Triton’s Variable Interaction With Neptune’s Magnetospheric Plasma

Abstract

Neptune's largest moon Triton (radius 𝐴𝐴 𝐴𝐴𝑇𝑇 = 1, 353 km) is thought to be an erstwhile Kuiper belt object that was captured by the ice giant (Agnor & Hamilton, 2006). Orbiting at a radial distance of 𝐴𝐴 14.4𝑅𝑅𝑁𝑁 (radius of Neptune 𝐴𝐴 𝐴𝐴𝑁𝑁 = 24, 622 km), Triton is always located within Neptune's magnetosphere (Curtis & Ness, 1986;Mejnertsen et al., 2016;Ness et al., 1989;Richardson, 1993). The moon's highly inclined orbit-tilted nearly 𝐴𝐴 157 • with respect to its parent planet's rotational equator-results in a retrograde orbital motion around Neptune. Triton possesses the second-most dense moon atmosphere in the solar system after Titan (Broadfoot et al., 1989;Strobel et al., 1990;Strobel & Zhu, 2017). Mainly comprised of neutral 𝐴𝐴 N2 , its maximum surface number density is on the order of 𝐴𝐴 10 15 cm −3 , with a scale height between 10 and 70 km (Broadfoot et al., 1989). Additionally, trace gases including methane are present, with surface densities below 𝐴𝐴 10 11 cm −3 (e.g., Summers & Strobel, 1991;Trafton, 1984;Krasnopolsky et al., 1992). This neutral envelope is predominantly ionized by a combination of magnetospheric electron impacts and photoionization, resulting in an ionospheric Pedersen conductance that may exceed 𝐴𝐴 10 4 S (Strobel et al., 1990). In addition to this global atmosphere, observations during the Voyager 2 encounter of Neptune in 1989 indicated localized, geyser-like vapor plumes emanating from the surface to an altitude of ∼10 km (Smith et al., 1989). Since the moon's interior is likely differentiated in a hydrosphere and rocky mantle, it is possible that these plumes originate from a global, deep ocean sustained via radiogenic heating and/or tidal forcing (Nimmo & Spencer, 2015).

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

“…In the Alfvénic far field, such an induced field would generate a slight shrinkage of the Alfvén wing tubes and a displacement away from an axis through Triton's center (Neubauer, 1999;Volwerk et al, 2007). However, it would not cause any qualitative changes of the flow pattern near the wings: while the model of Liuzzo et al (2021) does take into account an induced field, the displaced wake (and associated deflection pattern of the plasma) are clearly discernible in their results.…”

“…The angle ϑ is treated as a free parameter. At Triton, the expected values of ϑ range from (−43°) to (+43°) (see Liuzzo et al, 2021). However, in this study we consider the range 0° ≤ ϑ ≤ 90°.…”

“…The magnitude of the magnetospheric background field is set to B 0 = 5.14 nT and the field vector is assumed to be parallel to the 𝐴𝐴 (𝑥𝑥𝑥 𝑥𝑥) and 𝐴𝐴 ( x𝑥𝑥 x𝑥) planes. While the ϑ = 43° simulation of Liuzzo et al (2021) did include a magnetic field component B 0,y ≠ 0 perpendicular to these planes, its value was a factor of 200 weaker than the components parallel to them. In general, a non-zero B 0,y component of the background field can always be eliminated by rotating the Triton Interaction System around the x axis, that is, confining 𝐴𝐴 𝐴𝐴 0 to the (x, z) plane does not limit the validity of our results.…”

“…This is the major reason why such a shifted wake cavity has not yet been observed at any moon in the solar system. However, the displaced plasma wake identified by Liuzzo et al (2021) may indeed be a typical feature of Triton's magnetospheric interaction that occurs whenever the inclination angle ξ is smaller than a certain threshold. To corroborate this idea, the goal of our study is to prove analytically that the asymmetric flow deflection pattern proposed as the root cause of the displaced wake is indeed consistent with first principles.…”

“…Figure 3. (a) Number density of the magnetospheric upstream plasma and (b) u z component of the upstream flow velocity in the (x, z) plane, as obtained from the AIKEF hybrid model(Liuzzo et al, 2021). The results shown here are from the same AIKEF run as discussed in Section 3.2.2 of our companion paper; only the coordinate system has been adjusted to match the nomenclature used in this manuscript.…”