Statistical study of whistler waves in magnetic flux ropes in the Earth's magnetotail

Whistler waves are believed to have close relationship with the dynamics of the magnetic flux ropes (FRs) in the Earth's magnetotail. Previous studies about whistler waves in FRs were mainly based on event analysis. However, several key issues should be investigated from a statistical perspective, such as the spatial distributions of whistler waves in FRs with different motion directions, the frequency ranges of whistler waves, as well as the magnetic field power spectral densities (PSDs) and the excitation mechanisms of whistler waves. In this study, using the unprecedented high-resolution data from the magnetospheric multiscale (MMS) mission during the periods of magnetotail crossing between May and August in 2017, we perform a detailed statistical study on the whistler waves in the Earth's magnetotail FRs. Based on the magnetic field variation characteristics, the FRs are divided into three regions: the leading draping region, the core region, and the trailing draping region. Moreover, whistler waves are categorized into two types: low-band whistler waves with a frequency range between 0.1f_ce (f_ce is the local electron cyclotron frequency) and 0.5f_ce, and upper-band whistler waves with a frequency range from 0.5f_ce to f_ce. We separately investigate the spatial distribution characteristics, the magnetic-field PSDs, and excitation mechanisms of low-band whistler waves and upper-band whistler waves in earthward and tailward FRs. Finally, our research findings can be summarized as follows: (1) Whistler waves in the Earth’s magnetotail FRs are primarily lower-band ones; (2) More whistler waves along with more intense magnetic-field PSDs in the two frequency ranges both tend to occur in the trailing draping region; (3) The electron temperature with a dominant perpendicular anisotropy can account for the generation of low-band and upper-band whistler waves in the center region of core region of earthward FRs and low-band whistler waves in the center region of core region of tailward FRs; (4) In addition, the electron beams may provide free energy for upper-band whistler waves in the trailing draping region of earthward and tailward FRs. Our results may contribute to the interpretation of the impact of whistler waves on the evolution of Earth's magnetotail FRs and the magnetotail dynamics.