Observations of plasma waves in magnetic holes

We report the first observations of Langmuir waves excited within magnetic holes, which are characterized by a depression of magnetic field strength. This phenomenon is very common in the solar wind. In addition to Langmuir waves, waves at a few kHz, which are typical of Doppler‐shifted ion acoustic frequencies in the solar wind, and low frequency electromagnetic waves (below the electron cyclotron frequency) are sometimes observed simultaneously within the holes. In the holes with directional discontinuities at the boundary, the above plasma waves may occur at the edges of the holes. Possible physical mechanisms of the wave excitation are discussed.