Field-aligned beam observations at the quasi-perpendicular bow shock: Generation and shock angle dependence

[ 1] On 19 October 1995 the Geotail satellite skimmed along the quasi-perpendicular bow shock for more than 3 hours, and field-aligned ion beams (FABs) were continuously observed in the foreshock region. Also observed were 11 crossings with the bow shock where these FABs are thought to be generated. The upstream condition of the bow shock was in the state of low Mach number (M-A similar to 2.9) and low beta (beta similar to 0.02). By a detailed study of the evolution of the ion distribution across one of the crossings, we have found that leakage as origin of these ions from the magnetosheath is unlikely. Comparison of the observations with simulations suggests that FAB ions are generated by multiple interaction of incoming solar wind ions with the bow shock. We have then compared the FAB flux normalized by the solar wind flux, (F) over bar, with the shock angle theta(Bn) and have found that (F) over bar falls off rapidly just above theta(Bn) > 60 degrees, but it maintains significant level ((F) over bar similar to 0.01%) up to similar to 75 degrees.