Modulation of Whistler Waves by Ultra‐Low‐Frequency Perturbations: The Importance of Magnetopause Location

Abstract

Ultra-low-frequency (ULF, 0.001-1 Hz) perturbations are important aspect of the dynamics in the inner magnetosphere: They are responsible for radial transport (diffusion) of high-energy electrons and for energizing the ionosphere with field-aligned currents. This study is devoted to properties of ULF perturbations generated at the magnetopause, their propagation into the inner magnetosphere, and their modulation of whistler-mode very-low-frequency (VLF) waves. Taking advantage of the Time History of Events and Macroscale Interactions during Substorms (THEMIS) mission configuration in 2019, we investigate ULF perturbations simultaneously captured by three spacecraft at different distances from the magnetopause. Combining ground-based and THEMIS measurements of ULF perturbations to separate temporal and spatial variations of their properties, we show that their intensity decays exponentially with distance from the magnetopause as close as the geostationary orbit. Near the magnetopause ULF perturbations can modulate whistler-mode (VLF) waves effectively: Close to the magnetopause, VLF wave bursts have the same periodicity as the ULF perturbations. Our results demonstrate that almost the entire outer magnetosphere (from the geostationary orbit to L ∼ 12), including the outer radiation belts, is significantly influenced by ULF perturbations excited by magnetopause dynamic responses to the solar wind.

Role of Ducting in Relativistic Electron Loss by Whistler‐Mode Wave Scattering

“…These precipitations at have a transient nature that could be due to the temporal or spatial quasi‐periodicity of the whistlers. The ULF‐modulated electron anisotropy is expected to generate equatorial whistlers with some periodicity in both time and space (see Motoba et al., 2013; Zhang et al., 2020). This hypothesis is consistent with ELFIN A, METOP 1, and NOAA 19 observations of tens of keV electron precipitation from a broad MLT and ‐shell region (see Figure 3).…”

Energy Transfer Between Hot Protons and Electromagnetic Ion Cyclotron Waves in Compressional Pc5 Ultra‐low Frequency Waves

“…Many enhancements of He‐band EMIC waves (below f c_He+ ) were detected in the vicinity of the troughs of the magnetic field intensity (Figure 2e), although the one‐to‐one correspondence is less clear than in the cases of whistler‐mode waves observed in compressional waves (Baumjohann et al., 2000; Dubinin et al., 2007; W. Li et al., 2011; Xia et al., 2016; X.‐J. Zhang, Angelopoulos et al., 2020; X.‐J. Zhang, Chen et al., 2019).…”

“…Zhang, Mourenas et al. (2020) for whistler mode waves, can also produce wave packets. On the basis of the present study, we suggest that very localized waves can also become a candidate of the causes of observed wave packets.…”

“…The situation seems to be similar to that of whistler‐mode waves that are frequently modulated by compressional structures in the magnetosheath (e.g., Ahmadi et al., 2018; Breuillard et al., 2018; Kitamura et al., 2020; Maksimovic et al., 2001; Smith et al., 1969; Smith & Tsurutani, 1976; Y. Zhang et al., 1998) and sometimes even in the magnetosphere (Baumjohann et al., 2000; Dubinin et al., 2007; W. Li et al., 2011; Tenerani et al., 2013; Xia et al., 2016; X.‐J. Zhang, Angelopoulos et al., 2020; X.‐J. Zhang, Chen et al., 2019).…”

“…Similar compressional waves, which are identified as ultralow frequency (ULF) waves around the frequency range of Pc 5 (period: 150–600 s), are often observed in the magnetosphere (e.g., Baumjohann et al., 1987; Constantinescu et al., 2009; Lanzerotti et al., 1969; Takahashi, Lopez et al., 1987; X. Zhu & Kivelson, 1994), and some of them include whistler mode waves only in the vicinity of the troughs of magnetic field intensity (Baumjohann et al., 2000; Dubinin et al., 2007; W. Li et al., 2011; Xia et al., 2016; X.‐J. Zhang, Angelopoulos et al., 2020; X.‐J. Zhang, Chen et al., 2019).…”