Formation of a Displaced Plasma Wake at Neptune's Moon Triton

Abstract

Triton, the largest moon of Neptune (radius R T = 1,353 km), is located deep within the ice giant's magnetosphere at a distance of 14.4 Neptune radii (R N = 24,622 km), moving along a highly inclined and retrograde orbit around its parent planet. Similar to many large moons of Jupiter and Saturn, Triton is continuously exposed to a flow of sub-Alfvénic magnetospheric plasma that impinges at a relative velocity of about 40 km/s (Strobel et al., 1990). This magnetized flow sweeps particles out of Triton's ionosphere (Broadfoot et al., 1989;Majeed et al., 1990), generating ramside field pile-up and Alfvén wings in the process (Liuzzo et al., 2021;Strobel et al., 1990). Due to the combination of high ionospheric electron densities in excess of 10 4 cm −3 and a weak magnetospheric field along Triton's orbit (3-12 nT, see Connerney et al., 1991), the Pedersen conductance of the moon's ionosphere (Σ P ≈ 10 4 S) exceeds the Alfvén conductance of the ambient flow (Σ A ≈ 6 S) by four orders of magnitude (Strobel et al., 1990). Therefore, the interaction between Triton and Neptune's magnetospheric plasma is "saturated," that is, the streamlines of the impinging plasma are (almost) completely deflected around the Alfvén wing tubes

Influence of Titan's Variable Electromagnetic Environment on the Global Distribution of Energetic Neutral Atoms

“…With increasing inclination ϑ of the background field, the angle of the northern wing against the corotational flow direction increases and the angle of the southern wing decreases (this behavior is described analytically by Simon et al. [2022]). The strength of the B X perturbations also becomes asymmetric due to the different Alfvén conductances in the wings when $${\vec{B}}_{0}$$ is not perpendicular to the corotation direction (Neubauer, 1980).…”

“…While the Alfvénic Mach number is the same for all three cases, Figures 4b and 4c reveal an asymmetry between the Alfvén wings in the Z < 0 and Z > 0 half spaces due to the inclined background field. With increasing inclination ϑ of the background field, the angle of the northern wing against the corotational flow direction increases and the angle of the southern wing decreases (this behavior is described analytically by Simon et al [2022]). The strength of the B X perturbations also becomes asymmetric due to the different Alfvén conductances in the wings when 𝐴𝐴 ⃗ 𝐵𝐵0 is not perpendicular to the corotation direction (Neubauer, 1980).…”

A Statistical Study of the Moon's Magnetotail Plasma Environment

“…Of these, Tethys, Dione, and Rhea lack dense global exospheres and their interactions with the Saturnian plasma generate plasma absorption features—including a cavity in the plasma density and magnetic field compression signatures—and Alfvén wings that extend from the moons, connecting them with the polar Saturnian ionosphere (e.g., Khurana et al., 2008; Simon et al., 2015). Further similarities in the lunar magnetotail lobe plasma can be extended to Triton—whose interaction with the markedly sub‐Alfvénic and highly variable plasma environment may generate previously undetected Alfvén wing plasma absorption signatures (Liuzzo, Paty, et al., 2021; Simon et al., 2021), as well as to Io and Europa—which orbit within the strongly magnetized inner magnetosphere of Jupiter. Indeed, similar plasma interaction features well‐known at these outer‐planet moons have also been observed by ARTEMIS during individual encounters with the Moon while located within the magnetotail lobes (e.g., Liuzzo, Poppe, et al., 2021; Xu et al., 2019).…”

Energetic Magnetospheric Particle Fluxes Onto Callisto's Atmosphere

“…Instead, the density depletion is rotated out of the expected wake location toward the 𝐴𝐴  + (= u 0 + v A,0 ) Alfvén wing characteristic. For this case, the Alfvénic Mach number of the magnetospheric plasma is M A = 0.19, and the background field is tilted at an angle of 76° against the magnetospheric flow direction (i.e., rotated 14° away from the u 0 ⊥B 0 case, toward the moon's downstream hemisphere; see, e.g., Equation 4 of Neubauer, 1980 or Equations 1 and 2 of Simon et al, 2022). This minor deviation rotates the 𝐴𝐴  − (= u 0 − v A,0 ) Alfvén wing characteristic into Callisto's upstream hemisphere (see also Table 1).…”