The satellite along-track analysis in planetary magnetism

Erwan Thébault,Fotini Vervelidou,Mohamed Hamoudi,Vincent Lesur

doi:10.1111/j.1365-246x.2011.05281.x

Abstract

SUMMARY Many physical characteristics of planets, such as their topography, magnetic and gravitational fields, are routinely detected and measured by spacecrafts. At satellite altitudes, even if little is known about the measured signal, it is possible to separate the large-scale components from other contributions by a spectral analysis carried out along the spacecraft orbit. This procedure, which dates back to the early age of the satellite era, is routinely applied with spherical harmonics analysis for the study of large-scale planetary magnetic fields, particularly those that vary rapidly in time. In this paper, we review the approximations of this procedure for investigating internal and external magnetic fields of planets. We show that the magnetic field analyses along the orbits are limited by a finite frequency resolution that is known to introduce a spectral leakage. This leakage may lead to artificial spatio-temporal variations of the magnetic field, such as an apparent internal field secular variation, an asymmetry of the magnetospheric field and a regional distortion of the lithospheric field structures. We quantify the errors for the Earth’s lithospheric field and display its distribution in the spatial and the spectral domains. We also discuss these limits for the Moon and Mars lithospheric magnetic fields. Our analytical results illustrate the pros and cons of the orbit-by-orbit analyses and should allow us to avoid the pitfall of geophysical overinterpretation of some artificial magnetic field variations.

Highlights

Satellite missions continue to provide crucial measurements of the physical properties of the Earth and other planets in our solar system
The spectral leakage is the conversion of the energy normally carried by a term of the infinite series that was excluded from the solution into another one that belongs to truncated model selected a priori (Trampert & Snieder 1996 for a discussion in tomography)
We attempt to address the problem in a formal way to stay as general as possible we have to restrict ourselves to an ideal case that we mathematically describe in the first section

Summary

INTRODUCTION

Satellite missions continue to provide crucial measurements of the physical properties of the Earth and other planets in our solar system. A priori knowledge or measurements of the planetary magnetic field are available to guide the model parametrization, an effective technique called sequential analysis can be used. It requires a careful visual inspection of the data in which each source contributing to the total magnetic field is individually and empirically identified and subtracted from the measurements. We attempt to address the problem in a formal way to stay as general as possible we have to restrict ourselves to an ideal case (a satellite moving along a circular polar orbit) that we mathematically describe in the first section This case allows us to put a natural emphasis on the Earth, whose data coverage and quality are by far the best and the most comprehensive. We discuss the problems raised by the along-track analysis when these large-scale magnetic field estimates are used as a data correction with the purpose of discovering the smaller lithospheric field structures

DEFINITIONSANDHYPOTHESESOFTHEALONG - T R AC K A NA LY S I S

M AG NETICFIELDA NA LY SESALONGSINGLEORBITS

IMPLIC AT IONSFORC RU S TA LFIELDIDENTIFIC AT I O N

Data gridding and anomaly maps

Comparisons between initial and the estimated anomaly fields

DISCUSSION