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, it is proposed that these injections are caused by the explosive nonlinear growth of a shear flow-ballooning instability in the region where sub-auroral polarization streams (SAPS) also occur.
Abstract: Quasi-periodic energetic particle injections have been observed at geosynchronous orbit on the dusk-side during a steady magnetospheric convection event. We examine high-resolution auroral imager data and ground magnetometer data associated with the first of these injections and conclude that it was not associated with classical sub-storm signatures. It is proposed that these injections are caused by the explosive nonlinear growth of a shear flow-ballooning instability in the region where sub-auroral polarization streams (SAPS) also occur. It is suggested that interchange will occur preferentially in the low-conductivity SAPS region since the magnetic Richardson number is lowest there and the "line-tying" effect will also be least stabilizing there. We propose that the observed particle injection signatures and auroral morphology constitute a new type of SAPS-associated explosive "onset-like" disturbance that can occur during intervals of strong convection.
11 citations
Cites background from "Observational evidence for an insid..."
...Note that the onset of this “expansion phase” is delayed relative to the onset of the substorm (e.g., Akasofu, 1964, 1968; Henderson, 2009; Newell et al., 2001; Partamies et al., 2015) The expansion phase can last from a few to several tens of minutes until the intense activity subsides and the “recovery phase” begins....
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...…periodic auroral hot spots or “beads” (Donovan et al., 2006; Elphinstone et al., 1995; Henderson, 1994, 2009; Kalmoni et al., 2015; Motoba et al., 2012; Rae et al., 2010) that deform poleward and develop into the poleward expanding bulge (e.g., Henderson, 2009, shows a good example of this)....
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...Note that the onset of this “expansion phase” is delayed relative to the onset of the substorm (e.g., Akasofu, 1964, 1968; Henderson, 2009; Newell et al., 2001; Partamies et al., 2015) The expansion phase can last from a few to several tens of minutes until the intense activity subsides and the…...
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...…onset is often associated with the development of azimuthally periodic auroral hot spots or “beads” (Donovan et al., 2006; Elphinstone et al., 1995; Henderson, 1994, 2009; Kalmoni et al., 2015; Motoba et al., 2012; Rae et al., 2010) that deform poleward and develop into the poleward expanding…...
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10 citations
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...…on magnetic field topological change like MR (e.g., Akasofu, 2015; Antonova et al., 2009; Cheng, 2004; Friedrich et al., 2001; Haerendel, 2015; Henderson, 2009; Kozelov & Kozelova, 2013; Liu et al., 1991, 2012; Lui, 1991; Oberhagemann & Mann, 2020; Pu et al., 1997; Roux et al., 1991; Saito et…...
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TL;DR: In this paper, a multiple auroral onset substorm on 28 March 2010 provides an opportunity to understand the physical mechanism in generating auroral intensifications during a substorm expansion phase.
Abstract: A multiple auroral onset substorm on 28 March 2010 provides an opportunity to understand the physical mechanism in generating auroral intensifications during a substorm expansion phase. Conjugate observations of magnetic fields and plasma from the Time History of Events and Macroscale Interactions during Substorms (THEMIS) spacecraft, of field-aligned currents (FACs) from the Active Magnetosphere and Planetary Electrodynamics Response Experiment (AMPERE) satellites, and from ground-based magnetometers and aurora are all available. The comprehensive measurements allow us to further our understanding of the complicated causalities amongst dipolarization, FAC generation, particle acceleration and auroral intensification. During the substorm expansion phase, the plasma sheet expanded and was perturbed leading to the generation of a slow mode wave, which modulated electron flux in the outer plasma sheet. During this current sheet expansion, field-aligned currents formed, and geomagnetic perturbations were simultaneously detected by ground-based instruments. However, a magnetic dipolarization did not occur until about 3 minutes later in the outer plasma sheet observed by THEMIS-A spacecraft (THA). We believe this dipolarization led to an efficient Fermi acceleration to electrons, and consequently the cause of a significant auroral intensification during the expansion phase as observed by the All-Sky Imagers (ASIs). This Fermi acceleration mechanism operating efficiently in the outer plasma sheet during the expansion phase could be a common explanation of the poleward auroral development after substorm onset. These results also show a good agreement between the upward FAC derived from AMPERE measurements and the auroral brightening observed by the ASIs.
9 citations
TL;DR: In this article, the authors used a rare and close longitudinal conjunction of the THEMIS spacecraft with the Sondrestrom radar to present multipoint observations of important features of the time sequence of substorm pre-onset plasma flows in the ionosphere and magnetotail on March 5, 2008.
Abstract: [1] In this paper, we use a rare and close longitudinal conjunction of the THEMIS spacecraft with the Sondrestrom radar to present multipoint observations of important features of the time sequence of substorm pre-onset plasma flows in the ionosphere and magnetotail on March 5, 2008. We found that the onset was preceded sequentially by enhanced polar cap flows heading equatorward near the polar cap boundary, and then by tail fast flows from the mid-tail to the near-Earth region. We also observed in situ fluctuations in short-period Pi2 band (∼30–50 s) in both the magnetic field and plasma pressure during the initial couple of min of the fast flows, occurring nearly simultaneously with the dipolarization and the fast flows and propagating earthward. Our results suggest that these fluctuations may be triggered by the fast flows, and may play an important role in the substorm onset process. Our event suggests that localized tail reconnection may be triggered by the enhanced polar cap flows, though the reconnection location cannot be unambiguously determined. Earthward fast flows are generated as a result of the tail reconnection and reach the inner magnetosphere to initiate the substorm onset. The presented case is consistent with that predicted by the Nishimura et al. (2010a) scenario.
8 citations
01 Jun 2011
TL;DR: In this article, a review of the physical processes for magnetospheric substorm expansion onsets is presented, including the external conditions in the solar wind for substorm onset, observations prior to the onset of the substorm, observations immediately after the onset, time history approach, and system-wide approach.
Abstract: A major challenge in magnetospheric research is to identify the physical processes for magnetospheric substorm expansion onsets. Recent abundance of observations from Geotail, Cluster, and Themis missions has added impetus to substorm research. Observations that are linked to substorm expansion onsets are discussed. Topics encompassed in this review are (1) the external conditions in the solar wind for substorm onset, (2) observations prior to onset, (3) observations immediately after onset, (4) time history approach, and (5) system-wide approach that can reveal general characteristics of the physical processes for onset. The implications for the substorm onset processes are discussed based on these observations.
8 citations
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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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