Imaging the magnetosphere-ionosphere system with ground-based and in-situ magnetometers

Magnetometers provide high-fidelity observations of the Earth's magnetic field from single- and multipoint in-situ vantage points within the magnetosphere and from distributed arrays of ground-based instruments. These magnetometers can be used to diagnose wave processes and both large- and small-scale current systems throughout the coupled magnetosphere-ionosphere system. In this chapter we review the basic concepts of search coil and fluxgate magnetometers and discuss emerging magnetometer technologies. Following this we discuss the utility of ground-based and in-situ magnetometers in diagnosing magnetosphere and ionosphere phenomena and describe the analysis and signal processing techniques used to study magnetometer data. This includes a discussion of field-aligned and ionospheric current systems, wave power and imaging the magnetospheric system with waves which play a crucial role in energetic particle dynamics, field-line resonances and diagnosing magnetospheric plasma density, and finally substorm dynamics and characterizing the substorm current wedge and substorm expansion phase onset.