Plasma waves downstream of weak collisionless shocks

In September 1983 the ISEE 3/ICE spacecraft made a long traversal of the distant dawnside flank region of the Earth's magnetosphere and had many encounters with the low Mach number bow shock. These weak shocks excite plasma wave electric field turbulence with amplitudes comparable to those detected in the much stronger bow shock near the nose region. Downstream of quasi‐perpendicular (quasi‐parallel) shocks, the E field spectra exhibit a strong peak (plateau) at midfrequencies (1‐3 kHz); the plateau shape is produced by a low‐frequency (100‐300 Hz) emission which is more intense behind quasi‐parallel shocks. Polarization measurements made in the very steady magnetic field conditions downstream of two quasi‐perpendicular shocks show that the low frequency signals are polarized parallel to the magnetic field, whereas the midfrequency emissions are unpolarized or only weakly polarized. A new high frequency (10‐30 kHz) emission which is above the maximum Doppler shift frequency is clearly identified as a separate wave component. High time resolution spectra often exhibit a distinct peak at high frequencies; this peak is often blurred by the large amplitude fluctuations of the midfrequency waves. The high‐frequency component is strongly polarized along the magnetic field and varies independently of the lower‐frequency waves.