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New constraints on Mercury's internal magnetic field Uno, Hideharu

Abstract

Three-component vector magnetic field observations from MESSENGER’s first two flybys of Mercury have confirmed the presence of an internal field, along with external fields related to magnetospheric current systems. We use techniques in inverse theory to investigate structure in Mercury’s inter nal magnetic field permitted by the Mariner 10 and MESSENGER flyby data, and structures recoverable while spacecraft is in orbit. We remove external fields predicted by a parameterized magnetospheric model from the flyby observations. We estimate noise contributions from long-wavelength uncertainties in the external field and from un-modeled short-wavelength features. Internal field models are parameterized to spherical harmonic degree and order 8, with regularization constraints applied to the power spectrum. The field is predominantly dipolar but additional latitudinal and longitudinal structure is required to fit the data. Enhanced radial magnetic field in the region of the Mariner 10 and MESSENGER flybys latitudes is seen. Contributions to the internal field predicted by Aharonson and others for a long-wavelength crustal field are present (namely, the g⁰₁, g⁰₃, and g²₃ spherical harmonic coefficients), but our hypothesis testing has shown that the field is dominated by the g⁰₁ term rather than the proposed g⁰₃ term. Further hypothesis testing has determined that the dipole tilt of Mercury is in the range of 30 to 13° with 13° as a strict upper bound. Observations from the upcoming MESSENGER flyby will provide additional, and critical, low-latitude coverage. Analyses of flyby data have shown that much of the recovery internal field depends successfully characterizing the external field signatures that are present in the data. Currently, the limited ninnber of ob servations prevents us from constructing a reliable external field model, but these external field models will improve as data becomes more abundant. We also investigate recovery of three simulated core fields using synthetic data during MESSENGER’s orbital phase, under the assumption that long wavelength external fields can be modeled and removed. The results show excellent recovery of the dipole field and of field structure at mid-northern to high latitudes out to degree and order 10, providing encouraging results for future identification and characterization of core fields.

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