Ahmed M. Hamza

A review of field‐aligned beams observed upstream of the bow shock

For more than two decades the Earth’s bow shock and traveling interplanetary shocks have attracted much attention as researchers have attempted to understand the collisionless mechanisms that thermalize transmitted particles and accelerate those that are observed propagating away from the shock into the upstream. We are concerned here with the class of particles emerging from the shock that are field‐aligned and have energies of a few to several keV, and base our results on observations primarily from the Earth’s foreshock. While the basic empirical picture has been known for some time, fundamental questions about the underlying mechanisms producing them have resisted a comprehensive explanation. This review talk will begin with an overview of the observational framework, along with selected new results. The latter include recent refinements in the characterizations of upstream field‐aligned beams as a function of the shock geometry parameter θBn. Other observations from the Cluster spacecraft have shown ...

Effect of Shock Normal Orientation Fluctuations on Field-Aligned Beam Distributions

We address the unsolved question of how foreshock field-aligned beam (FAB) parallel temperatures are produced. Studies including numerical simulations and recent observations have indicated that shocks can be nonstationary and include embedded spatial structures with varied scales. As a first step towards assessing the impact of such variability on backstreaming ions, we examine how a randomly distributed shock normal direction will affect FAB parallel velocity (vk) distribu- tions. Assuming that the FABs are produced in a quasi-adiabatic reflection process at the shock, we derive a probability distribution function for vk. These derived distributions exhibit second, third and fourth order moments that agree well with the observations for a large range of reflection efficiencies i, and depend strongly upon the average angle between the magnetic field and the shock normalBn0 .B est agreement is obtained for fluctuations of the normal orientation of a few degrees about a nominal direction. The derived model predicts a strong correlation between the shock geometry (� Bn0) and the moments of the parallel velocity distribution, but with stronger tails extending to higher values ofBn0, a trend opposite to the observations.