Introduction

When the phrase solar–terrestrial activity is used, the intent is to describe those changes of energetic particles and electromagnetic fields that originate at the Sun, travel to the Earth's magnetosphere, and have drastic effects upon the Earth's atmosphere and geomagnetic field. The activity is on time scales that are short in the human perception of events. The Sun is said to be “active” when the magnitude of such changes is distinguishably large with respect to the average behavior over tens of years. A specific region or process on the Sun is said to be an active source region when a particle or field disturbance in the Earth's magnetosphere can be traced to some special change in that region of the Sun. The vagueness in these definitions should disappear as we become more specific in the description of such phenomena as sunspots, flares, coronal holes, coronal mass ejections, solar wind, geomagnetic storms, ionospheric disturbances, auroras, and substorm processes.

We call the moving plasma of ionized particles and associated magnetic fields that are expanding outward from the Sun the solar wind. Its associated field is the interplanetary magnetic field (IMF). The wind exists out past 150 times the Sun–Earth distance because the pressure of the interstellar medium is insufficient to confine the energetic particles coming from the hot solar corona. We call this solar-wind dominated region the heliosphere.

Outer space is filled with particles and fields originating from the formation of the universe and from stars.