Vertical evolution of auroral acceleration at substorm onset

Morioka, A.; Miyoshi, Yoshizumi; Tsuchiya, F.; Misawa, Hiroaki; Yumoto, K.; Parks, G. K.; Anderson, R. R.; Menietti, J. D.; Honary, F.

doi:10.5194/angeo-27-525-2009

Cited by 18 publications

(19 citation statements)

References 45 publications

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“…This altitude is roughly consistent with previous observations [e.g., Cattell et al , ; Mozer and Hull , ; Chaston et al , ]. The acceleration altitude is also in reasonably agreement with a result of Morioka et al [] based on auroral kilometric radiation, demonstrating that the low‐altitude acceleration region (~4000–5000 km) evolves between initial brightening and breakup of aurora.…”

Section: Observations and Discussionsupporting

confidence: 92%

High‐resolution auroral acceleration signatures within a highly dynamic onset arc

Motoba

Hirahara

2016

Geophysical Research Letters

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On 16 March 2006, the Reimei spacecraft observed dynamic small‐scale vortex‐like structures along an east‐west elongated onset arc, subsequently followed by the auroral expansion. Within a few seconds when Reimei crossed over the arc, detailed comparisons of the simultaneous conjugate auroral imaging and particle observations with high resolution were made to reveal the fine‐scale acceleration processes acting within/around the flux tube of the bright auroral structures. In the surrounding dark region, Alfvénic accelerated electron fluxes—time‐dispersive, field‐aligned electron bursts spanning a broad range of energies—became most noticeable. The dominant contributor to the central bright region, on the other hand, was monoenergetic electron fluxes in 3–7 keV at larger pitch angles (>30°) rather than the Alfvénic electron fluxes. The Reimei observations suggest that the Alfvénic acceleration process operates in the vicinity of small‐scale auroral structures but plays a less significant role in powering the central bright emissions.

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Section: Observations and Discussionsupporting

confidence: 92%

High‐resolution auroral acceleration signatures within a highly dynamic onset arc

Motoba

Hirahara

2016

Geophysical Research Letters

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“…Preceding the substorm expansion in Figure , one can see a gradually enhancing low‐altitude AKR in the altitude range from 3000 to 5000 km beginning at about 05:24 UT (blue dotted line). This enhancement usually corresponds to auroral initial brightening, as illustrated in Figure , which is a common feature in typical substorms [ Morioka et al ., , ]. Note that, before substorm expansion, dH/dt data demonstrated a gradual and negative decrease in the baseline (the red thin line in Figure c) with small‐amplitude oscillations of about 70 s. This declining DC component of ultralow frequency (ULF) pulsation is called prebreakup DC‐ULF in this paper.…”

Section: Substorm Onset Phasesmentioning

confidence: 77%

Substorm onset process: Ignition of auroral acceleration and related substorm phases

et al. 2014

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The substorm onset process was studied on the basis of the vertical evolution of auroral acceleration regions derived from auroral kilometric radiation (AKR) spectra and Pi pulsations on the ground. The field-aligned auroral acceleration at substorm onset demonstrated two distinct phases. Low-altitude acceleration (h~3000-5000 km), which accompanied auroral initial brightening, prebreakup Pi2, and direct current of ultralow frequency (DC-ULF) pulsation were first activated and played an important role (precondition) in the subsequent substorm expansion phase onset. Prebreakup Pi2 is suggestive of the ballooning mode wave generation, and negative decrease in DC-ULF suggests increasing field-aligned current (FAC). We called this stage the substorm initial phase. A few minutes after this initial phase onset, high-altitude acceleration, which accompanied auroral breakup and poleward expansion with breakup Pi1 and Pi2 pulsations, suddenly broke out in an altitude range from 8000 to 16,000 km. Thus, substorm expansion onset originated in the magnetosphere-ionosphere (M-I) coupling region, i.e., substorm ignition in the M-I coupling region. Statistical investigations revealed that about 65% of earthward flow bursts observed in the plasma sheet were accompanied by enhanced low-altitude AKR, suggesting that flow braking of bursts causes FAC and resulting low-altitude field-aligned acceleration in the M-I coupling region. On the basis of these observations, we propose a substorm onset scenario in which FAC that originated from the braking of plasma flow bursts first enhances low-altitude acceleration (substorm initial phase onset) and then the increasing FAC induces current-driven instability in the M-I coupling region, which leads to high-altitude acceleration and resulting substorm expansion phase onset.

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“…Thus, it is highly possible that the Pi2 perturbations are related to the arc formation. The Pi1 perturbations are excited after the Pi2 pulsations grow to larger amplitude, and co-exist with Pi2 perturbations in the expansion phase (Samson, 1982;Lester, 1984;Cheng and Lui, 1998;Lessard et al, 2007;Morioka et al, 2009Morioka et al, , 2010. Thus, Pi1 pulsations are not directly related to the arc formation.…”

Section: Summary and Discussionmentioning

confidence: 99%

Behavior of substorm auroral arcs and Pi2 waves: implication for the kinetic ballooning instability

et al. 2012

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We present synoptic observations of the 21 December 2006 substorm event by the THEMIS ground-based All-Sky-Imagers, the ISUAL CCD Imager aboard the FORMOSAT-2 satellite, the geosynchronous satellites and the ground-based magnetometers, and discuss the implication of the observations. There are three subsequent arc breakups with time separation of <1 min during the substorm expansion phase. In particular, we investigated the mode number of the substorm arc bead-like structure and the concurrent behavior of the arc intensity, the westward electroject intensity, and the ground Pi2 pulsation amplitude. Prior to each arc breakup there was a clear azimuthally-spaced bright spot structure along the arc with high mode number (~140–180) and the arc intensity increased together with the westward electrojet and the ground Pi2 pulsation amplitude under the arc. The Pi1 perturbations observed under the arc appeared at or after the arc breakup started. This suggests that the Pi2 pulsation is related to the arc formation. The Pi2 pulsation may be caused by the kinetic ballooning instability (KBI) that is excited in the strong cross-tail current region. The longitudinal extent of the earthward expansion front of the substorm dipolarization region at the geosynchronous orbit is estimated from timings of the energetic proton and electron injections and is roughly located between ~19.50 MLT and ~23.00 MLT, which is consistent with the corresponding longitudinal extent of the auroral substorm activity

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Vertical evolution of auroral acceleration at substorm onset

Cited by 18 publications

References 45 publications

High‐resolution auroral acceleration signatures within a highly dynamic onset arc

High‐resolution auroral acceleration signatures within a highly dynamic onset arc

Substorm onset process: Ignition of auroral acceleration and related substorm phases

Behavior of substorm auroral arcs and Pi2 waves: implication for the kinetic ballooning instability

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