講演情報
[PPS01-P08]A Case Study of EMIC Waves Observed During an Injection Event in Jupiter's Magnetosphere Based on Juno’s Observation
*野口 智史1、加藤 雄人1、熊本 篤志1、土屋 史紀1、佐藤 晋之祐1,2、Collet Brieuc1 (1.東北大学大学院理学研究科地球物理学専攻、2.Aix-Marseille Université, CNRS, CNES, Institut Origines, LAM, Marseille, France)
キーワード：
木星、磁気圏、EMIC波動、Juno
The dynamics of Jupiter's giant magnetosphere are dominated by the planet's rapid rotation and its interaction with heavy ions supplied by the moon Io. Plasma waves in this environment are a crucial subject of study for understanding energy transport and particle acceleration. This research focuses on a case in which electromagnetic ion cyclotron (EMIC) waves were observed in Jupiter's inner magnetosphere, associated with intermittent flux enhancements of electrons and ions in a broad energy range from several keV to hundreds of keV.
One excitation mechanism for plasma waves, such as EMIC waves, is temperature anisotropy in the velocity distribution function of energetic ions, where their temperature is predominantly perpendicular to the ambient magnetic field. In Jupiter's magnetosphere, injection events are considered as a primary mechanism for generating this anisotropy. It is thought that injection events in Jupiter's magnetosphere occur when high-energy particles are injected into the inner magnetosphere by phenomena such as the interchange instability (Mauk et al., 1999). Adiabatic heating associated with inward radial transport during these injection events is expected to create a strong temperature anisotropy in the velocity distribution of high-energy particles. While recent study has observed whistler-mode chorus waves caused by injection events in Jupiter's magnetosphere (Ma et al., 2024), there have been no reports of observational cases showing a clear association between injection events and EMIC waves. Investigating whether injection-associated EMIC waves, which are frequently observed in Earth's magnetosphere, also occur at Jupiter is important for clarifying the details of wave-particle interactions in the Jovian magnetosphere and for exploring universal physical laws through the comparison with Earth's magnetosphere.
This study aims to elucidate the details of Jupiter's magnetospheric dynamics and wave generation, confirm the contribution of wave-particle interactions, and compare with Earth. For this purpose, we identified and investigated a case where both injection and EMIC waves were simultaneously observed by the Juno spacecraft. First, we used the data acquired by the Jupiter Aurora Distribution Experiment (JADE) (McComas et al., 2017). This data was obtained around 05:00 UT on 22 May 2022, at 15-20 Jupiter radii (RJ) from Jupiter's center in the equatorial region of Jupiter. During this period, signs of an injection event were confirmed by a sharp increase in the energy flux of electrons and ions in the 0.1-several tens of keV range. Fluctuations in the flux were observed at 30-second intervals. Using the time-of-flight (TOF) analysis and an improved method for estimating ion species (Kim et al., 2020), it was found that O+ or S++ ions around several keV were dominant during this period. Meanwhile, concurrent magnetic field observations by the Magnetometer (MAG) (Connerney et al., 2017) revealed significant magnetic field fluctuations.
A detailed analysis of these magnetic field fluctuations was conducted using wavelet analysis and the singular value decomposition (SVD) method (Santolik et al., 2003). The results confirmed a wave phenomenon with an intensity exceeding 102 nT2/Hz lasting for about 5 minutes. The intensity before and after was 50 nT2/Hz or below. The observed waves exhibited left-hand polarization, quasi-parallel wave normal angles, and a frequency below the local O+ or S++ ion cyclotron frequency. Based on these results, it is considered that the identified event captured EMIC waves. While previous studies reported EMIC waves in Jupiter's magnetosphere generated by pickup ions from the moon Io (Cao et al., 2025) and in the outer magnetosphere (90 RJ) (Yuan et al., 2024), this is the first report of EMIC waves associated with an injection event in the inner magnetosphere.
One excitation mechanism for plasma waves, such as EMIC waves, is temperature anisotropy in the velocity distribution function of energetic ions, where their temperature is predominantly perpendicular to the ambient magnetic field. In Jupiter's magnetosphere, injection events are considered as a primary mechanism for generating this anisotropy. It is thought that injection events in Jupiter's magnetosphere occur when high-energy particles are injected into the inner magnetosphere by phenomena such as the interchange instability (Mauk et al., 1999). Adiabatic heating associated with inward radial transport during these injection events is expected to create a strong temperature anisotropy in the velocity distribution of high-energy particles. While recent study has observed whistler-mode chorus waves caused by injection events in Jupiter's magnetosphere (Ma et al., 2024), there have been no reports of observational cases showing a clear association between injection events and EMIC waves. Investigating whether injection-associated EMIC waves, which are frequently observed in Earth's magnetosphere, also occur at Jupiter is important for clarifying the details of wave-particle interactions in the Jovian magnetosphere and for exploring universal physical laws through the comparison with Earth's magnetosphere.
This study aims to elucidate the details of Jupiter's magnetospheric dynamics and wave generation, confirm the contribution of wave-particle interactions, and compare with Earth. For this purpose, we identified and investigated a case where both injection and EMIC waves were simultaneously observed by the Juno spacecraft. First, we used the data acquired by the Jupiter Aurora Distribution Experiment (JADE) (McComas et al., 2017). This data was obtained around 05:00 UT on 22 May 2022, at 15-20 Jupiter radii (RJ) from Jupiter's center in the equatorial region of Jupiter. During this period, signs of an injection event were confirmed by a sharp increase in the energy flux of electrons and ions in the 0.1-several tens of keV range. Fluctuations in the flux were observed at 30-second intervals. Using the time-of-flight (TOF) analysis and an improved method for estimating ion species (Kim et al., 2020), it was found that O+ or S++ ions around several keV were dominant during this period. Meanwhile, concurrent magnetic field observations by the Magnetometer (MAG) (Connerney et al., 2017) revealed significant magnetic field fluctuations.
A detailed analysis of these magnetic field fluctuations was conducted using wavelet analysis and the singular value decomposition (SVD) method (Santolik et al., 2003). The results confirmed a wave phenomenon with an intensity exceeding 102 nT2/Hz lasting for about 5 minutes. The intensity before and after was 50 nT2/Hz or below. The observed waves exhibited left-hand polarization, quasi-parallel wave normal angles, and a frequency below the local O+ or S++ ion cyclotron frequency. Based on these results, it is considered that the identified event captured EMIC waves. While previous studies reported EMIC waves in Jupiter's magnetosphere generated by pickup ions from the moon Io (Cao et al., 2025) and in the outer magnetosphere (90 RJ) (Yuan et al., 2024), this is the first report of EMIC waves associated with an injection event in the inner magnetosphere.