A quantitative theory for terrestrial foreshock radio emissions

We present the first quantitative theoretical results for the spatial distribution and flux levels of fundamental ( f p ) and second harmonic (2 f p ) plasma radiation generated in Earth's foreshock by beam‐driven Langmuir waves. The theory predicts that both the f p and 2 f p source regions typically extend over several hundred Earth radii ( R E ) along the tangent magnetic field line. In the direction perpendicular to the magnetic tangent, however, the source regions are more localized, with the peak emission confined to ≲10 R E for 2 f p radiation and ≲1 R E for f p radiation. The flux levels predicted by our model are in close agreement with the levels measured in situ by various spacecraft, with typical values ∼10 −14 W m −2 for 2 f p radiation. The f p flux densities are typically 2–3 orders of magnitude lower.