Seufert, Mario (2012) Callisto: Induction Signals, Atmosphere and Plasma Interaction. PhD thesis, Universität zu Köln.
Callisto’s magnetic field environment and ionosphere are examined using a model for the magnetic fields induced in the satellite’s interior and 3D magnetohydrodynamic (MHD) simulations for Callisto’s interaction with the Jovian magnetospheric plasma. The induction model is also applied to the other Galilean moons Io, Europa and Ganymede to investigate the inductive responses of the satellites assuming the existence of interior conductive ocean and core layers. The first part of this thesis includes a thorough study of the frequencies and amplitudes of the temporary variable part of the magnetospheric field i.e., the inducing or primary fields and the strength of the induced or secondary fields originating in the interiors of the satellites. The primary fields are determined by using models for Jupiter’s intrinsic field, fields generated by the magnetospheric current sheet and fields caused by Chapman-Ferraro currents at the magnetopause boundary. A Fourier analysis of the magnetic field time series along the Galilean moons’ orbits predicted by this composite magnetospheric model yields the frequencies and amplitudes of the primary fields. A second model for the inductive response of a multi-layered conductivity structure based on two separate interior models for each satellite is applied to study the strength of the secondary fields at the surface. The synodic rotation period of Jupiter (∼10 h), the orbital periods of the satellites (from 42 h at Io to 400 h at Callisto) and the solar rotation period (642 h) are identified as the primary periodicities for the inducing fields at the Galilean moons. It is further shown that conductive ocean layers at Callisto and the other satellites should generate detectable magnetic signals for several frequencies in the vicinity of the satellites. The inferred strength of the signals at the surface ranges from 16 nT at Callisto to 210 nT for a magma ocean at Io. Possible conductive core layers, however, do not significantly modify the signals outside the satellites. The interaction of the magnetospheric plasma with the atmosphere and interior of the satellite has been extensively studied for the cases of Io, Europa and Ganymede. The second part of this work represents the first in depth numerical study of Callisto’s plasma interaction. A 3D MHD model, taking into account collision, ionization and recombination processes due to the neutral atmosphere of the satellite, is used to examine Callisto’s ionosphere and magnetic field environment. In addition to the expected modifications of the magnetospheric plasma flow embodied e.g., by the generation of Alfvén wings and the upstream pileup of the magnetic field, the model results indicate a complex behavior of the plasma flow in Callisto’s tail. The model predicts the existence of extended regions where an oppositely directed plasma flow is associated with vertical eddy structures and disturbances of the regions downstream of the Alfvén wings. The plasma densities within the simulation are compared to measured electron density profiles to investigate the underlying reason for the inferred dependence of the generation of an ionosphere on the solar illumination of Callisto’s ram side. A parameter study performed for different configurations of the neutral atmosphere shows that an atmosphere primarily confined to the upstream hemisphere of Callisto suitably explains the measured variability of the ionospheric plasma densities. Additional reasons for this variability are varying conditions for the magnetospheric plasma flow, differences in the solar photon flux and differences in the plasma particle transport towards Callisto’s flanks depending on the solar illumination geometry. So far, observations of Callisto’s airglow yield no direct evidence for the existence of an O2 atmosphere. The interaction model predicts a disk integrated auroral intensity of ∼6 R for earthbound measurements. This value is in agreement with the upper limit of 15 R determined by Strobel et al. (2002). The simulation results for several flyby scenarios are compared to magnetic field data taking into account the predicted induced fields. Even though the plasma interaction signatures give rise to ambiguities regarding the secondary field strength, the existence of a conductive interior ocean layer is inevitable in order to explain the measured data. An ocean at a maximum depth of ∼150 km, with a thickness of ∼10 km and a conductivity of sea water (∼5 S/m), which is in agreement with an interior model by Kuskov and Kronrod (2005a), yields induced signals within the plausible range suggested by the observations. The hypothesis of an asymmetric neutral atmosphere is consistent with both radio occultation and magnetometer measurements performed by the Galileo spacecraft.
Actions (login required)