Gyrophase‐restricted 100 keV–2 MeV ion beams near the foreshock boundary

We report on gyrophase‐restricted ion beams with energies extending from ∼100 keV up to ∼2 MeV, observed by Wind in the Earth's distant (∼65 R E ) foreshock. The ion gyrophases seen were nearly constant during periods when the distance to the shock contact point could be expected to vary by several R E , when there was no significant wave activity. At times the distributions had two peaks ∼180° apart in gyrophase. These were consistent with a remotely‐sensed energetic ion foreshock region having a thickness <2 gyroradii (∼1.5 R E for 0.5 MeV protons with pitch‐angle α = 30°). In this picture, gaps in phase space would correspond to particles with guiding centers outside of the energetic foreshock region. Similar observations over a decade of energies (100 keV–2 MeV) suggest that the region thickness scales with gyroradius. According to this interpretation, we have determined a rough range of geometries for which energetic particle production is favored. (Eg., θ B n ∼70–80° for 500 keV ions with α = 30°.)