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Discussion papers | Copyright
https://doi.org/10.5194/angeo-2018-39
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 4.0 License.

Regular paper 14 May 2018

Regular paper | 14 May 2018

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This discussion paper is a preprint. A revision of the manuscript is under review for the journal Annales Geophysicae (ANGEO).

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, Qiugang Zong4, Mengmeng Wang1, Xiaochen Shen1, Weijie Sun5, Shichen Bai1, Ruilong Guo6, Huizi Wang1, Andrew Fazakerly3, Suiyan Fu4, and Zuiyin Pu4 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
  • 4School of Earth and Space Sciences, Peking University, Beijing 100871, China
  • 5Department of Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, USA
  • 6Institute of Geology and Geophysics Chinese Academy of Sciences, Be ijing 100029, 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 Time History of Events and Macroscale Interactions during Substorms (THEMIS) during the period from 2008 to 2015, the Pc5-6 ULF waves in the tail region with X*GSM<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 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 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 just between 8 and 16 RE, it 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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