The satellite along-track analysis in planetary magnetism

Abstract

S U M M A R YMany 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.

Swarm SCARF Dedicated Lithospheric Field Inversion chain

“…This difficulty is illustrated, for instance, by the expression of the zonal external field coefficient q 0 l that would be estimated from the magnetic field vector data measured only along a complete meridian and that would then be used to correct the data along this meridian. It can be shown that (Thébault et al, 2012) …”

“…Fortunately, one can further lessen such bias along satellite tracks (Eq. (6)) by applying the correction on large portions of orbits (Thébault et al, 2012). Therefore, all satellite tracks containing segments longer than about 2000 km without data are rejected.…”

“…This approach successfully led to a suite of lithospheric field models with unprecedented spatial resolution (SH degree 133 for the MF7 model). Even though specific problems related to spectral leakage can arise (Sabaka and Olsen, 2006;Thébault et al, 2012), implying that all SH coefficients are not necessarily well resolved, these models clearly revealed striking regional correlations between magnetic anomalies and continental geological bodies at small spatial scales.…”

A Swarm lithospheric magnetic field model to SH degree 80

“…The signal can furthermore be masked by time-varying external fields that create anisotropic errors which are aligned along the satellite orbits. Attempts to correct for these contaminations can be done by along-track analysis (Maus et al 2006a;Thébault et al 2012) or statistically , but the transient nature of these disturbances makes these approaches imperfect. As a result, satellite lithospheric field models often show disagreements in their sectorial SH terms.…”

“…For each low-latitude satellite track, we fit an SH external dipole (three parameters) with its internally induced zonal dipole part (one parameter) in the inclined internal dipole reference frame and remove this model from the measurements. Such along-track filtering must be applied with care as it also tends to filter out genuine lithospheric magnetic field structures (e.g., Maus et al 2006a, b) and to introduce artifacts (Thébault et al 2012). To minimize its adverse effect, we first subtract the lithospheric field contribution from SH degrees 16-45 of the model derived in process 2 from the scalar and vector measurements before estimating the alongtrack correction.…”