Ion sound wave packets at the quasiperpendicular shock front

Electric field measurements from a single spacecraft have been used to study ion‐sound turbulence observed within the Earth's bow shock. The observed frequency of the ion‐sound waves can be both lower and higher than the local electron cyclotron frequency depending upon the direction of wave propagation in the plasma rest frame. The ion‐sound waves observed upstream of the ramp can not be generated either by an instability related to the gradient in the electron temperature or an electric current within the ramp. A comparison of wave vectors for distinctive wave packets indicate that non‐stationary, short scale current layers formed in the processes of the ramp evolution might be the source of the free energy for such waves.