The boundary region between a plasma stream and a dipole magnetic field with dipole axis normal to the flow is investigated by means of probes. Simultaneous measurements of the transverse electric field, particle density, magnetic field, and rate of change of magnetic field are made. Oscillations in the electric and magnetic fields are found in the megacycle range. The electric‐field oscillations take place in the region of the particle‐density gradient, and the magnetic‐field oscillations are located at the magnetic‐field gradient. The electric‐field oscillations are thought to be ion acoustic or magnetoacoustic waves excited by a streaming instability.