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 θ_Bn. Other observations from the Cluster spacecraft have shown the occurence of a very sharp boundary separating FABs and gyrating ion populations in the foreshock. The Wind spacecraft has seen FABs at distances in excess of ∼100 R_E from the Earth, indicating lifetimes greater than expected from linear theory of the ion‐ion streaming instability. These observations prompt new questions. Some analytic calculations will be reviewed briefly. Models based upon the guiding center approximation and those which introduce diffusion as a means of enhancing the fluxes of upstream beams fail to produce the properties observed.