Origin of Foreshock Electron Waves Below the Local Plasma Frequency

Electrostatic plasma waves near the plasma frequency are a typical feature of the foreshock region. These waves are known to be generated by electrons reflected by the shock, but the form of the electron distribution function varies greatly with local shock properties, distance to the shock and foreshock geometry. Far upstream in the electron foreshock, narrowband Langmuir and beam mode waves close to plasma frequency are generated by weak and fast electron beams. A more complex scenario appears deeper in the foreshock, in particular near the foot region of oblique and quasi-parallel shock, where electrostatic waves are observed well below the plasma frequency. We use Cluster data to investigate the properties of such waves and the associated electron distribution function. Cluster consistently observes electron beams at weakly suprathermal energies and loss-cone features associated with generation of the waves. We investigate a dependence of beam energy on the location within the foreshock and compare the results with an existing shock acceleration model. We perform a statistical comparison between electric field spectrum and observed electron distributions showing a correlation between beam energy and frequency of the wave emission relative to the local plasma frequency. We show that the origin of low frequency waves can in most cases be interpreted in terms of beam mode instability, but in some very low frequency cases, other physical mechanism is needed to explain the observations, such as generation of electron-acoustic waves or electrostatic electron-cyclotron instability.