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Annales Geophysicae An interactive open-access journal of the European Geosciences Union
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Volume 36, issue 5 | Copyright
Ann. Geophys., 36, 1335-1346, 2018
https://doi.org/10.5194/angeo-36-1335-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Regular paper 08 Oct 2018

Regular paper | 08 Oct 2018

Statistical study of ULF waves in the magnetotail by THEMIS observations

Shuai Zhang1,2, Anmin Tian1, Quanqi Shi1, Hanlin Li1, Alexander W. Degeling1, I. Jonathan Rae3, Colin Forsyth3, Mengmeng Wang1, Xiaochen Shen1, Weijie Sun4, Shichen Bai1, Ruilong Guo5, Huizi Wang1, Andrew Fazakerley3, Suiyan Fu6, and Zuyin Pu6 Shuai Zhang et al.
  • 1Shandong Provincial Key Laboratory of Optical Astronomy and Solar-Terrestrial Environment, School of Space Science and Physics, Shandong University, Weihai, 264209, China
  • 2State Key Laboratory of Space Weather, Chinese Academy of Sciences, Beijing 100190, China
  • 3University College London, Mullard Space Science Laboratory, Space and Climate Physics, Dorking, UK
  • 4Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, USA
  • 5Institute of Geology and Geophysics Chinese Academy of Sciences, Beijing 100029, China
  • 6School of Earth and Space Sciences, Peking University, Beijing 100871, China

Abstract. Ultra-low-frequency (ULF) waves are ubiquitous in the magnetosphere. Previous studies mostly focused on ULF waves in the dayside or near-Earth region (with radial distance R<12RE). In this study, using the data of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission during the period from 2008 to 2015, the Pc5–6 ULF waves in the tail region with XGSM<0, 8RE<R<32RE (mostly on the stretched magnetic field lines) are studied statistically. A total of 1089 azimuthal oscillating events and 566 radial oscillating events were found. The statistical results show that both the azimuthal and radial oscillating events in the magnetotail region (12RE<R<32RE) are more frequently observed in the post-midnight region. The frequency decreases with increasing radial distance from Earth for both azimuthal oscillating events (8RE<R<16RE) and radial oscillating events (8RE<R<14RE), which is consistent with the field line resonances theory. About 52% of events (including the azimuthal and radial oscillating events) are standing waves in the region of 8–16RE, while only 2% are standing waves in the region of 16–32RE. There is no obvious dawn–dusk asymmetry of ULF wave frequency for events in 8RE<R<32RE, which contrasts with the obvious dawn–dusk asymmetry found by previous studies in the inner magnetosphere (4RE<R<9RE). An examination for possible statistical relationships between the ULF wave parameters and substorm occurrences is carried out. We find that the wave frequency is higher after the substorm onset than before it, and the frequency differences are more obvious in the midnight region than in the flank region.

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The features of ULF waves are statistically studied on the magnetotail stretched magnetic field lines (8 RE < R < 32 RE) by using 8 years of THEMIS data. The occurrence rates of ULF waves are higher in the post-midnight region than pre-midnight region. The frequency decreases with increasing radial distance of 8–16 RE and could be explained by much more standing waves in this region than in the region of 16–32 RE. The wave frequency is higher after the substorm onset than before it.
The features of ULF waves are statistically studied on the magnetotail stretched magnetic field...
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