Using ultra-low frequency waves and their characteristics to diagnose key physics of substorm onset

Rae, I. J.; Murphy, K. R.; Watt, Clare E. J.; Mann, I. R.; Yao, Zhonghua; Kalmoni, N. M. E.; Forsyth, C.; Milling, D. K.

doi:10.1186/s40562-017-0089-0

Cited by 9 publications

(15 citation statements)

References 61 publications

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“…Pi1 period waves, with their shorter periods, are often observed at similar times to the Pi2 pulsations (Bosinger & Yahnin, ; Jacobs et al, ). More recent work has often used a combination of the two period bands, often denoted Pi1‐2 and spanning the period band 24–96 s (e.g., Milling et al, ; Murphy et al, ; Rae et al, , ), which have been shown to be coincident with auroral beads (Rae, Mann, Murphy, et al, ; Walsh et al, ). Additionally, the spectra of ULF waves around substorm onset has been shown to be a power law, with no clear break separating the Pi1 and Pi2 wave bands (Murphy et al, ).…”

Section: Methodsmentioning

confidence: 99%

On the Magnetospheric ULF Wave Counterpart of Substorm Onset

et al. 2020

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One near-ubiquitous signature of substorms observed on the ground is the azimuthal structuring of the onset auroral arc in the minutes prior to onset. Termed auroral beads, these optical signatures correspond to concurrent exponential increases in ground ultralow frequency (ULF) wave power and are likely the result of a plasma instability in the magnetosphere. Here, we present a case study showing the development of auroral beads from a Time History of Events and Macroscale Interactions during Substorms (THEMIS) all-sky camera with near simultaneous exponential increases in auroral brightness, ionospheric and conjugate magnetotail ULF wave power, evidencing their intrinsic link. We further present a survey of magnetic field fluctuations in the magnetotail around substorm onset. We find remarkably similar superposed epoch analyses of ULF power around substorm onset from space and conjugate ionospheric observations. Examining periods of exponential wave growth, we find the ground-and space-based observations to be consistent, with average growth rates of ∼0.01 s −1 , lasting for ∼4 min. Cross-correlation suggests that the space-based observations lead those on the ground by approximately 1-1.5 min. Meanwhile, spacecraft located premidnight and ∼10 R E downtail are more likely to observe enhanced wave power. These combined observations lead us to conclude that there is a magnetospheric counterpart of auroral beads and exponentially increasing ground ULF wave power. This is likely the result of the linear phase of a magnetospheric instability, active in the magnetotail for several minutes prior to auroral breakup.

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Section: Methodsmentioning

confidence: 99%

On the Magnetospheric ULF Wave Counterpart of Substorm Onset

et al. 2020

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“…The preonset aurora scenario suggests that the fast earthward flow evolves an onset instability, such as ballooning instability, leading to initial auroral brightening (Nishimura et al, 2014). Ballooning mode waves may be excited a few minutes before initial brightening (as determined with a spacecraft-borne auroral imager) (Saito et al, 2008). The time difference between the arrival of the preonset aurora at the onset arc and initial brightening that Nishimura et al (2010) showed seems consistent with the growth time of an instability.…”

Section: Summary and Open Questionsmentioning

confidence: 77%

Revisiting substorm events with preonset aurora

Miyashita

Ieda

2018

Ann. Geophys.

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Abstract. Nishimura et al. (2010) proposed a new plasma intrusion or preonset aurora scenario of substorm triggering. In this scenario, a substorm is triggered by a fast earthward flow generated at the distant neutral line which corresponds to a preonset auroral streamer or arc in the ionosphere propagating from the auroral poleward boundary to the initial auroral brightening site, i.e., “preonset aurora”. In the present paper, we revisited three substorm events reported as being triggered by such a mechanism related to preonset auroras, based on THEMIS ground-based all-sky imager data. Unlike previous studies, we examined the arrival timing of the preonset aurora relative to the three steps of auroral onset arc development (initial brightening, enhancement of the wave-like structure, and poleward expansion) to make the role of the preonset aurora in the auroral steps clearer. Our detailed timing analysis found that preonset auroral streamers reached the auroral onset arc but away from the initial brightening site after initial brightening for two events, while no preonset aurora reaching the initial brightening site could be identified for the other event. This result suggests that the processes associated with auroral streamers are unlikely to affect at least initial brightening, even if we consider not only the presence and arrival timing and location of the auroral streamers but also the scale of the corresponding flow and flow vortices. We list a series of open questions for testing the preonset aurora scenario further in future studies. Keywords. Magnetospheric physics (storms and substorms; auroral phenomena; magnetotail)

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“…Rae et al (2017) determine the relative timing between sudden increases in amplitude, or onsets, of different ultra-low-frequency (ULF) wave bands during substorms. They show that differing onset times and spatial expansion exist for Pi1, Pi1-2, and Pi2 waves in the ionosphere during substorms.…”

Section: Ground Magnetic Perturbationmentioning

confidence: 99%