Abstract
AbstractOne 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 RE 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.
Highlights
During a magnetospheric substorm, magnetic energy is stored and explosively released in the magnetotail (Akasofu, 1964; McPherron, 1970)
We explore the presence, location, and properties of increased ultralow frequency (ULF) wave power around substorm onset
We could define T0 as the time identified by SOPHIE as representing expansion phase onset, which would yield a repeatable and quantitatively defined epoch based on the SML index
Summary
Magnetic energy is stored and explosively released in the magnetotail (Akasofu, 1964; McPherron, 1970). The newly opened magnetic flux is dragged across the polar caps by the ambient solar wind flow and stored in the magnetotail lobes; this process forms the growth phase of a substorm (McPherron, 1970). Later, during the substorm expansion phase, the open magnetospheric flux is rapidly closed through reconnection in the nightside magnetotail (Hones, 1976). The start of the expansion phase (often called substorm onset) is classically defined from ground-based optical observations as the time at which the most equatorward auroral arc brightens and is observed to move poleward (Akasofu, 1964, 1977). Substorm onset is associated with a large growth in ultralow frequency (ULF) wave power (Rae, Mann, Murphy, et al, 2009; Rae et al, 2012; Rae & Watt, 2016; Voronkov et al, 2003)
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