Predictions for the Shape and Orientation of Earth's Foreshock Radiation Sources

Abstract

Radio emission produced in Earth's foreshock is due to the bow shock reflecting some electrons back upstream into the foreshock, where they produce Langmuir waves and radio emissions near the electron plasma frequency $f_p$ and near $2f_p$. Here we predict the source shapes and fluxes of Earth's foreshock radio emissions, with associated implications for other radio sources at multiples of $f_p$. We use an extended model for Earth's foreshock radiation to calculate the electron distributions and the energy flows into Langmuir waves and radiation for specific emission processes. One extension is the addition of linear mode conversion of Langmuir waves into $f_p$ radiation at density gradients, found to produce $\approx 20-30\%$ of the total radio flux near $f_p$. We produce the first 2D images of Earth's foreshock radio sources for $f_p$ and $2f_p$ radiation observed remotely from the L1 point and other locations. These images confirm a prediction for the 2–3 kHz outer heliospheric radiation that the radio source should be strongly elongated along the shock perpendicular to the upstream magnetic field and plasma flow velocity.