Moritz Feyerabend

The impact of Callisto's atmosphere on its plasma interaction with the Jovian magnetosphere

The interaction between Callistos atmosphere and ionosphere with the surrounding magnetospheric environment is analyzed by applying a hybrid simulation code, in which the ions are treated as particles and the electrons are treated as a fluid. Callisto is unique among the Galilean satellites in its interaction with the ambient magnetospheric plasma as the gyroradii of the impinging plasma and pickup ions are large compared to the size of the moon. A kinetic representation of the ions is therefore mandatory to adequately describe the resulting asymmetries in the electromagnetic fields and the deflection of the plasma flow near Callisto. Multiple model runs are performed at various distances of the moon to the center of Jupiters magnetospheric current sheet, with differing angles between the corotational plasma flow and the ionizing solar radiation. When Callisto is embedded in the Jovian current sheet, magnetic perturbations due to the plasma interaction are more than twice the strength of the background field and may therefore obscure any magnetic signal generated via induction in a subsurface ocean. The magnetic field perturbations generated by Callistos ionospheric interaction are very similar at different orbital positions of the moon, demonstrating that local time is only of minor importance when disentangling magnetic signals generated by the magnetosphere-ionosphere interaction from those driven by induction. Our simulations also suggest that deflection of the magnetospheric plasma around the moon cannot alone explain the density enhancement of 2 orders of magnitude measured in Callistos wake during Galileo flybys. However, through inclusion of an ionosphere surrounding Callisto, modeled densities in the wake are consistent with in situ measurements.

Hybrid simulation of Titan's interaction with the supersonic solar wind during Cassini's T96 flyby

By applying a hybrid (kinetic ions and fluid electrons) simulation code, we study the plasma environment of Saturns largest moon Titan during Cassinis T96 flyby on 1 December 2013. The T96 encounter marks the only observed event of the entire Cassini mission where Titan was located in the supersonic solar wind in front of Saturns bow shock. Our simulations can quantitatively reproduce the key features of Cassini magnetic field and electron density observations during this encounter. We demonstrate that the large-scale features of Titans induced magnetosphere during T96 can be described in terms of a steady state interaction with a high-pressure solar wind flow. About 40min before the encounter, Cassini observed a rotation of the incident solar wind magnetic field by almost 90 degrees. We provide strong evidence that this rotation left a bundle of fossilized magnetic field lines in Titans ionosphere that was subsequently detected by the spacecraft.

Disentangling plasma interaction and induction signatures at Callisto: The Galileo C10 flyby

We apply a combination of data analysis and hybrid modeling to study Callistos interaction with Jupiters magnetosphere during the Galileo C10 flyby on 17 September 1997. This encounter took place while Callisto was located near the center of Jupiters current sheet. Therefore, induction in Callistos subsurface ocean and magnetospheric field line draping around the moons ionosphere both made non-negligible contributions to the observed magnetic perturbations. The induction signal during C10 was obscured by plasma currents to a significant degree, in contrast to previously studied Callisto flybys. Our analysis reveals that at large distances to Callisto, its magnetic environment was dominated by field line draping, leading to the formation of Alfven wings. Closer to the surface and in Callistos wake, Galileo encountered a quasi-dipolar “core region” that was partially shielded from the plasma interaction and was dominated by the induced field. When exiting this “core region,” the spacecraft crossed a rotational discontinuity where the magnetic field vector rotated by approximately 50°. The hybrid model is able to quantitatively explain numerous key features of the observed magnetic signatures, especially the transitions between draping- and dipole-dominated regimes along the C10 trajectory. The model also reproduces the electron number density enhancement by 3–4 orders of magnitude detected in Callistos wake, requiring a substantial ionosphere to surround the moon during C10. For flybys with non-negligible plasma currents, comprehensive knowledge of the incident flow conditions and properties of Callistos atmosphere is required to refine existing constraints on the subsurface ocean (conductivity, thickness, depth) based on magnetic field data. These findings are highly relevant for the upcoming JUICE mission, which will include multiple Callisto flybys.

Magnetic signatures of plasma interaction and induction at Callisto: The Galileo C21, C22, C23, and C30 flybys

We apply a combination of analytical modeling, hybrid simulations, and data analysis techniques to provide a comprehensive study of magnetometer data from four Galileo flybys of Callisto (C21, C22, C23, and C30) that have never been discussed in the literature before. Callisto’s distance to the center of Jupiter’s magnetospheric current sheet varied considerably from flyby to flyby. Therefore, the relative strength of the magnetic field perturbations due to Callisto’s plasma interaction with Jupiter’s magnetosphere and induction within Callisto’s subsurface ocean drastically changed as well. During C21, a strong magnetic field perturbation along the corotation direction was detected in Callisto’s geometric plasma shadow. This enhancement can be explained with Callisto’s steady state plasma interaction only, if the upstream flow possessed a nonnegligible component away from Jupiter. During C22, Galileo only grazed Callisto’s Alfven wings which were elevated out of the flyby plane due to the ambient magnetospheric field orientation. During C23, the combination of an inclined flyby trajectory and finite gyroradius effects caused Callisto’s observed Alfven wings to be slightly asymmetric between both hemispheres. During C30, a discontinuity with a surface normal pointed toward Jupiter was detected within Callisto’s geometric plasma shadow, similar to the earlier C10 flyby. Due to strong plasma interaction and an unfavorable flyby geometry (C21), a large closest approach altitude (C22), or weak inducing field (C23 and C30), no discernible induction signatures were observed during these four flybys. Based on data from all available Galileo flybys, we determine requirements on future flyby geometries that must be satisfied for an identification of Callisto’s subsurface ocean in magnetometer data.

A Three‐Dimensional Model of Pluto's Interaction With the Solar Wind During the New Horizons Encounter

We apply a hybrid (kinetic ions, fluid electrons) simulation model to study Plutos plasma environment during the New Horizons encounter on 14 July 2015. We show that Plutos plasma interaction is dominated by significant north-south asymmetries, driven by large pickup ion gyroradii on the order of 200 Pluto radii. The transition region from the ambient solar wind to the population of plutogenic ions (called the “Plutopause”) also shows considerable asymmetries that cannot be explained by a fluid picture. Since the New Horizons spacecraft does not carry a magnetometer, we use our model to estimate the strength and direction of the interplanetary magnetic field (IMF) at the time of the flyby by comparing output from the hybrid simulation to the plasma signatures observed during the New Horizons encounter. We find that an IMF strength of at least 0.24 nT is required to generate the observed plasma signatures. An IMF orientation either parallel or anti-parallel to Plutos orbital motion is able to explain the observed plasma densities and velocities along the New Horizons trajectory. Our simulations are able to quantitatively reproduce all key features of the plasma observations, specifically the gradual slowing of the solar wind, as well as the location and thickness of the Plutopause and bow shock.