Foreshock Preconditioning and its Magnetospheric/Ionospheric Effects

David Sibeck
JHU/Applied Physics Laboartory

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. Case studies confirm that pressures associated with the energetic ions depress foreshock magnetic field strengths and densities and that the magnitude of the depression is proportional to the flux of energetic ions. By contrast, narrow regions of enhanced densities and magnetic field strengths, but depressed velocities, lie just outside the foreshock cavities. The statistical survey indicates that magnetic field strengths typically decrease by 20% within the cavities, but rise by 10% on their boundaries. The cavities are most likely in high-speed solar wind streams. The distribution of burst durations is similar to that for interarrival times for interplanetary magnetic field discontinuities, magnetopause motion, and flux transfer events, suggesting that these processes are interrelated.