The region upstream of the earth's bow shock has been known for more than a decade to be rich in both wave and particle phenomena. This region of geospace has proven to be a natural laboratory for the study of particle acceleration mechanisms, wave-particle interactions, and collisionless-shock-associated phenomena. Interest in upstream waves and particles has recently experienced an increase in popularity coincident with the arrival of data from new and sophisticated instruments carried by the three International Sun-Earth Explorer (Isee) spacecraft.
The upstream ion population is now known to consist of at least two distinct components, the ‘reflected’ and ‘diffuse’ components. The reflected component is basically an ion beam that is moving upstream, away from the bow shock, with a typical speed that is approximately 2-3 times the solar wind speed. The diffuse component is characterized by a much broader pitch angle distribution (roughly isotropic in the spacecraft frame) and an energy spectrum that extends to much higher energies (> 100 keV). The density in both populations is ∼0.1/ cm3 (∼1% of the solar wind density).