Abstract
AbstractMagnetic field observations from low-Earth-orbiting satellites provide a unique means of studying ionospheric current systems on a global scale. Such studies require that estimates of other sources of the Earth’s magnetic field, in particular, the dominant main field generated primarily in Earth’s core but also due to the magnetized lithosphere and large-scale magnetospheric currents, are first removed. Since 1999 multiple low-Earth-orbit satellites including Ørsted, CHAMP, SAC-C, and most recently the Swarm trio have surveyed the near-Earth magnetic field in increasing detail. This chapter reviews how models of the main magnetic field are today constructed from multiple satellites, in particular discussing how to take advantage of estimated field gradients, both along-track and across-track. A summary of recent results from the Swarm mission regarding the core and lithospheric field components is given, with the aim of informing users interested in ionospheric applications of the options available for high accuracy data reduction. Limitations of the present generation of main field models are also discussed, and it is pointed out that further progress requires improved treatment of ionospheric sources, in particular at polar latitudes.
Highlights
Ionospheric current systems produce magnetic fields that are measured by magnetometers on low-Earth-orbit satellites, together with the magnetic fields produced by a wide range of other natural sources
As example, constructing three rows of Gi associated with three components of a vector magnetic field measurement in the vector fluxgate magnetometer (VFM) frame involves (i) taking the spherical polar gradients of the expansions for internal and external potentials (12.8) and (12.11), (ii) multiplying these with the Euler rotation matrices R1 · R2 · R3 from the previous iteration to rotate to the VFM frame, (iii) taking the derivatives with respect to each model parameter in turn, such that each column of Gi refers to a derivative with respect to a different model parameter), and evaluating the elements using the model parameters from the previous iteration mi
In order to illustrate the form of signals seen in field gradient data, and how these are related to field data, we present here scalar field and scalar field difference signals collected by the Swarm satellites on example day-side (Fig. 12.4) and night-side (Fig. 12.5) half orbits
Summary
Ionospheric current systems produce magnetic fields that are measured by magnetometers on low-Earth-orbit satellites, together with the magnetic fields produced by a wide range of other natural sources The largest of these sources is the so-called ‘main’ magnetic field, generated in Earth’s liquid metal outer core through motional induction in a process known as the geodynamo (e.g. Roberts and King 2013). For those interested in precise studies of ionospheric currents it is important to remove a high-resolution estimate of the internally-generated field, capturing as far as possible its small-scale structure and secular time dependence (see e.g. Stolle et al 2016). Limitations of all existing main field models, and opportunities to improve them using our expanding knowledge of ionospheric processes are discussed at the end of this chapter
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