Field‐aligned ion beams upstream of the Earth's bow shock: Evidence for a magnetosheath source

High time resolution ISEE‐1 and ‐2 observations of upstream field‐aligned ion beams at several crossings of the Earth’s bow shock indicate that some beams are due to high‐energy magnetosheath particles leaking through the shock into the upstream region. The distribution immediately downstream of these oblique (θ Bn ∼ 50°) shocks consists of a ‘core’ of directly transmitted, slightly heated ions plus a crescent‐shaped, high‐velocity distribution, centered roughly on the magnetic field in the direction toward the upstream region, with a fairly well‐defined low velocity cut‐off.