GP013-0006
A New Method for Inferring Paleopoles using Spacecraft Magnetic Field Measurements

Wednesday, 16 December 2020
Poster
Foteini Vervelidou, Massachusetts Institute of Technology, Cambridge, MA, United States, Vincent Lesur, Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, UMR 7154 CNRS/INSU, Paris, France, Benjamin P Weiss, MIT, Earth, Atmospheric and Planetary Sciences, Cambridge, MA, United States, Eduardo A. Lima, MIT-Earth & Planetary Sciences, Cambridge, MA, United States and France Lagroix, IPGP, Paris, France
Abstract:
Inferring paleopoles using spacecraft magnetic field measurements has been a long standing tool to study the evolution of planetary interiors. Under the simplified assumption that the planetary rocks have been magnetized by a central dipole, converting the magnetization direction into paleopole locations helps us track the rotational motion of this imaginary dipole. This in turn can inform us about the presence of a planetary dynamo and its evolution in time.

The key step in obtaining paleopole positions is inverting the magnetic field measurements for the magnetization direction. This inversion is well known to be non-unique. Several techniques have been proposed to tackle this issue but none of them has allowed for concordant paleopole estimates among different studies until now.

Here, we propose a new methodology that relies on the use of vector spherical harmonics. Projecting the magnetization on these basis functions, separates the magnetization into the part that generates the observable magnetic field, the so-called visible magnetization, and the part that generates no magnetic field outside of the sources. We obtain the magnetization direction by searching for the reference system in which the sectorial harmonics of the visible magnetization over a region of interest get minimized. We present the theory behind this method and show results of a series of synthetic tests. Finally, we apply this method to infer paleopoles on Mars and discuss the implications for the Martian dynamo.

Figure 1: A test case. (a) A thermoremanence magnetization distribution is simulated as a magnetization distribution generated by an internal central dipole and a susceptibility distribution (here we use a terrestrial model). The region of interest is shown with the red circle. (b) The radially downward component of the resulting magnetic field (our input). (c) The north and south paleopole locations correspond to the 2 minima of the normalized power of the sectorial harmonics.

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