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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20 Nov 2015
40 citations
TL;DR: In this article, the authors examined a near-Earth current disruption/dipolarization (CDD) event in which two THEMIS satellites were located at nearly identical equatorial projections but separated by the distance from the neutral sheet.
Abstract: [1] We examine a near-Earth current disruption/dipolarization (CDD) event in which two THEMIS satellites were located at nearly identical equatorial projections but separated by the distance from the neutral sheet. One satellite was very close to the neutral sheet with ∣Bx∣, ∣By∣ –8.1 RE) and was not due to magnetic flux pileup (requiring frozen-in condition) or arrival of a dipolarization front from mid-tail (Xgsm < –15 RE) disturbances.
38 citations
TL;DR: In this paper, the authors reviewed magnetospheric mechanisms for driving quiescent auroral arcs and predicted the driving of auroral-arc currents in large-scale computer simulations.
Abstract: One of the longstanding questions of space science is: How does the Earth’s magnetosphere generate auroral arcs? A related question is: What form of energy is extracted from the magnetosphere to drive auroral arcs? Not knowing the answers to these questions hinders our ability to determine the impact of auroral arcs on the magnetospheric system. Magnetospheric mechanisms for driving quiescent auroral arcs are reviewed. Two types of quiescent arcs are (1) low-latitude non-Alfvenic (growth-phase) arcs magnetically connecting to the electron plasma sheet and (2) high-latitude arcs magnetically connecting near the plasma-sheet boundary layer. The reviews of the magnetospheric generator mechanisms are separated for the two types of quiescent arcs. The driving of auroral-arc currents in large-scale computer simulations is examined. Predicted observables in the magnetosphere and in the ionosphere are compiled for the various generator mechanisms.
37 citations
Cites background from "Observational evidence for an insid..."
...Raeder et al. (2012) demonstrated a connection between ballooning instability in the tail and auroral beads, which are frequently observed in preexisting arcs prior to substorm onset (e.g., Liang et al. 2008; Henderson 2009)....
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TL;DR: In this paper, the authors argue that ballooning modes are viable candidates for the generation of high-latitude Pi2 pulsations on conjugate, stationary ground stations, where ballooning wave perturbations are connected via field-aligned currents to the ionosphere causing Pi2 perturbation in the ground magnetic field.
Abstract: [1] With the evidence set forth in this study, we argue that ballooning modes are viable candidates for the generation of high-latitude Pi2 pulsations. In the scenario proposed here, it is the spatial separation ballooning wave perturbations along an energized boundary in the near-Earth plasma sheet combined with a westward ion diamagnetic drift, which imprint a temporal signature, namely, Pi2 pulsation, on conjugate, stationary ground stations. The ballooning wave perturbations are connected via field-aligned currents to the ionosphere causing Pi2 perturbations in the ground magnetic field. The observational evidence for this mechanism was conjugate spacecraft and ground data, showing correlated diamagnetic plasma perturbations and ground magnetic field oscillations, for two spatially separated, simultaneously occurring Pi2 events on 23 March 2007. The plasma perturbations were westward-drifting waves with azimuthal mode number, m, of approximately 40 consistent with drift ballooning modes. During the Pi2 events, particle energy and energy flux gradually increased until the onset of major particle injections, causing auroral intensifications and additional larger-amplitude Pi2s. The ground Pi2s, also shown to be westward-traveling, were superposed on a slowly decreasing H component which abruptly turned into a substorm bay. A ballooning-driven, high-latitude Pi2 differs from a high-latitude Pi2 that starts synchronized with substorm onset and that has been attributed to the transient response mechanism in that it is the frequency of drift ballooning mode in the near-Earth plasma sheet that determines the Pi2 frequency and not the bounce frequency of Alfven waves along a field line. The geophysical context for this type of Pi2 is a class of substorms that develop from a plasma instability in the near-Earth plasma sheet. If this instability does not lead to a substorm breakup, it could lead to a pseudo-breakup. Hence, it is also likely that some Pi2s associated with pseudo-breakup are caused by the mechanism described here.
36 citations
TL;DR: In this article, the authors show that the auroral dawn storm is the manifestation of magnetic reconnection in the dawn side magnetosphere. But it is unclear if the contemporaneous nature of the observations is a coincidence or part of an underlying physical connection.
Abstract: Jupiter displays many distinct auroral structures, among which auroral dawn storms and auroral injections are often observed contemporaneously. However, it is unclear if the contemporaneous nature of the observations is a coincidence or part of an underlying physical connection. We show six clear examples from a recent Hubble Space Telescope campaign (GO‐14634) that each display both auroral dawn storms and auroral injection signatures. We found that these conjugate phenomena could exist during intervals of either relatively low or high auroral activity, as evidenced by the varied levels of total auroral power. In situ observations of the magnetosphere by Juno show a strong magnetic reconnection event inside of 45 Jupiter radii (RJ) on the predawn sector, followed by two dipolarization events within the following 2 hr, coincident with the auroral dawn storm and auroral injection event. We therefore suggest that the auroral dawn storm is the manifestation of magnetic reconnection in the dawnside magnetosphere. The dipolarization region is mapped to the auroral injection, strongly suggesting that this was associated with the auroral injection. Since magnetic reconnection and dipolarization are physically connected, we therefore suggest that the often‐conjugate auroral dawn storm and auroral injection events are also physically connected consequences.
32 citations
Cites background from "Observational evidence for an insid..."
...In addition, plasma instabilities can also cause magnetic dipolarizations (Henderson, 2009; Lui, 1996; Lui et al., 1992)....
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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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