Space‐Ground Observations of Dynamics of Substorm Onset Beads. Issue 2 (5th February 2022)

• Record Type: Journal Article
• Authors: Nishimura, Y.; Artemyev, A. V.; Lyons, L. R.; Gabrielse, C.; Donovan, E. F.; Angelopoulos, V.
• Is Part Of: Journal of geophysical research. Volume 127:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-05
• Subjects: THEMIS all‐sky imager (ASI) -- Substorm onset -- Plasma sheet and magnetotail -- Plasma flow -- Pressure and electric field -- Kinetic effects; Magnetospheric physics -- Periodicals; Space environment -- Periodicals; Cosmic physics -- Periodicals; Planets -- Atmospheres -- Periodicals; Heliosphere (Astrophysics) -- Periodicals; Geophysics -- Periodicals; 523.01
• DOI: 10.1029/2021JA030004
• Languages: English
• ISSNs: 2169-9380

Abstract

Abstract: We present observations during two substorms using simultaneous Time History of Events and Macroscale Interactions During Substorms satellites and all‐sky imagers to determine plasma sheet dynamics associated with substorm auroral onset beads. The multi‐satellite observations showed that the cross‐tail current decreased and the field‐aligned currents increased at the substorm auroral onset, indicating that the satellites detected an initiation of the currents being deflected to the ionosphere. For duskward‐propagating beads, the electric field was tailward, and ions were accumulated closer to the Earth than electrons. The mapped bead propagation speed was close to energetic ion drift speed. The E × B $E\times B$ and electron drift speeds increased duskward and reduced the cross‐tail current at the onset. For dawnward‐propagating beads, the electric field was equatorward/earthward, and electrons were inferred to accumulate earthward of ions. The mapped bead propagation speed was comparable to the dawnward E × B $E\times B$ and electron drift speeds. The duskward ion drift and tail current were reduced, and electrons became the dominant current carrier. We suggest that the plasma species that is responsible for the bead propagation changes with the electric field configuration and that the tail current reduction by the enhanced E × B $E\times B$ drift at onset destabilizes the plasma sheet. Ion and electron outflows substantially increased low‐energy plasma density and may have increased the role of E × B $E\times B$ drifts. The bead wavelength was comparable to ion gyroradius and thus ion kinetic effects are important for determining the wavelength. In the dawnward‐propagating event, the mode of oscillation in the plasma sheet was suggested to be the sausage‐mode flapping oscillations. Plain Language Summary: Aurora in the night sky often begins with a sudden intensification called substorms, and the auroral intensification shows a wave‐like display (beads). It is critical to understand the origin of beads for revealing the mechanism of the sudden auroral intensifications, but it has been extremely difficult to find satellite observations in space to explore the magnetospheric source region of substorms. We identified two simultaneous observations of auroral beads at the beginning substorm auroral intensification by all‐sky imagers and related plasma dynamics by the Time History of Events and Macroscale Interactions During Substorms satellites in the plasma sheet. We found that the bead structure and propagation are related to charge accumulation and fast plasma streams in the plasma sheet. The plasma flow speed coincided with the bead speed, and the charge distribution and plasma stream direction reversed when the auroral beads moved to an opposite direction, giving strong evidence of the connection between the observations at the two locations. The plasma streams reduced the electric currents in the plasma sheet, and we suggest that the current reduction destabilizes the plasma sheet and initiates the substorm auroral intensification. Key Points: Duskward beads are related to energetic ion drift. The duskward E × B and electron drifts reduced the cross‐tail current at onset Dawnward beads are related to E × B and electron drifts. The duskward ion drift and tail current were reduced at onset Species involved in bead propagation changes with the electric field. Tail current reduction by enhanced E × B destabilizes the plasma sheet