Search for pick‐up ion generated Na+ cyclotron waves at Mercury

Magnetometer data was examined from the two Mariner 10 passages through Mercury's magnetosphere for evidence of Na+ electromagnetic ion cyclotron waves (ICW). These waves are expected to be produced if the thermalization of newly created Na+ pick‐up ions occurs. We found no evidence of Na+ ICW. The spacecraft's rapid passage through Mercury's magnetosphere meant that the ambient magnetic field was only relatively constant for at most ∼3–4 Na+ ICW wave periods throughout each transversal, therefore limiting the spectral resolution of the Na+ cyclotron frequency. Our computations indicate that the wavelengths of Na+ ICW may be on the order of the system scale lengths. It is, therefore, questionable whether these waves can grow to sufficient amplitude to effectively thermalize these pick‐up ions and incorporate them into Mercury's magnetosphere.