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

[1] 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.