On the Field-Aligned Beam Thermal Energy

Abstract

The parallel and perpendicular reduced distribution functions of field-aligned beams (FABs) observed upstream of the Earth's bow shock using the Cluster spacecrafts are examined. A previous study revealed that FABs, observed in oblique shock geometries, exhibit reduced distribution functions with high-energy tails. A selection of FABs with weak-energy tails are considered, and the associated reduced distributions are fit with Maxwellians. First, we have found that the FABs full width at half maximum (FWHM), σ_∥ and σ_⊥ derived from the fit, are linearly correlated with the solar wind speed (or equivalently to solar wind temperature). Moreover, the parallel beam σ_∥ has a very weak dependence upon the beam parallel speed which reflects the shock geometry; we have found that σ_∥∼0.23V_sw. In contrast, we have found that the perpendicular beam σ_⊥, in the range of beam speeds investigated, depends on the shock geometry. These new results indicate that the parallel σ_∥ is essentially controlled by the solar wind while the shock geometry plays, along with the solar wind, a role in the perpendicular σ_⊥. These results also put some strong constraints on theoretical models as far as field-aligned beam production mechanisms are concerned. One potential explanation for the significant perpendicular broadening of the FAB distribution reported in this study could be the presence of kinetic Alfvèn (or/and whistler) turbulence at the shock.