Abstract: Geomagnetic data assimilation aims at constraining the state of the geodynamo working at the Earth’s deep interior by sparse magnetic observations at and above the Earth’s surface. Due to difficulty separating the different magnetic field sources in the observations, spectral models of the geomagnetic field are generally used as inputs for data assimilation. However, the assimilation of raw pointwise observations can be relevant within certain configurations, specifically with paleomagnetic and historical geomagnetic data. Covariance localisation, which is a key ingredient to the assimilation performance in an ensemble framework, is relatively unexplored, and differs with respect to spectral and pointwise observations. This chapter introduces the main characteristics of geomagnetic data and magnetic field models, and explores the role of model and observation covariances and localisation in typical assimilation set-ups, focusing on the use of 3D dynamo simulations as the background model.

In Chapter 6.1 we briefly review the main instruments used in today’s ground-based geomagnetic observations, focusing on their performances and working principles (from a user’s point of view). Next, the major measurement methods and systems currently in use will be introduced, with a focus on the latest developments in the field. In Chapter 6.2 electromagnetic (EM) methods will be discussed to study the electrical conductivity structure within Earth in a wide depth range and can be measured at the Earth’s surface by magnetometers and telluric electrodes. In Chapter 6.3 a new technique based on differences in instrument responses from ground-based magnetic measurements that extracts the frequency content of the magnetic field with periods ranging from 0.1 to 100 seconds will be discussed. This method enables the study of field line oscillations using the publicly available, worldwide database of geomagnetic observatories.