On the Magnetospheric ULF Wave Counterpart of Substorm Onset

Abstract

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.

Diagnosing the Time‐Dependent Nature of Magnetosphere‐Ionosphere Coupling via ULF Waves at Substorm Onset

“…Together, these observations have been used to suggest that the mechanism of auroral beading and ULF waves at substorm expansion phase onset is in the magnetotail. The results here confirm that the ULF wave power is first observed in the magnetotail and then subsequently in the ionosphere (e.g., Smith et al., 2020). Additionally, we have presented ground ASI images showing the conjugate development of auroral beads in the brightening arc.…”

“…Empirically, Smith et al. (2020) cross‐correlated bandpass‐filtered wavepower between 10 and 100 s, measured in space and at conjugate ground stations at substorm onset, estimating a 1‐ to 1.5‐min delay on average.…”

“…Numerous studies have shown ULF wave activity in this region close to the time of substorm onset (e.g., Chang & Cheng, 2015;Chaston et al, 2012;Ergun et al, 2015;Keiling, 2012;Lui et al, 2008;Motoba et al, 2015;Panov et al, 2012;Saito et al, 2008;Uritsky et al, 2009;Uozumi et al, 2020;Walsh et al, 2010). Most recently, Smith et al (2020) showed excellent statistical correspondence between ground-and space-based observations of increasing Pi1-2 wave power around substorm onset, demonstrating that there is a consistent source region in the magnetotail. Despite these earlier works, there remain significant uncertainties about the sequence of events surrounding magnetospheric substorm onset.…”

“…Most recently, Smith et al. (2020) showed excellent statistical correspondence between ground‐ and space‐based observations of increasing Pi1–2 wave power around substorm onset, demonstrating that there is a consistent source region in the magnetotail.…”

“…It is notoriously difficult to calculate accurate travel times for magneto-hydrodynamic (MHD) waves from the magnetotail to the ionosphere at substorm onset. Empirically, Smith et al (2020) cross-correlated bandpass-filtered wavepower between 10 and 100 s, measured in space and at conjugate ground stations at substorm onset, estimating a 1-to 1.5-min delay on average. Several studies have approached the issue of magnetosphere-ionosphere coupling through ULF waves theoretically.…”