11:45 AM - 12:00 PM
[PEM17-11] Contribution of the background plasma number density gradient in nonlinear wave–particle interactions
Keywords:nonlinear wave–particle interaction, particle acceleration process, Jovian magnetosphere, plasma number density gradient
Nonlinear wave–particle interactions in planetary magnetospheres include, for example, auroral electron acceleration through Landau resonance with kinetic Alfvén waves [e.g., Saito et al., 2025] and the excitation of whistler mode chorus waves through cyclotron resonance with high energy electrons [e.g., Omura, 2021]. These particle trapping processes can be understood by introducing the second-order resonance theory [e.g., Matsumoto and Omura, 1981]. The motion of particles trapped by coherent electromagnetic waves via Landau resonance (cyclotron resonance) in velocity phase space is described by considering a simple harmonic motion of the wave phase ψ (the relative phase ζ = φ-ψ with the particle gyro phase φ) and the effect of an inhomogeneity factor S arising from the background gradient.
In general, evaluations of the inhomogeneity factor S often consider only the time variation of the wave frequency and the background magnetic field gradient, neglecting the background plasma density gradient [Omura et al., 2008; Saito et al., 2025]. However, in systems such as Jupiter's Io torus, where the plasma density varies significantly over the centrifugal scale height near the magnetic equator [e.g., Thomas et al., 2004], the number density gradient may contribute more to S than the magnetic field gradient.
Saito et al. (2025) investigated the auroral electron acceleration by kinetic Alfvén waves along the L = 9 field line in the terrestrial magnetosphere. In the plasma sheet, where the plasma temperatures range from several hundred eV to a few keV, the number density variation along the field line near the magnetic equator is minimal [e.g., Ergun et al., 2000; Saito et al., 2023]; hence, the contribution of the background magnetic field gradient only becomes important in S. The electron acceleration process by kinetic Alfvén waves is also considered important in the Jovian magnetosphere, such as in the Io torus. Observations indicated a centrifugal scale height of Hc = 1.16 RJ [Phipps et al., 2018]. We newly derive the inhomogeneity factor considering the magnetic field gradient (B) and the number density gradient (n) S = SB+Sn, and found that the ratio Sn/SB is estimated to be under 2.88 in the Io torus, suggesting that the number density gradient can be a dominant component of S.
A similar argument applies to whistler mode chorus waves. The inhomogeneity factor affecting the motion of energetic electrons is characterized by a coefficient Λ, which includes not only unity but also a term related to the ratio of the number density gradient to the magnetic field gradient [Omura et al., 2009]. Strong whistler mode waves have been observed near the Jovian moon Europa [Shprits et al., 2018]. Based on the plasma environment around Europa [Bagenal et al., 2015], Hc is estimated as 1.61 RJ, and approximating the Jovian intrinsic magnetic field as a dipole leads to Λ = 1+7.5(Ωe-ω)/Ωe, where Ωe is the electron cyclotron frequency and ω is the wave frequency. This indicates that the number density gradient contributes several times more to S than the magnetic field gradient.
Thus, in regions such as the Jovian magnetosphere where number density gradients are pronounced, the number density gradient may become the primary contributor to the inhomogeneity factor S. In this presentation, we report the details of these theoretical considerations.
In general, evaluations of the inhomogeneity factor S often consider only the time variation of the wave frequency and the background magnetic field gradient, neglecting the background plasma density gradient [Omura et al., 2008; Saito et al., 2025]. However, in systems such as Jupiter's Io torus, where the plasma density varies significantly over the centrifugal scale height near the magnetic equator [e.g., Thomas et al., 2004], the number density gradient may contribute more to S than the magnetic field gradient.
Saito et al. (2025) investigated the auroral electron acceleration by kinetic Alfvén waves along the L = 9 field line in the terrestrial magnetosphere. In the plasma sheet, where the plasma temperatures range from several hundred eV to a few keV, the number density variation along the field line near the magnetic equator is minimal [e.g., Ergun et al., 2000; Saito et al., 2023]; hence, the contribution of the background magnetic field gradient only becomes important in S. The electron acceleration process by kinetic Alfvén waves is also considered important in the Jovian magnetosphere, such as in the Io torus. Observations indicated a centrifugal scale height of Hc = 1.16 RJ [Phipps et al., 2018]. We newly derive the inhomogeneity factor considering the magnetic field gradient (B) and the number density gradient (n) S = SB+Sn, and found that the ratio Sn/SB is estimated to be under 2.88 in the Io torus, suggesting that the number density gradient can be a dominant component of S.
A similar argument applies to whistler mode chorus waves. The inhomogeneity factor affecting the motion of energetic electrons is characterized by a coefficient Λ, which includes not only unity but also a term related to the ratio of the number density gradient to the magnetic field gradient [Omura et al., 2009]. Strong whistler mode waves have been observed near the Jovian moon Europa [Shprits et al., 2018]. Based on the plasma environment around Europa [Bagenal et al., 2015], Hc is estimated as 1.61 RJ, and approximating the Jovian intrinsic magnetic field as a dipole leads to Λ = 1+7.5(Ωe-ω)/Ωe, where Ωe is the electron cyclotron frequency and ω is the wave frequency. This indicates that the number density gradient contributes several times more to S than the magnetic field gradient.
Thus, in regions such as the Jovian magnetosphere where number density gradients are pronounced, the number density gradient may become the primary contributor to the inhomogeneity factor S. In this presentation, we report the details of these theoretical considerations.