Azimuthal auroral expansion associated with fast flows in the near-Earth plasma sheet: Coordinated observations of the THEMIS all-sky imagers and multiple spacecraft
Abstract:[1] Fast azimuthal auroral expansion and poleward expansion are characteristic features of the expansion phase of substorms. In the first study of its kind, we have investigated the azimuthal auroral expansion and its magnetospheric counterpart using data from Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky imagers and multiple spacecraft. During the tail season in 2008-2009, we found 16 events of azimuthally expanding aurora that passed near the magnetic footprints of the … Show more
“…The speed match along the azimuthal direction among the dipolarization, density structure, and plasma motion suggests that the dipolarization propagated along with plasma motion. This speed will be further discussed and compared to previous studies (Angelopoulos et al., 2008; Nagai, 1982; Ogasawara et al., 2011; Ohtani et al., 2018) in Section 4. RBSP‐A and ‐B were in the PSBL before around 05:05 UT, when both spacecraft saw increases in the electron temperature from several keV to several 10s of keV and electron density from several 0.1s to above 1 cm −3 (not shown).…”
Section: Observationsmentioning
confidence: 77%
“…This speed corresponds to dipolarizations propagate at 150 km/s at geosynchronous orbit along the azimuthal direction. Although such “fast” dipolarizations are reported in many events (Angelopoulos et al., 2008; Ogasawara et al., 2011), statistical studies show that the typical value of the azimuthal phase speed of the dipolarizations is only 30–50 km/s (Nagai, 1982; Ohtani et al., 2018) at geosynchronous orbit. In Section 4, we discuss the nature of the “fast” dipolarizations, the apparent speed difference between the “fast” and “slow” dipolarizations, and their relation to the auroral motion and Alfvenic Poynting flux.…”
Auroral substorms involve the sudden brightening and spatial expansion of auroral activity in the nightside ionosphere (Akasofu, 1964). Low-altitude in-situ observations have shown that the auroral activity is directly connected to the magnetic flux tubes of the upward current region (Chapter 4 in Paschmann et al., 2003, and references therein). Within these flux tubes, electrons are accelerated toward the ionosphere in the auroral acceleration region (about 2-4 Re geocentric distances). This occurs primarily through two major mechanisms: quasi-static potential drops (McFadden et al., 1999) and kinetic Alfven waves (Chaston et al., 2004). Although strong wave-particle interaction occurs in the auroral acceleration region, it is clear that the free-energy source for the energy conversion lies at higher altitude.In-situ measurements in the magnetotail have shown that the auroral substorm is one aspect of the more general concept of geomagnetic substorm, which involves sudden release of the magnetic energy stored in the tail. There are a variety of interrelated phenomena resulting from the energy release that can be observed including magnetic field dipolarization (Runov et al., 2009), enhanced earthward Poynting flux (Ergun et al., 2015), and plasma flow channels (Angelopoulos et al., 1992). As the magnetic flux tubes connect the ionosphere to the plasma sheet and its boundary layer in the magnetotail, the coupling between the ionosphere and magnetotail during substorms has been a focus of research for decades.Conjunction studies between high-altitude in-situ observations and ionospheric or low-altitude measurements have been a powerful tool to study the coupling between the auroral ionosphere and magnetotail. The Polar spacecraft routinely sampled the plasma sheet boundary layer (PSBL) at 4-6 Re, providing in-situ measurements in magnetic conjunction with the auroral zone. Polar conjunctions have highlighted enhanced earthward Poynting fluxes as an important energy source for the auroral acceleration (Keiling et al., 2003). The Poynting fluxes are carried in the form of Alfven waves and kinetic Alfven waves (Wygant et al., 2000(Wygant et al., , 2002. However, one-to-one correlation has not been reported due to limited spatial and temporal resolution of auroral measurements. In this study, with improved auroral measurements in conjunction
“…The speed match along the azimuthal direction among the dipolarization, density structure, and plasma motion suggests that the dipolarization propagated along with plasma motion. This speed will be further discussed and compared to previous studies (Angelopoulos et al., 2008; Nagai, 1982; Ogasawara et al., 2011; Ohtani et al., 2018) in Section 4. RBSP‐A and ‐B were in the PSBL before around 05:05 UT, when both spacecraft saw increases in the electron temperature from several keV to several 10s of keV and electron density from several 0.1s to above 1 cm −3 (not shown).…”
Section: Observationsmentioning
confidence: 77%
“…This speed corresponds to dipolarizations propagate at 150 km/s at geosynchronous orbit along the azimuthal direction. Although such “fast” dipolarizations are reported in many events (Angelopoulos et al., 2008; Ogasawara et al., 2011), statistical studies show that the typical value of the azimuthal phase speed of the dipolarizations is only 30–50 km/s (Nagai, 1982; Ohtani et al., 2018) at geosynchronous orbit. In Section 4, we discuss the nature of the “fast” dipolarizations, the apparent speed difference between the “fast” and “slow” dipolarizations, and their relation to the auroral motion and Alfvenic Poynting flux.…”
Auroral substorms involve the sudden brightening and spatial expansion of auroral activity in the nightside ionosphere (Akasofu, 1964). Low-altitude in-situ observations have shown that the auroral activity is directly connected to the magnetic flux tubes of the upward current region (Chapter 4 in Paschmann et al., 2003, and references therein). Within these flux tubes, electrons are accelerated toward the ionosphere in the auroral acceleration region (about 2-4 Re geocentric distances). This occurs primarily through two major mechanisms: quasi-static potential drops (McFadden et al., 1999) and kinetic Alfven waves (Chaston et al., 2004). Although strong wave-particle interaction occurs in the auroral acceleration region, it is clear that the free-energy source for the energy conversion lies at higher altitude.In-situ measurements in the magnetotail have shown that the auroral substorm is one aspect of the more general concept of geomagnetic substorm, which involves sudden release of the magnetic energy stored in the tail. There are a variety of