Auroras and Auroral Particles

Akasofu, S.‐I.

doi:10.1007/978-94-010-1164-8_3

Cited by 4 publications

(2 citation statements)

References 188 publications

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“…This is followed by expansion phase onset, which can be visualized as auroral activity in the ionosphere where the most equatorward arc suddenly brightens. Subsequently, the aurora expands poleward and westward during auroral breakup (Akasofu, 1964(Akasofu, , 1977. At or before expansion phase onset, structures known as auroral beads often develop in the brightening arc and are observed in 90% of all substorms (Kalmoni et al, 2017).…”

Section: Introductionmentioning

confidence: 99%

Plasma Sheet Counterparts for Auroral Beads and Vortices in Advance of Fast Flows: New Evidence for Near‐Earth Substorm Onset

Babu,

Mann,

Donovan

et al. 2024

JGR Space Physics

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The relationship between auroral, ground, and plasma sheet signatures in the late growth phase is crucial for understanding the sequence of events during a substorm expansion phase onset. Here we show conjugate ground‐auroral‐satellite observations of a substorm that occurred on 18 September 2021, between 04:45 and 05:00 UT, where four auroral activations were detected in the all‐sky imagers. An initial activation showed the brightening of an equatorward arc within the cutoff of the 630 nm emissions, indicating activity on closed field lines well inside the open‐closed field line boundary (OCFLB). During a second activation, auroral beads were observed on a brightening arc, equatorward and within the OCFLB, followed by the transformation from small‐scale to large‐scale vortices. The tail current sheet was highly disturbed during the auroral vortex evolution, including pressure and magnetic disturbances, an apparent broadening of a previously thin current sheet, and a breakdown of the frozen‐in condition. Our observations clearly show late growth phase dynamics, including arc brightenings, the formation of auroral beads, and auroral vortex development, can occur well in advance of fast Earthward flows in the tail. Indeed, it is only during that later activity that auroral breakup and strong Earthward flows, which we associate with magnetic reconnection further down the tail, are observed together with strong magnetic bays on the ground. The sequence of events is consistent with an inside‐to‐outside model at substorm expansion phase onset, most likely via a shear‐flow ballooning instability in the transition region from dipole to tail‐like fields in the near‐Earth plasma sheet.

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Section: Introductionmentioning

confidence: 99%

Plasma Sheet Counterparts for Auroral Beads and Vortices in Advance of Fast Flows: New Evidence for Near‐Earth Substorm Onset

Babu,

Mann,

Donovan

et al. 2024

JGR Space Physics

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“…The expansion phase ultimately results in the release of stored magnetic energy in the magnetotail, the injection of energetic particles toward the Earth (e.g., McPherron, 1979; McPherron et al., 1973) and a dipolarization of the tail magnetic field (e.g., McPherron et al., 1973). Correspondingly, the expansion phase in the ionosphere is associated with a brightening and poleward and westward expansion of the aurora (Akasofu, 1965; McPherron, 1979), and the onset is usually marked by the sudden brightening of the most equatorward arc (Akasofu, 1977), and the structuring of the onset arc in the form of auroral beads (e.g., Kalmoni et al., 2017, and others).…”

Section: Introductionmentioning

confidence: 99%

Evolution of Energetic Proton Parallel Pressure Anisotropy at Geosynchronous Altitudes: Potential Role in Triggering Substorm Expansion Phase Onset

Babu,

Mann,

Dimitrakoudis

et al. 2024

Geophysical Research Letters

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The sequence of events associated with the triggering of energy release during substorm expansion phase onset is still not well‐understood. Oberhagemann and Mann (2020b, https://doi.org/10.1029/2019gl085271) proposed a new substorm onset mechanism, where the transition toward parallel proton pressure anisotropy during tail stretching in the late growth phase could trigger a pressure anisotropic ballooning instability. Here we examine the evolution of energetic proton parallel pressure anisotropy at geosynchronous altitudes, seeking evidence in support of the proposed substorm onset mechanism. We use the Geostationary Operational Environment Satellite (GOES) proton flux and magnetometer data combined with substorm onset indicators derived from ground‐based magnetometers. Superposed epoch analysis of substorm onset times for 2014 using the isolated substorm list (Ohtani & Gjerloev, 2020, https://doi.org/10.1029/2020ja027902) clearly shows signatures of energetic proton parallel pressure anisotropy immediately before substorm onset, potentially supportive of the Oberhagemann and Mann theory.

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Plasmas in the Earth’s Magnetotail

Frank

1985

Space Plasma Simulations

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Auroras and Auroral Particles

Cited by 4 publications

References 188 publications

Plasma Sheet Counterparts for Auroral Beads and Vortices in Advance of Fast Flows: New Evidence for Near‐Earth Substorm Onset

Plasma Sheet Counterparts for Auroral Beads and Vortices in Advance of Fast Flows: New Evidence for Near‐Earth Substorm Onset

Evolution of Energetic Proton Parallel Pressure Anisotropy at Geosynchronous Altitudes: Potential Role in Triggering Substorm Expansion Phase Onset

Plasmas in the Earth’s Magnetotail

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