Observations of two distinct populations of bow shock ions in the upstream solar wind

Observations upstream of the earth's bow shock with the LASL/MPI fast plasma experiments on ISEE 1 and 2 reveal the presence of two distinct and mutually exclusive populations of low energy (≤ 40 keV) ions apparently accelerated at the bow shock. The first of these, the "reflected" population, is characterized by 1) sharply peaked spectra seldom extending much above ∼ 10 keV/ion and 2) relatively collimated flow coming from the direction of the shock. On the other hand, the "diffuse" ions are distinguished by relatively flat energy spectra above ∼ 10 keV and broad angular distributions. They are by far the most commonly observed upstream ion event. A close causal association is suggested between the diffuse ion population in the upstream solar wind and energetic plasma ions observed within the magnetosheath.