The Sun-Earth interaction is a complex system of multi-scale processes. The spatial scales of interest vary from the mega-meter size of solar corona structures to the few hundred kilometers of the terrestrial magnetopause and even less when kinetic effects need to be considered. The temporal variations also span a wide range of scales, from thousands of years for the hydrological ocean cycles driven by the total solar radiation to scales of minutes and below for particle acceleration in magnetic reconnection. In this chapter we introduce the building blocks of the Sun-Earth system and briefly describe its important components. Solar disturbances such as solar flares and coronal mass ejection (CME) have the largest impact on geomagnetic activity, especially magnetic storms. Magnetic storms are responsible for large depressions in the horizontal (H) component of the Earth’ surface magnetic field. The strength of a storm is quantified by the Dst index, which is a local time average of the depression measured along the magnetic equator. The depression during a storm is caused by a ring current around the Earth with additional contributions from the magnetopause and tail currents. We review recent developments of empirical prediction algorithms for the Dst index using observations made upstream of the Earth, and alternative procedures based on the same concept including neural networks and the NARMAX method. Future improvements in empirical prediction will require more data from extreme events, additional physical insight to identify the role of other processes, and better measurements of the inputs to the system.

In this chapter we provide an introduction to different fields of geomagnetism studies rather than to this book as a whole. The aim is to present the wide, temporaly variable and spatialy heterogeneous subjects covered by geomagnetism studies. Individual parts of this chapter address subjects extending from geodynamo through historical records of the geomagnetic field, its present dynamics, its interaction with solar wind and coronal mass ejections, its influence on the electrodynamics of plasma in near-Earth space,hazards of space weather, and finally to the importance of the geomagnetic field for the biosphere. Individual parts, written by top experts in these fields, are different in both style and length; and in view of the vast range of topics covered, some of the subjects introduced here are not presented in greater details in the whole volume. We believe that this chapter shows the many facets of the role of the geomagnetic field and associated physical phenomena, taking place in our natural environment, some of which may have significant effects on human lives and the technology that we use today. The chapter demonstrates the attractiveness and usefulness of geomagnetism and aeronomy studies in a form suitable for both experts and non-specialists.

The ionosphere boundary between the magnetosphere and atmosphere is often considered thin in the magnetosphere-ionosphere-thermosphere system. This approximation is not valid at the inner boundary, where height variation is important in ionosphere-thermosphere (I-T) coupling, particularly with respect to momentum/energy transfer. Here the Cowling channel and energy coupling between regions are better modelled including altitude variations. In the equatorial region the equatorial plasma fountain results from a field perpendicular ExB drift and field aligned plasma diffusion, while the equatorial ionisation anomaly is formed by removal of equatorial plasma by upward ExB drift. Under magnetic storm conditions an eastward prompt penetration electric field and neutral winds contribute. The polar cap ionosphere and auroral zones transfer solar wind energy into the magnetosphere. In the polar cap key indicators for energy/momentum transfer to the solar wind I-T system are the cross-polar cap potential/electric field, and the relationship to the interplanetary magnetic field where linear and non-linear relationships may occur. Models have been produced to describe various aspects of the coupled system. In the auroral zones aurora are associated with different regions and processes; substorm-associated aurora, shock associated aurora, pulsation aurora, cusp aurora and mid-latitude aurora. These categories and recent models are referenced.

In the context of space weather effects, magnetosphere-ionosphere coupling is one of the fundamental processes controlling energy transfer and dissipation in geospace. Alfvén waves appear to play a key role in this coupling, specifically in coupling the dynamics of magnetospheric convection to the ionosphere and in generating the region 1 and region 2 global field-aligned current systems. The momentum transport from the magnetosphere to the ionosphere can be described as the result of the generation and propagation of Alfvén waves, for example as arising along newly reconnected magnetic field-lines, and in general in terms of their incidence on and reflection from the ionosphere. The thermosphere experiences dramatic changes in density and composition during magnetic storms. Intense Joule heating and particle precipitation at auroral latitudes cause intense thermal expansion, air upwelling and strong wind circulations. The Joule heating at E-layer altitudes can cause both density enhancements and depletions at higher altitudes, and complicate the interpretation of mass density anomalies at high latitudes. The thermospheric response to storms at middle and low latitudes is less complicated, where the averaged density enhancement is linearly proportional to the solar wind input. Magnetic substorms during active periods also cause mass density perturbations. Magnetic storms and substorms can cause disturbances up to thousands of nT at the Earth’s surface. The time derivative of the magnetic field provides a proxy for the associated geoelectric field, which can drive geomagnetically induced currents in Earthed conductors. The geoelectric field is thus a key quantity for space weather effects on technological systems such as power grids, and it can be obtained by modelling the magnetic field using ionospheric currents and model ground conductivity as inputs.

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.

The geomagnetic field supports a wide range of magnitudes, spatial scales and temporal variations. Outlined here are particular recent advances in temporal variability, stretching from geomagnetic field polarity reversals over millions of years, through secular field variations and ultra-low frequency (ULF) waves (1mHz – 5Hz), to very low frequency(VLF) emissions with frequencies in the kHz range. Long-term variations are discussed with respect to paleomagnetic, geological and archaeological records. Both external and internal fields contribute to temporal variations on decadal to daily time scales. More rapid oscillations at ULF wave frequencies associated with Sun-Earth connection contribute to weather in space. These involve the magnetosphere, ionosphere and atmosphere system, and may affect charged/neutral particle populations. Waves are generated external and internal to the magnetosphere and through integration of global magneto-hydrodynamic or local magneto-ionic modelling with satellite and ground observations, progress has been made in understanding the dynamics of waves and energy transfer within the coupled system. Equally important to space weather is the understanding of ULF and VLF waves on energetic charged particles in the Van Allen radiation belts during geomagnetic storms.

Since the discovery of the magnetosphere-magnetotail system in the1950s-1960s), and the associated beginning of the satellite era, we have gained a well-informed understanding of this space plasma region permeated by the geomagnetic field and home to a variety of charged particle populations and plasma waves. Over the last six decades, IAGA has played an important role in supporting international magnetospheric research. Here we provide an overview of recent developments in energy transport from the solar wind into the Earth’s environment. Topics include, magnetosphere energy input, the role of the boundary layer. Solar wind interaction with the magnetosphere creates geomagnetic activity and the response of the region leading to sub-storms and steady magnetospheric convection are discussed. The charged particle energy (eV to MeV) inherent/contained in the magnetospheric ring current and Van Allen radiation belts establish many properties of the region, giving rise to boundary regions and waves. Results from recent state of the art and currently operating Earth orbiting satellites (Cluster, THEMIS, Van Allen Probes, Magnetosphere MultiScale), are providing exciting new results. Waves from magnetospheric scale ultra-low frequency (ULF) from a few milliHertz, up to upper hybrid waves and continuum radiation in the 1-2 MHz band. Finally, current understanding of the plasmasphere and associated boundary the plasmapause, are considered.