On the theoretical/observational comparison of field‐aligned ion beams in the Earth's foreshock

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Abstract
We demonstrate that the de Hoffman‐Teller frame, the transformation for which removes the motional V × B electric field while keeping a planar shock at rest, is a logical choice for data analysis. Previous comparisons between local planar theories and observations of backstreaming ion beam velocities are streamlined and related directly to the one physically meaningful angle, between the magnetic field and shock normal, by transforming the data into this frame. Our limited data set suggests that both reflection processes and leakage ones may be operating at different times. A larger survey employing the techniques described here could provide confirmation of our tentative conclusions.