed during this type of event. In Chapter 4 we present results of an RCM-E simulation of an isolated substorm, which is done by imposing depleted PV (a bubble) in the expansion phase. The results of this simulation reproduce typical features of a substorm and agree fairly well with multipoint observations. Chapter 6 presents a detailed analysis of the RCM-E expansion phase simulation which indicates that the reconfigurations of PV, plasma pressure and magnetic field in an idealized bubble injection event can be quite complicated. Chapter 7 presents results of a superposed epoch study using Geotail data showing that the time variations of PV are different in isolated substorms, pseudo-breakups and convection bay events, suggesting that bubbles have different characteristics in different modes of earthward transport. We follow this up with three corresponding RCM-E simulations by representing a sustained bubble, a transient bubble and sustained low PV plasma along the boundary. The simulations are roughly consistent with theoretical suggestions, superposed epoch results and some other observations. These simulations provide a systematic description of inner magnetospheric configuration during various active events, suggesting the temporal and spatial characteristics of PV in the plasma sheet as a key in the magnetospheric convection. Acknowledgements First of all, I would like to express my heartfelt gratitude to my advisor Dr. Frank Toffoletto for his invaluable advice and guidance. He shows unlimited patience in our discussions. He is not only tolerant to my unconstraint thoughts, but also expresses interests and encouragement, and unselfishly contributes his creative ideas, which always help me out of troubles and push me towards brighter directions in the research. I would like to thank Dr. Richard Wolf for many of his profound thoughts and warm encouragement during these years. I am very grateful to Dr. Stan Sazykin and Dr. Bob Spiro for their help and many useful discussions. I would like to thank Dr. Colin Zelt for serving as my committee member. This work cannot be done without the great data provided by generous Pontus Brandt, Michael Henderson, Harald Frey and Michelle Thomsen. The sawtooth event simulation cannot be done without Nikolai Tsyganenko's substorm current wedge model source code. My work is also benefited by intuitive discussions with them, and Gary Erickson, Xiaoyan Xing, Xin Tao, Liang Wu, Colby Lemon, Chaosong Huang, Geoff Reeves, Mei-Ching Fok, Shin Ohtani, Larry Lyons, Mikhail Sitnov, Dae-Young Lee, Yukinaga Miyashita, Richard Kaufmann. I am deepest grateful to my wife Huang, Hui and my mother Liu, Yuying, for their greatest support, encouragement and love.
11 citations
Cites background or methods from "Observational evidence for an insid..."
...(5) Although from the simulation point of view, the size of the pearls of FACs may be dependent on the equilibrium code grid resolution and the numerical noise effect on the formation of these FACs is still under further investigation, they are observationally associated with the quasi-periodic spaced spot-like aurora observed by IMAGE-WIC and Polar-UVI [Henderson, 2009; Keiling et al, 2008]....
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...(6) The string-of-pearl-like aurora are suggested to be associated with near-Earth ballooning instability near the transition region [Keiling et al., 2008; Henderson, 2009]....
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...12) may be associated with spot-like aurora measured by aurora remote sensing instrument [Henderson, 2009; Keiling et al, 2008]....
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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.
1,460 citations
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
1,138 citations
Journal Article•
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
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.
552 citations
"Observational evidence for an insid..." refers background in this paper
...…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 and Eastman,…...
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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.
480 citations
"Observational evidence for an insid..." refers background in this paper
...The deceleration or “braking” of these flow bursts in the near-Earth region produces the familiar current wedge, Pi2 pulsations and breakup activity on or near the most equatorward arc (Haerendel, 1992; Shiokawa et al., 1997, 1998)....
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