Introduction

The solar–terrestrial environment, nowadays sometimes called geospace, includes the upper part of the terrestrial atmosphere, the outer part of the geomagnetic field, and the solar emissions which affect them. It could be defined as that region of space closest to the planet Earth, a region close enough to affect human activities and to be studied from the Earth, but remote enough to be beyond everyday experience. Clearly, it is not the familiar atmosphere of meteorology; nor is it the inter-planetary space of astronomy, though it interacts with both. The material found there is mainly terrestrial in origin and strictly a part of the atmosphere of the Earth, though it is greatly affected by energy arriving from the Sun. Starting some 50–70 km above the Earth's surface and extending to distances measured in tens of Earth radii, geospace is a region of interactions and of boundaries: interactions between terrestrial matter and solar radiation, between solar and terrestrial magnetic fields, between magnetic fields and charged particles; and boundaries between solar and terrestrial matter, and between regions dominated by different patterns of flow.