Impact of Satellite Orbit Altitudes on the Global Induced Magnetic Fields

Satellite magnetic data contain significant information about the Earth's interior electrical structure. However, the altitudes of satellites vary over time and latitude. Theoretically, the signals from the external magnetosphere and ionosphere, along with the induced magnetic field from the Earth, exhibit considerable variation at different altitudes. The specific effects of altitude variations on the global electromagnetic induction signal, and their subsequent impact on the transfer function, are not yet fully understood. Given that the orbital altitude of the Challenging Minisatellite Payload (CHAMP) satellite changes continuously, we have calculated the Q-response and the C-response at different altitudes through numerical simulations and CHAMP satellite data, respectively. The Q-response represents the ratio of coefficients of the internal and external fields, while the C-response, derived from the Q-response, is altitude-dependent. The findings indicate that the amplitudes of the induced (internal) magnetic field decrease with increasing altitude, with discrepancies exceeding 10 nT, particularly in mid-latitude regions. Conversely, the inducing (external) fields increase with altitude. The Q-responses exhibit distinct behaviors across different degrees of Gaussian spherical harmonics (SHs). Specifically, the real part of the orbital C-response increases with satellite altitude, although higher degrees tend to yield lower magnitudes. This study offers insights into the design of satellite orbits in magnetic field measurements.