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- Title
On the Magnetospheric ULF Wave Counterpart of Substorm Onset.
- Authors
Smith, A. W.; Rae, I. J.; Forsyth, C.; Watt, C. E. J.; Murphy, K. R.
- 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 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. Key Points: Conjugate ionospheric and magnetospheric measurements show exponential growth of ULF waves at rates of ∼0.01 s−1Magnetospheric ULF wave growth leads the ionosphere by 1–1.5 min, identifying a magnetospheric sourceLarge magnetotail ULF power is most commonly observed premidnight (at XGSM∼10RE), confirming the conjugate location of auroral beads
- Subjects
MAGNETIC storms; AZIMUTHAL projection (Cartography); AURORAL substorms; MAGNETIC fields; SPACE vehicles
- Publication
Journal of Geophysical Research. Space Physics, 2020, Vol 125, Issue 4, p1
- ISSN
2169-9380
- Publication type
Article
- DOI
10.1029/2019JA027573