The satellite along-track analysis in planetary magnetism

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 Earths 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.