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 eff ects of energetic ion bursts on the ambient solar wind for comparison with model predictions. Owing to the spiral interplanetary magnetic field orient ation, the events are far more common upstream from the prenoon than postno on bow shock. Pressures associated with the energetic ions depress foreshoc k magnetic field strengths and plasma densities. The magnitude of the depre ssion is proportional to the intensity of energetic ions. The excavated pla sma and magnetic field sometimes pile up in narrow regions of enhanced plas ma densities and magnetic field strengths, but depressed flow velocities, j ust outside the foreshock cavities. Typical amplitudes of the depressions a nd 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 obser ved just outside the bow shock. The cavities occur preferentially during hi gh-speed solar wind streams but show no clear dependence upon other solar w ind parameters. The distribution of burst durations resembles those for int erarrival times for interplanetary magnetic field discontinuities, magnetop ause motion, and flux transfer events, suggesting causal relationships betw een these phenomena.