Deceleration of the solar wind upstream from the Earth's bow shock and the origin of diffuse upstream ions

Observations with the Los Alamos Scientific Laboratory/Max‐Planck‐Institut crossed‐fan solar wind ion experiment on ISEE 1 reveal that the solar wind is decelerated and deflected away from the direction of the earth’s bow shock as it enters that portion of the upstream region populated by ‘diffuse’ bow shock ions and long‐period (10–60 s) waves. Typically, the average directed velocity vector changes by 7–10 km s −1 as it enters the wave region. At times, average speed changes as large as 25–40 km s −1 are observed. Superposed upon these changes in average flow speed are large amplitude (∼±15 km s −1 ) fluctuations in flow speed associated with the waves themselves. The observations suggest that the solar wind deceleration is the result of momentum transfer from ‘reflected’ bow shock ions to the wind via the long‐period waves as the reflected ion beams go unstable. The broad angular distributions of the diffuse ions thus appear to be produced as a consequence of the disruption of reflected ion beams.