Solar wind preconditioning in the flank foreshock: IMP 8 observations

We use IMP 8 plasma, magnetic field, and energetic ion observations within the Earth's foreshock from January through August 1995 to determine the effects of energetic ion bursts on the ambient solar wind for comparison with model predictions. Owing to the spiral interplanetary magnetic field orientation, the events are far more common upstream from the prenoon than postnoon bow shock. Pressures associated with the energetic ions depress foreshock magnetic field strengths and plasma densities. The magnitude of the depression is proportional to the intensity of energetic ions. The excavated plasma and magnetic field sometimes pile up in narrow regions of enhanced plasma densities and magnetic field strengths, but depressed flow velocities, just outside the foreshock cavities. Typical amplitudes of the depressions and enhancements at IMP 8 far upstream from the bow shock are far less (20% and 10%, respectively) than those seen in past case studies of events observed just outside the bow shock. The cavities occur preferentially during high-speed solar wind streams but show no clear dependence upon other solar wind parameters. The distribution of burst durations resembles those for interarrival times for interplanetary magnetic field discontinuities, magnetopause motion, and flux transfer events, suggesting causal relationships between these phenomena.