Observations and measurement techniques

There have now been over 50 years of in situ measurements of the ring current. As the measurement capabilities have increased, and more missions have contributed, a remarkably consistent picture has developed of the characteristics of the ring current. In this paper, we review the observations of the ring current, starting from the satellites in the early 60’s, the breakthrough of Explorer 45 identifying the particle population that creates the ring current, and the surprises when the first composition measurements at low energies were made, showing the importance of the ionospheric plasma. We present the measurements of composition of the bulk of the ring current that became possible with the AMPTE/CCE and CRRES missions. We then discuss the new global view of the ring current provided by energetic neutral atom imaging. Finally, we present the results of the Cluster, Van Allen Probes, and Arase missions, that have allowed unprecedented observations of both the small-scale and large-scale temporal, and spatial dynamics of the ring current.