Intrinsic magnetic field is one of the fundamental properties of a planetary body. The existence of a dipole-dominant, global-scale magnetic field at saturn was discovered with in-situ magnetic…

This is a $202,919.76 grant awarded by the National Aeronautics and Space Administration (NASA) to the California Institute of Technology (Caltech) to conduct research and analysis on Saturn's intrinsic magnetic field. The research aims to characterize key features of Saturn's magnetic field using data from the Cassini mission's magnetometer instrument, and interpret these characteristics in terms of Saturn's interior structure and dynamics. The research will involve magnetic equator crossing analysis, forward calculations, and numerical magnetohydrodynamics modeling. The work is in support of the Cassini Solstice Mission's objective to determine Saturn's rotation rate and internal structure. This grant does not have a set-aside designation.

Intrinsic magnetic field is one of the fundamental properties of a planetary body. The existence of a dipole-dominant, global-scale magnetic field at saturn was discovered with in-situ magnetic field measurements made by pioneer 11 flyby in 1979. Our knowledge about the features of saturns magnetic field has been subsequently refined with in-situ magnetic field measurements made during voyager 1, voyager 2 flybys and those made with the ongoing cassini mission. Spherical harmonic analysis based on these data have shown that saturns magnetic field is weaker than expected, is extremely axisymmetric, does possess non-negligible north-south asymmetry, features extremely slow secular variation, and features strong pole-to-equator field strength contrasts. Here we propose to conduct novel analysis based on cassini magnetometer (MAG) data to characterize key features of saturns intrinsic magnetic field and to interpret these characteristics in terms of interior structural and dynamical features of saturn. For measurements near the equatorial plane during each cassini orbit within 6 saturn radii, we will analyze magnetic equator crossings. Here magnetic equator is defined as where the cylindrical radial component of the magnetic field vanishes. Such analysis will yield ratios between the first few axial moments of saturns intrinsic magnetic fields, and suffers significantly less uncertainties compared to spherical harmonic analysis given the non-circular nature of cassini orbits. Well-organized longitudinal variations of the magnetic equator positions, if identifiable when searching the range of possible rotation periods of saturn, will serve as the first evidence of nonaxisymmetry in saturns intrinsic magnetic field and yield the rotation rate of saturns deep interior. Monitoring the time evolution of the magnetic equator positions throughout the cassini ERA will enable a direct assessment of the time evolution of the intrinsic magnetic field. We will then turn to measurements made at high-latitudes yet close enough to the planet. We will conduct forward calculations as well as regularized inversion based on these high-latitudes measurements to estimate the acceptable equator-to-pole magnetic field variations as well as possible fine-scale latitudinal modulations. To interpret the observed characteristics of saturns magnetic field (e.g. high degree of axisymmetry, strong pole-to-equator field strength contrast), we will conduct numerical magnetohydrodynamics (MHD) calculations using the community dynamo code magic. Here we emphasize one feature that distinguishes the magnetic field generation process inside gas giants from those inside terrestrial planets: the smooth transition of electrical conductivity from the deep interior towards the outer region inside gas giants. We will investigate to what extent do the properties of the observable part of saturns intrinsic magnetic field depend on the properties of and the dynamics within the transition region. We will in particular address the MHD consequences of equatorial zonal jets and multiple high latitude zonal jets (these deep zonal jets can be orders of magnitude weaker than those observed at the cloud level and yet have significant effects on the magnetic field in the transition region) employing numerical MHD calculations. The proposed investigation incorporates analyzing cassini magnetometer (MAG) data and modeling efforts to facilitate the interpretation of data, thus supports the objective of the NASA cdaps program (c.10) to enhance the scientific return of the cassini mission by broadening the scientific participation in the analysis and interpretation of the data returned by the mission. Moreover, the proposed work directly responses to the mission-level science objective, SN1A, of the cassini solstice mission: determine saturns rotation rate and internal structure despite the planets unexpected high degree of axisymmetry.