LAUSR

Magnetosonic (MS) waves are dominant plasma waves causing severe Martian ionospheric erosion. They are generally considered to originate upstream of Martian bow shock with frequencies near the upstream proton gyrofrequency. However, whether MS waves can be locally excited lacks theoretical analysis. Here we present an event of MS waves with frequencies above and closely related to the local proton gyrofrequency in the Martian ionosphere. Concurrently, ring beam hot proton distributions are observed due to the penetration of magnetosheath protons. By employing the observed plasma and magnetic field data, the calculated linear growth rates for MS waves agree well with the observed wave power spectra, demonstrating that they can be locally excited by unstable ring beam hot protons at Mars. Our results could be of great help in understanding the excitation of MS waves in a heavy ion‐rich environment around unmagnetized planets.