Measurement of the electron beam mode in Earth's foreshock

Abstract

High frequency electric field measurements from the AMPTE IRM plasma wave receiver are used to identify three simultaneously excited electrostatic wave modes in the Earth's foreshock region: the electron beam mode, the Langmuir mode, and the ion acoustic mode. A technique is developed which allows the rest frame frequency and wave number of the electron beam waves to be determined. Plasma wave and magnetometer data are used to determine the interplanetary magnetic field direction at which the spacecraft becomes magnetically connected to the Earth's bow shock. From the knowledge of this direction, the upstreaming electron cutoff velocity can be calculated. We take this calculated cutoff velocity to be the flow velocity of an electron beam in the plasma. Assuming that the wave phase speed is approximately equal to the beam speed and using the measured electric field frequency, we determine the plasma rest frame frequency and the wave number. We then show that the experimentally determined rest frame frequency and wave number agree well with the most unstable frequency and wave number predicted by linear homogeneous Vlasov theory for a plasma with Maxwellian background electrons and a Lorentzian electron beam. From a comparison of the experimentally determined and theoretical values, approximate limits are put on the electron foreshock beam temperatures. A possible generation mechanism for ion acoustic waves involving mode coupling between the electron beam and Langmuir modes is also discussed.