3:30 PM - 3:45 PM
[PEM12-17] Controlling factors of the occurrence of multi-harmonic toroidal ULF waves: A statistical study of the Arase satellite observations
Keywords:ULF waves, magnetosphere, solar wind, upstream waves, Kelvin-Helmholtz instability
Ultralow frequency (ULF) waves can coherently or stochastically interact with charged particles through drift/drift-bounce resonance (e.g., Claudepierre et al., 2013; Schulz & Lanerotti, 1974). In the wave-particle interaction, wave frequency is one of the parameters which determine the energy where the interaction occurs. Because differences of frequency spectra may cause different results of wave-particle interaction, as numerically simulated by Sarris et al. (2017), ULF waves with a discrete frequency and those with broadband frequencies are investigated separately (e.g., Murphy et al., 2020). Therefore, classification of ULF waves according to their frequency spectra is important to accurately understand the wave-particle interaction.
Toroidal ULF waves with multiple discrete frequencies were also reported in event studies (e.g., Takahashi & McPherron, 1984; Engebretson et al., 1986; Takahashi et al., 2020). It was suggested that multi-harmonic toroidal waves (hereinafter MTWs) occur when the cone angle (θxB) of the solar wind is small; however, controlling factors of MTWs are not fully investigated. One of the difficulties in investigation of the relation between ULF waves and the solar wind is that solar wind parameters are correlated with each other and it is difficult to determine which one controls wave excitation. To circumvent this difficulty, we conducted statistical analysis of the occurrence of MTWs under four conditions for the first time: high/low speed solar wind (VSW greater/less than 450 km/s) and large/small cone angle (θxB greater/less than 45°).
From the ULF wave observations by the Arase/Magnetic Field Experiment (MGF) instrument, we found that not only θxB but also VSW is correlated with the occurrence of MTWs. MTWs are frequently observed around noon sector when the cone angle is small, while the occurrence frequency and wave amplitude of MTWs on the flank side increase as VSW increases. This result suggests that both the upstream waves excited in the ion foreshock region and the Kelvin-Helmholtz (KH) instability at the magnetopause can be the energy sources of MTWs at each sectors in the magnetosphere. We also examined the influence of the plasmasphere on the occurrence of MTWs. The occurrence frequency dramatically increases as the cold electron density (Ne) increases. This is probably because the interval of eigenfrequencies of wave harmonics decreases with the increase of Ne, and field line resonance (FLR) at multiple harmonic frequencies may be caused by disturbances with a finite frequency band. Outside the plasmasphere (Ne < 50 cm-3), the occurrence frequency of MTWs is relatively low, but the wave amplitudes are large on the flank side in the case of high speed solar wind. Thus, the plasmasphere has negative effect on the amplitudes of the waves driven by the KH instability. We further examined the relation between MTWs and fluctuation of the solar wind dynamic pressure (Pdyn), and revealed that Pdyn fluctuation can be the energy source of MTWs especially in the case of low speed solar wind. On the basis of our statistical analysis, VSW, θxB and background plasma density are the principal controlling factors of the occurrence of MTWs, and Pdyn fluctuation is categorized into the second group of the controlling factors.
Toroidal ULF waves with multiple discrete frequencies were also reported in event studies (e.g., Takahashi & McPherron, 1984; Engebretson et al., 1986; Takahashi et al., 2020). It was suggested that multi-harmonic toroidal waves (hereinafter MTWs) occur when the cone angle (θxB) of the solar wind is small; however, controlling factors of MTWs are not fully investigated. One of the difficulties in investigation of the relation between ULF waves and the solar wind is that solar wind parameters are correlated with each other and it is difficult to determine which one controls wave excitation. To circumvent this difficulty, we conducted statistical analysis of the occurrence of MTWs under four conditions for the first time: high/low speed solar wind (VSW greater/less than 450 km/s) and large/small cone angle (θxB greater/less than 45°).
From the ULF wave observations by the Arase/Magnetic Field Experiment (MGF) instrument, we found that not only θxB but also VSW is correlated with the occurrence of MTWs. MTWs are frequently observed around noon sector when the cone angle is small, while the occurrence frequency and wave amplitude of MTWs on the flank side increase as VSW increases. This result suggests that both the upstream waves excited in the ion foreshock region and the Kelvin-Helmholtz (KH) instability at the magnetopause can be the energy sources of MTWs at each sectors in the magnetosphere. We also examined the influence of the plasmasphere on the occurrence of MTWs. The occurrence frequency dramatically increases as the cold electron density (Ne) increases. This is probably because the interval of eigenfrequencies of wave harmonics decreases with the increase of Ne, and field line resonance (FLR) at multiple harmonic frequencies may be caused by disturbances with a finite frequency band. Outside the plasmasphere (Ne < 50 cm-3), the occurrence frequency of MTWs is relatively low, but the wave amplitudes are large on the flank side in the case of high speed solar wind. Thus, the plasmasphere has negative effect on the amplitudes of the waves driven by the KH instability. We further examined the relation between MTWs and fluctuation of the solar wind dynamic pressure (Pdyn), and revealed that Pdyn fluctuation can be the energy source of MTWs especially in the case of low speed solar wind. On the basis of our statistical analysis, VSW, θxB and background plasma density are the principal controlling factors of the occurrence of MTWs, and Pdyn fluctuation is categorized into the second group of the controlling factors.