2019 THEMIS SCIENCE NUGGETS
A statistical study of near-Earth magnetotail evolution during pseudosubstorms and substorms with THEMIS data
by Kento Fukui, Institute for Space-Earth Environmental Research, Nagoya University
and Yukinaga Miyashita, Korea Astronomy and Space Science Institute
Introduction
Pseudosubstorms, also called pseudobreakups, are similar to substorms. During pseudosubstorms and substorms, the same phenomena occur in the magnetotail: for example, near-Earth magnetic reconnection, plasmoid, formation of the current wedge, dipolarization (persistent increase in the northward magnetic field), and energetic particle injection.
The early stage of auroral onset arc development is also similar. As shown in Figure 1, after initial auroral brightening, the onset arc gradually extends westward and eastward with its wave-like structure growing for both a pseudosubstorm and substorm. However, they differ appreciably in terms of the subsequent auroral development. The onset arc is suppressed without progressing to poleward expansion for the pseudosubstorm, whereas poleward expansion or auroral breakup begins for the substorm.
To understand what causes this difference, we studied temporal and spatial development of the near-Earth magnetotail at X=-7 to -11 Re around pseudosubstorm and substorm onsets, based on superposed epoch analysis of THEMIS data. The results are expected to give us some clues to understanding of substorm onset and the development mechanism, or what suppresses further development of poleward expansion after initial brightening for pseudosubstorms.
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| Figure 1. Examples of auroral onset arc development for (left) the pseudosubstorm and (right) the substorm observed by THEMIS GBO all-sky imagers. |
Results
We found that the earthward flow (Figure 2b) begins to increase at X=-10 to -11 Re just before onset and dipolarization for both pseudosubstorms and substorms, possibly due to near-Earth magnetic reconnection or the preceding relaxation of the thin current sheet in a tailward region, but the earthward flow is slower for pseudosubstorms than for substorms. Dipolarization (Figure 2c) and associated magnetic field fluctuation (Figure 2d) are nearly the same at X=-8 Re for both cases, but they are weaker at other distances for pseudosubstorms than for substorms. The total pressure (Figure 2e) at X=-8 to -11 Re is larger for several minutes before onset of substorms than for pseudosubstorms. After onset the total pressure gradient increases more for substorms than for pseudosubstorms.
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| Figure 2. Results of superposed epoch analysis of (a,b) the earthward component of the ion flow Vx, (c) the residual northward magnetic field after subtracting the International Geomagnetic Reference Field (IGRF) from the observed value Bz,res, (d) magnetic field fluctuation related to waves in a frequency range of 1/3 to 64 Hz, and (e) the total (ion plus magnetic) pressure Pt (left column) for pseudosubstorms and (right column) for substorms. Zero epoch time (t=0) is the onset (initial auroral brightening) time. |
Conclusion
We suggest that for pseudosubstorms, the current disruption related to dipolarization does not expand tailward and hence auroral poleward expansion does not occur. We also suggest that the differences in the total pressure are important factors for determining whether ballooning instability causing current disruption grows in a wide area, that is, whether the initial action develops into a substorm or subsides as a pseudosubstorm.
Identifying a more detailed cause and effect relationship between near-Earth magnetotail conditions, dipolarization, pseudosubstorms, and substorms requires further statistical and theoretical studies. Besides the near-Earth magnetotail conditions, the effects of near-Earth magnetic reconnection and the ionosphere on pseudosubstorms and substorms need further investigation.
Reference
Fukui, K., Miyashita, Y., Machida, S., Miyoshi, Y., Ieda, A., Nishimura, Y., and Angelopoulos, V. (2020). A statistical study of near-Earth magnetotail evolution during pseudosubstorms and substorms with THEMIS data. Journal of Geophysical Research: Space Physics, 125, e2019JA026642. https://doi.org/10.1029/2019JA026642Biographical Note
Kento Fukui was a graduate student at Institute for Space-Earth Environmental Research, Nagoya University in Nagoya, Japan. Yukinaga Miyashita is a principal researcher at Korea Astronomy and Space Science Institute in Daejeon, South Korea. Their research interests include substorm triggering mechanism, the magnetotail, and aurora.
Please send comments/suggestions to
Emmanuel Masongsong / emasongsong @ igpp.ucla.edu