2022 THEMIS SCIENCE NUGGETS

Space-Ground Observations of Dynamics of Substorm Onset Beads

by Toshi Nishimura
Boston University

Introduction

Substorm auroral onset is characterized by an initial brightening along an auroral arc in the equatorward portion of the nightside auroral oval, and the initial brightening involves ray structures (beads) during the first few minutes after the onset. Beads show characteristic duskward or dawnward propagation. Beads are thought to be the ionospheric signature of substorm onset instability, but it is not understood why the beads propagate duskward or dawnward. In addition, in-situ observations associated with substorm onset beads have not been reported to date, because it is extremely rare to have satellite observations in the plasma sheet in the possible onset region during the short lifetime of beads that are measured by auroral imagers. Observationally, it is not understood what plasma sheet structures are associated with the beads, and what determines the direction and speed of beads. In the present study, we determine what structures in the near-Earth plasma sheet are associated with substorm onset beads. Particularly, we will identify what are the magnetotail counterpart of the duskward and dawnward propagation of beads using two conjunction events between the THEMIS satellites and all-sky imagers (ASIs).

Figure 1. The left column shows (a) a north-south keogram near the central meridian of the Yellow Knife (YKNF) THEMIS ASI, (b) ground magnetometer data at Fort Smith (FSMI), and (c) THEMIS ASI snapshot at 06:52 UT during the substorm on 29 March 2008. The THEMIS satellite footprints are shown as the colored dots. The right column shows (d) the east-west keogram along the onset arc from the FSMI ASI, (e) magnetic field, (f) electric field, (g) duskward velocities (red: ion, blue: electron, black: E×B), (h) duskward current densities (red: ion, blue: electron), and (i) net duskward current density (gray: unfiltered, black: 60-sec smoothed).

Results

Figure 1a shows an auroral keogram at Yellow Knife (YKNF) in the pre-midnight sector for a substorm on 29 March 2008. The substorm auroral onset occurred at ~06:50 UT. The H-component of the ground magnetic field at Fort Smith (FSMI) near the arc decreased by ~200 nT associated with the arc brightening (Figure 1b). As shown in the ASI snapshot right after the onset at 06:52 UT (Figure 1c), the auroral onset was fully covered by the THEMIS ASI FOV near the zenith of the FSMI and Fort Simpson (FSIM) ASIs, and the onset arc showed a wave-like luminosity modulation (beads; hereinafter referred to as onset waves). The T96 footprints of THEMIS-B, C, D and E were located within the MLT range of the onset waves. An east-west keogram along the onset arc in the FSMI ASI shows duskward propagation of the beads (Figures 1d). THEMIS-D was closest to the neutral sheet and detected magnetic field dipolarization with wave-like oscillations (Figure 1e). After the substorm onset, the electron velocity increased to ~200-300 km/s and remained close to the E×B speed, while the ion velocity only increased to ~120 km/s (Figure 1g). The partial velocity moment for >10 keV ions was duskward throughout the event and increased to ~300 km/s after the onset. Because the E×B speed was small until a few minutes after the onset, the duskward drift of the energetic ions was most consistent with the duskward bead propagation. The cross-tail current decrease to nearly zero at substorm onset (Figure 1h-i) due to the duskward drifting electrons by E×B. The reduced current could contribute to disturb the force balance in the near-Earth magnetotail and could enhance the substorm instability.

Figure 2. Same as Figure 1 but for the 20 November 2014 event, except that Panels h and i are combined.

Figure 2 shows the second substorm event, which occurred on 20 November 2014. The substorm auroral onset occurred at ~09:14 UT (Figure 2a) and was associated with a drop in the H-component of the ground magnetic field by ~270 nT (Figure 2b). The substorm auroral onset was associated with clear auroral beads along the onset arc (Figure 2c), and the footprints of THEMIS-A and E were within the MLT range of the onset waves. The beads propagated dawnward (Figure 2d). THEMIS-A was in the central plasma sheet and detected quasi-periodic oscillations of a ~30-60 sec period, which was close to the period of the auroral onset waves. A major difference from the previous event is the direction of the dawn-dusk plasma velocities. Figure 2g shows that the ion velocity before the onset was weakly duskward, and then it became nearly stagnant soon after the onset followed by a dawnward drift. The electron velocity was initially weakly dawnward, and then it increased dawnward after the onset. The dawnward electron velocity was due to the E×B drift associated with the equatorward electric field. The dawnward bead propagation was most consistent with the dawnward electron E×B drift. The ion current decreased and then turned dawnward, while the electron current remained duskward with variable magnitudes (Figure 2h). The electrons became the main carrier of the cross-tail current after the onset. The net cross-tail current also decreased substantially.

Figure 3. Schematic illustration of the magnetotail plasma, drift, current and electric field structures associated with the (a) duskward and (b) dawnward auroral beads.

Conclusion

We have examined plasma dynamics in the near-Earth plasma sheet that is associated with substorm onset beads using two substorm events with simultaneous THEMIS satellite and ASI observations. The key results are summarized in Figure 3. In the duskward-propagating bead event, the electron pressure gradient was anti-parallel (tailward and dawnward) to the ion pressure gradient (earthward and duskward). The electric field was outward (tailward and poleward), corresponding to the duskward E×B drift. The duskward bead speed was most consistent with the drift speed of >10 keV ions that are partially magnetized. The cross-tail current became nearly zero in the first few minutes after the onset due to the increased speed of duskward drifting electrons. The reduction of the cross-tail current would weaken the magnetic field stretching in the magnetotail and may contribute to destabilize the plasma sheet as an initiation of the substorm expansion phase. In the dawnward-propagating bead event, the electric field was earthward (or equatorward at off-equatorial locations), corresponding to the dawnward E×B drift. The dawnward bead propagation speed was most consistent with the E×B drift speed. The electrons became the main carrier of the cross-tail current and the net current decreased soon after the onset, again indicating that the tail force balance changes at onset. The THEMIS data in this event provide an observational indication that the charged current sheet forms at substorm onset and plays a critical role in substorm onset instability. The different orientations of the electric field in the two events suggest that the ion and electron distributions and magnetic field configurations in the two substorms are substantially different, and that the differences in the electric field orientations distinguish the duskward and dawnward propagating onset waves. We suggest that the plasma species that is responsible for the bead propagation changes with the electric field configuration and current sheet structure in the magnetotail.

Reference

Nishimura, Y., Artemyev, A. V., Lyons, L. R., Gabrielse, C., Donovan, E. F.,& Angelopoulos, V. (2022). Space-ground observations of dynamics of substorm onset beads. Journal of Geophysical Research: Space Physics, 127, e2021JA030004.

Biographical Note

Toshi Nishimura is a Research Associate Professor in the Department of Electrical and Computer Engineering and Center for Space Physics at Boston University. His research interest is magnetosphere-ionosphere coupling using satellite and ground-based instruments particularly all-sky imagers.

Please send comments/suggestions to

Emmanuel Masongsong / emasongsong @ igpp.ucla.edu