Foreshock Electrons Impact Ionization Effect on the Amplitude of Pickup Proton Generated Waves: Consequence on Exosphere Density Determination

Electromagnetic waves at the local proton cyclotron frequency are frequently observed upstream from the Martian bow shock. They are excited by unstable velocity distributions of newborn protons continuously produced locally by ionization of exospheric hydrogen atoms (pickup protons).  The analysis of MAVEN magnetic field data demonstrates for the first time that the amplitude of these waves undergoes a sharp gradient when crossing the electron foreshock boundary. Moreover, a decrease of the amplitude with the increasing distance from the shock along the ambient magnetic field is observed inside the foreshock. Both signatures are correlated with the variations of the energetic electron fluxes. These two properties connecting the wave growth to electron physics raise an issue since the waves are excited purely through an ion-ion instability. We propose that the extra free energy necessary to increase the wave amplitude be due to additional ionization of hydrogen atoms by electron impact ionization inside the foreshock. These results imply that extreme caution is needed when directly deriving the exospheric densities at Mars and other similar environments from the local pickup ion wave amplitude, especially in the foreshock region.