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ISSN  2096-3955

CN  10-1502/P

Citation: Song, Y. X., and Chen, C. X. (2022). Observation evidence for the entropy switch model of substorm onset. Earth Planet. Phys., 6(2), 161–176.

2022, 6(2): 161-176. doi: 10.26464/epp2022020


Observation evidence for the entropy switch model of substorm onset

CAS Key Laboratory of Geospace Environment, Department of Geophysics and Planetary Sciences, School of Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China

Corresponding author: ChuXin Chen,

Received Date: 2021-10-11
Web Publishing Date: 2022-01-28

The cause of substorm onset is not yet understood. Chen CX (2016) proposed an entropy switch model, in which substorm onset results from the development of interchange instability. In this study, we sought observational evidence for this model by using Time History of Events and Macroscale Interactions during Substorms (THEMIS) data. We examined two events, one with and the other without a streamer before substorm onset. In contrast to the stable magnetosphere, where the total magnetic field strength is a decreasing function and entropy is an increasing function of the downtail distance, in both events the total magnetic field strength and entropy were reversed before substorm onset. After onset, the total magnetic field strength, entropy, and other plasma quantities fluctuated. In addition, a statistical study was performed. By confining the events with THEMIS satellites located in the downtail region between ~8 and ~12 Earth radii, and 3 hours before and after midnight, we found the occurrence rate of the total magnetic field strength reversal to be 69% and the occurrence rate of entropy reversal to be 77% of the total 205 events.

Key words: substorm onset, entropy switch model, interchange or ballooning instability, Time History of Events and Macroscale Interactions during Substorms (THEMIS) data

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Observation evidence for the entropy switch model of substorm onset

YunXiang Song, ChuXin Chen