Longitudinally propagating arc wave in the pre‐onset optical aurora

Uritsky, V. M.; Liang, J.; Donovan, E.; Spanswick, E.; Knudsen, D. J.; Liu, W.; Bonnell, J. W.; Glaßmeier, K. H.

doi:10.1029/2009gl040777

Cited by 56 publications

(68 citation statements)

References 26 publications

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“…Azimuthally spaced auroral forms (AAF) have recently been considered as ionospheric phenomena of kinetic ballooning/interchange instability [ Elphinstone et al , 1995; Uritsky et al , 2009], though they are auroral signatures in the “pre‐onset” of substorms. Typical longitudinal mode number of such preexisting auroral arcs ranged from 100 to 300 [ Liang et al , 2008], and it is in agreement with the spatial scale of our observed pulsating patches.…”

Section: Discussionmentioning

confidence: 99%

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

Nishiyama¹,

Sakanoi²,

Miyoshi³

et al. 2012

J. Geophys. Res.

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We have carried out ground‐based observations, optimized to temporal and spatial characteristics of pulsating auroras (PAs) in the micro/meso scale, using an electron multiplying charge coupled device (EMCCD) camera with a wide field of view corresponding to 100 × 100 km at an altitude of 110 km and a high sampling rate up to 100 frames per second. We focus on transient PAs propagating southward around 1100 UT, in the early recovery phase of the substorm, on 4th March 2011. Three independent patches (PA1–3) each with different periods between 4 and 7 s were observed, which means that the periodicity was not explained by the electron bounce motion and strongly depended on local plasma conditions in the magnetosphere or in the ionosphere. One more insight is that only PA1 had also a sharp peak of modulations around 1.5 Hz, with a narrow frequency width of 0.30 Hz, and the strong modulations existed as a small spot in the center of PA1. We have also conducted cross spectrum analysis and have obtained coherence and phase distributions for auroral variations between 0.1 and 3.0 Hz. The results indicated that low frequency variations from 0.2 to 0.5 Hz inside PA1–3 propagated as a collective motion in well‐defined directions. The estimated horizontal propagation velocities ranged from 50 to 120 km/s at the auroral altitude. The velocities are almost consistent with the Alfven speed at the magnetic equator, which suggests that compressional waves have an effect on PA via modulations of the ambient plasma environment.

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Section: Discussionmentioning

confidence: 99%

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

Nishiyama¹,

Sakanoi²,

Miyoshi³

et al. 2012

J. Geophys. Res.

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“…We define the auroral "activeness" of each ASI bin in a 1 min interval as the integral of the square of the high-passed luminosity fluctuations of this bin. This auroral "activeness" by this definition may not purely represent the PsA activity during the early substorm expansion phase when the substorm auroral bulge dominates the auroral dynamics, since the substorm auroral brightening is found to also contain highfrequency (10-30 s) wave fluctuations [Uritsky et al, 2009]. However, during the late expansion phase and the recovery phase when the substorm auroral bulge fades and the diffuse auroras begin to dominate (roughly after 1100 UT in our event), the auroral "activeness" by our definition should presumably be mainly contributed by the PsA fluctuations and thus give a proper measure of the PsA activity.…”

Section: Psa Observations and Their Relationship To The Ulf Waves Andmentioning

confidence: 99%

THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

et al. 2010

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[1] We present multiprobe, multi-instrument observations of the electron cyclotron harmonic (ECH) emissions and ultralow-frequency (ULF) waves from Time History of Events and Macroscale Interactions during Substorms (THEMIS) and explore their potential linkage to the concurrent ground-observed pulsating auroras (PsA) on 4 January 2009. The ECH emissions were observed as discrete packets modulated by the ULF flapping motion of the neutral sheet around the probes. Combining different data sets of the ECH observations, we infer that the ECH emission intensities were strongly fluctuating and contained multi-time scale fine structures. The distribution of PsA patches featured longitudinal "wavelength" in concert with the in situ ULF wave characteristics inferred from a cross-phase analysis. The overall activeness of the PsA correlated with the in situ-measured energetic electron fluxes and ECH wave intensities. We suggest that ECH waves played the key role in the pitch angle diffusion of the plasma sheet electrons that led to the PsA, while the ULF waves structured the plasma sheet and imposed a macroscopic effect over the spatial distribution of the PsA.

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“…Uritsky et al [2009] suggested that the azimuthally propagating arc wave prior to substorm onset was an auroral footprint of the drift wave mode in the stretched magnetotail. The same process might be expected to work during the expansion phase, leading to auroral expansion.…”

Section: Introductionmentioning

confidence: 99%

Azimuthal auroral expansion associated with fast flows in the near-Earth plasma sheet: Coordinated observations of the THEMIS all-sky imagers and multiple spacecraft

et al. 2011

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[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 multiple spacecraft operating in the near-Earth plasma sheet. In the magnetosphere, these events commonly showed fast azimuthal and earthward flows associated with intense electric fields and magnetic dipolarization. The speed of the propagating structure, which was estimated from the time difference of the depolarization observed by the multiple spacecraft, was close to the measured azimuthal plasma flow velocity. We also found that this azimuthal plasma transport was dominated by the E × B drift speed associated with the enhanced electric field. In a statistical analysis, the averaged speeds of the leading edge of the westward and eastward auroral expansion were 8.8 and 5.3 km/s, respectively. When mapped onto the equatorial magnetosphere, these speeds (267 and 162 km/s) were comparable to the averaged azimuthal plasma (E × B) flow speeds observed by the spacecraft, which were 193 (239) km/s in the westward direction and 112 (139) km/s in the eastward direction. Our events showed that E × B flows and auroral expansion predominantly propagated westward, indicating an effect of westward background convection in the Harang flow shear. From these results, we concluded that the azimuthal auroral expansion was closely related to magnetic dipolarization which expanded azimuthally at the E × B drift speed. On the basis of the abrupt formation of the fast E × B flows and their propagation away from the onset location, we suggest that the effects of the intense large-scale electric fields, which are possibly generated through substorm onset turbulence, propagate to the ionosphere along the magnetic field lines and lead to azimuthal expansion of an auroral arc.

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Longitudinally propagating arc wave in the pre‐onset optical aurora

Cited by 56 publications

References 26 publications

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

Fine scale structures of pulsating auroras in the early recovery phase of substorm using ground‐based EMCCD camera

THEMIS observations of electron cyclotron harmonic emissions, ULF waves, and pulsating auroras

Azimuthal auroral expansion associated with fast flows in the near-Earth plasma sheet: Coordinated observations of the THEMIS all-sky imagers and multiple spacecraft

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