A Statistical Study of the Solar Wind Dependence of Multi‐Harmonic Toroidal ULF Waves Observed by the Arase Satellite

Abstract

Toroidal standing Alfvén wave is one of the ultra‐low frequency waves that are frequently observed in the terrestrial magnetosphere. They sometimes exhibit multi‐harmonic frequency spectra, indicating wide energy range input in the magnetosphere. However, their energy source has not been fully understood due to the lack of statistical studies. Here we used the data of the Arase satellite observations for ∼3.5 years and conducted a statistical analysis of the solar wind dependence of the occurrence rate, wave power, and frequency of the multi‐harmonic toroidal waves. We automatically detected the multi‐harmonic waves and categorized them into four groups according to the solar wind velocity and the cone angle of the interplanetary magnetic field. We found that the occurrence rate and wave power of the multi‐harmonic waves increase with the solar wind velocity on the flank sides. In the noon sector, the occurrence rate of the multi‐harmonic waves increases with the decrease of the cone angle. The median frequency of the multi‐harmonic waves on the dayside is positively correlated with the upstream wave frequency predicted by the theory of the ion beam instability for a small cone angle. The occurrence rate also increases with the solar wind dynamic pressure fluctuations. Therefore, we suggest that the Kelvin‐Helmholtz instability, the upstream waves, and the dynamic pressure fluctuations are possible sources of the multi‐harmonic waves. This study sheds light on the activity of the multi‐harmonic waves which can affect radiation belt electrons under various solar wind conditions.

Polarization and Spatial Distribution Features of Pc4 and Pc5 Waves in the Magnetosphere

“…Yamamoto et al. (2022) analyzed solar wind control of dayside toroidal waves using Arase data and found that monochromatic toroidal waves are observed several times more often than multi‐harmonic waves. They showed that multi‐harmonic toroidal wave frequencies and occurrence rate correlate with V SW , SW pressure, and cone angle.…”

“…Likewise, this instability may be an amplifier of waves excited on the dayside magnetopause (Archer et al, 2019(Archer et al, , 2021Klimushkin et al, 2019). Yamamoto et al (2022) analyzed solar wind control of dayside toroidal waves using Arase data and found that monochromatic toroidal waves are observed several times more often than multi-harmonic waves. They showed that multi-harmonic toroidal wave frequencies and occurrence rate correlate with V SW , SW pressure, and cone angle.…”

“…We used L, MLT, and MLAT to present spatial distribution of ULF wave occurrence rate. L‐MLT diagrams show an equatorial distribution, and were used in many previous studies (Anderson, 1993; Dai et al., 2015; Liu et al., 2009; Yamamoto et al., 2022). A high inclination of Arase orbit enhances our study by adding meridional distributions (L‐MLAT).…”

“…As a result, we obtained a list of wave events with the information about event duration, spacecraft position during an observation, wave amplitude averaged over the interval of wave observation for all three magnetic field components, and wave frequency. Thus, we can identify multi‐harmonic waves (Li et al., 2021; Yamamoto et al., 2022), coupling of Alfvén and compressional waves (Mager et al., 2015), and simultaneous observation of different waves (Korotova et al., 2020). Comparing wave amplitudes among three components, we revealed the dominating oscillation direction for each wave event, that is, the polarization of the wave.…”

Definition of the Alfvén mode in inhomogeneous magnetic field

“…С другой стороны, ММЗ-волна испытывает сильное бесстолкновительное затухание при взаимодействии с частицами. С альфвеновскими волнами обычно отождествляют значительную часть МГД-колебаний в земной магнитосфере [Леонович, Мазур, 2016;Alperovich, Fedorov, 2007] и солнечном ветре [Belcher, Davis, 1971;Tu, Marsch, 1995]. В атмосфере Солнца эти моды наблюдать технически труднее, но и здесь имеются свидетельства их существования [Jess et al, 2009;Кобанов и др., 2017;Челпанов и др., 2018].…”