In Situ Electron Density From Active Sounding: The Influence of the Spacecraft Wake

Results obtained in the Martian ionosphere by the Langmuir Probe and Waves instrument aboard the Mars Atmosphere and Volatile EvolutioN Mission spacecraft are presented. The results include ionospheric electron densities determined from the frequency of Langmuir waves. Since the amplitude of thermal Langmuir waves is often below the instrument's detection level, Langmuir Probe and Waves excites these waves by injecting into the plasma a 3.3‐V white noise signal. Electric field spectral measurements obtained shortly after the excitation show a resonance line at frequencies slightly below the local plasma frequency. The observed resonance line is interpreted to originate from plasma waves excited in the wake behind the spacecraft. These results reveal an important phenomenon in electron density estimation from stimulated Langmuir waves. The observed phenomenon, not previously reported by earlier missions, may be a common process in active sounding that can affect in situ electron density measurements. Key Points Electric field spectra in active sounding show an anomalous resonance line at frequencies below the local electron plasma frequency The observed resonance line is interpreted as the signature of the plasma waves excited within the spacecraft wake The observed phenomenon may have affected results in similar active missions that were incapable of identifying this process