interrelated phenomena resulting from the energy release that can be observed including magnetic field dipolarization (Runov et al., 2009), enhanced earthward Poynting flux (Ergun et al., 2015), and plasma flow channels (Angelopoulos et al., 1992). As the magnetic flux tubes connect the ionosphere to the plasma sheet and its boundary layer in the magnetotail, the coupling between the ionosphere and magnetotail during substorms has been a focus of research for decades.Conjunction studies between high-altitude in-situ observations and ionospheric or low-altitude measurements have been a powerful tool to study the coupling between the auroral ionosphere and magnetotail. The Polar spacecraft routinely sampled the plasma sheet boundary layer (PSBL) at 4-6 Re, providing in-situ measurements in magnetic conjunction with the auroral zone. Polar conjunctions have highlighted enhanced earthward Poynting fluxes as an important energy source for the auroral acceleration (Keiling et al., 2003). The Poynting fluxes are carried in the form of Alfven waves and kinetic Alfven waves (Wygant et al., 2000(Wygant et al., , 2002. However, one-to-one correlation has not been reported due to limited spatial and temporal resolution of auroral measurements. In this study, with improved auroral measurements in conjunction
“…The sustained dipolarization's azimuthal propagation speed fully agrees with the statistical results of Liou et al (), who found that dipolarization propagation velocities are ~60 km/s eastward and ~75 km/s westward at geosynchronous orbit. Ogasawara et al () found consistency between auroral azimuthal expansion and dipolarization propagation. Their velocities were larger (267 km/s); however, they focused on bulge expansion in the vicinity of substorm onset where changes occur rapidly.…”
The injection region's formation, scale size, and propagation direction have been debated throughout the years, with new questions arising with increased plasma sheet observations by missions like Cluster and THEMIS. How do temporally and spatially small‐scale injections relate to the larger injections historically observed at geosynchronous orbit? How to account for opposing propagation directions—earthward, tailward, and azimuthal—observed by different studies? To address these questions, we used a combination of multisatellite and ground‐based observations to knit together a cohesive story explaining injection formation, propagation, and differing spatial scales and timescales. We used a case study to put statistics into context. First, fast earthward flows with embedded small‐scale dipolarizing flux bundles transport both magnetic flux and energetic particles earthward, resulting in minutes‐long injection signatures. Next, a large‐scale injection propagates azimuthally and poleward/tailward, observed in situ as enhanced flux and on the ground in the riometer signal. The large‐scale dipolarization propagates in a similar direction and speed as the large‐scale electron injection. We suggest small‐scale injections result from earthward‐propagating, small‐scale dipolarizing flux bundles, which rapidly contribute to the large‐scale dipolarization. We suggest the large‐scale dipolarization is the source of the large‐scale electron injection region, such that as dipolarization expands, so does the injection. The >90‐keV ion flux increased and decreased with the plasma flow, which died at the satellites as global dipolarization engulfed them. We suggest the ion injection region at these energies in the plasma sheet is better organized by the plasma flow.
“…In particular, we find that within ~60 s prior to the substorm onset the auroral wave-like arc structure moves westward with average moving velocity about the same as the phase velocity of the westward propagating Pi2 δ B y disturbance observed by the THEMIS D and THEMIS E (Th D and Th E) satellites if the Pi2 disturbance activity is mapped by using the T96 model (Tsyganenko 1995 ) to the ionosphere. This substorm event was previously examined by Pu et al ( 2010 ) in relation with magnetic reconnection in the magnetotail and by Ogasawara et al ( 2011 ) on the relation between the auroral features and magnetispheric plasma flow and B z dipolarization which occurred just after the auroral expansion onset. Here, we focus on the dynamics of the wave-like substorm arc prior to the substorm onset and its relationship with the corresponding Pi2 disturbance activity in the near-Earth plasma sheet.…”
Section: Methodsmentioning
confidence: 88%
“…But, the Th E spacecraft observes the magnetic B z dipolarization at ~0405:00 UT probably because it is located further away from the substorm initiation region in the plasma sheet. In both Th D and Th E spacecraft observations, the magnetic B z dipolarization is accompanied by a significant increase in plasma flow, wave fluctuations, and high energy ion flux (Pu et al 2010 ; Ogasawara et al 2011 ). Fig.…”
Wave-like substorm arc features in the aurora and Pi2 magnetic disturbances observed in the near-Earth plasma sheet are frequently, and sometimes simultaneously, observed around the substorm onset time. We perform statistical analyses of the THEMIS ASI auroral observations that show wave-like bright spot structure along the arc prior to substorm onset. The azimuthal mode number values of the wave-like substorm arcs are found to be in the range of ~100–240 and decrease with increasing geomagnetic latitude of the substorm auroral arc location. We suggest that the azimuthal mode number is likely related to the ion gyroradius and azimuthal wave number. We also perform correlation study of the pre-onset wave-like substorm arc features and Pi2 magnetic disturbances for substorm dipolarization events observed by THEMIS satellites during 2008–2009. The wave-like arc brightness structures on the substorm auroral arcs tend to move azimuthally westward, but with a few exceptions of eastward movement, during tens of seconds prior to the substorm onset. The movement of the wave-like arc brightness structure is linearly correlated with the phase velocity of the Pi2 δBy disturbances in the near-Earth plasma sheet region. The result suggests that the Pi2 transverse δBy disturbances are related to the intensifying wave-like substorm onset arcs. One plausible explanation of the observations is the kinetic ballooning instability, which has high azimuthal mode number due to the ion gyroradius effect and finite parallel electric field that accelerates electrons into the ionosphere to produce the wave-like arc structure.
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