Role of the Ionospheric Conductance Profile in Sub‐Alfvénic Moon‐Magnetosphere Interactions: An Analytical Model

Abstract

The Galilean moons of Jupiter and the largest satellites of Saturn are located within their parent planets' magnetospheres where they are continuously overtaken by a partially corotating plasma flow. The obstacle that a moon represents to this incident magnetospheric plasma can consist mainly of its dense atmosphere and ionosphere, as is the case at Jupiter's volcanic moon Io (Blöcker et al., 2018) and Saturn's largest moon Titan (e.g., Neubauer et al., 1984, 2006Simon et al., 2015). At Europa, Ganymede, and Callisto the resulting ionosphere-magnetosphere interaction is modified to a varying degree by permanent or induced magnetic moments from the objects' interiors and ionospheres (e.g.

Effect of the Magnetospheric Plasma Interaction and Solar Illumination on Ion Sputtering of Europa’s Surface Ice

“…Deflection of thermal ions around Europa's Alfvén wings reduces the ion surface flux by over an order of magnitude at nearly all locations on the upstream hemisphere (Addison et al, 2021). As shown by Simon et al (2021), the small number of ions which penetrate the wing tubes move almost completely in the corotation direction, since the ionospheric Hall Effect is too weak at Europa to drastically break the symmetry of the ion flow pattern between the sub-Jovian and anti-Jovian hemispheres. In analogy to the case of uniform fields, the influx pattern of these ions onto the upstream hemisphere is therefore determined by the cosine of the angle between the corotation direction and the local surface normal.…”

“…Hence, even when the field perturbations are taken into account, the spatial distribution of the thermal ion flux onto the upstream hemisphere retains something of the bullseye-like distribution seen in the case of uniform fields (Figure 5a). We note that the model of Simon et al (2021) applies a fluid approach to describe ion dynamics. Hence, their model does not capture individual ions from the "edges" of the Maxwellian upstream distribution that impinge onto Europa at large angles against the corotation direction.…”

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

“…where m and n 0 denote the ion mass and number density of the undisturbed upstream plasma (see Equation 23in Simon et al, 2021). Since the flow in the Alfvénic far field is incompressible, the plasma density in that region has a constant value n 0 (Neubauer, 1980).…”

“…In agreement with the estimations of Strobel et al (1990), the average Pedersen conductance in the model ionosphere is set to 〈Σ P 〉 ≈ 4.5 ⋅ 10 4 S. The radius of the obstacle to the flow is R = R T + 3H, with H = 70 km approximately representing the scale height of Triton's atmosphere (Broadfoot et al, 1989). The inclusion of a realistic shape for the ionospheric Pedersen conductance profile comes at the expense of having to set the Hall conductance to Σ H = 0 (Simon, 2015;Simon et al, 2021). However, this limitation does not affect our conclusions since the Hall effect would merely introduce a rotation of the flow pattern in planes perpendicular to the wing characteristics, that is, it would only break the symmetry between the y > 0 and y < 0 hemispheres (Saur et al, 1999;Simon, Saur, Kriegel, et al, 2011;Simon, Kriegel, et al, 2013).…”

Triton’s Variable Interaction With Neptune’s Magnetospheric Plasma

“…Solving for the flow velocity 𝐴𝐴 𝐮𝐮 in the vicinity of the Alfvén wings (see also Simon, 2015;Simon et al, 2021) yields…”