Global modeling of ULF waves at Mercury

ULF waves in the ion cyclotron frequency range waves are regularly observed at Mercury's magnetosphere. Although previous statistical studies have shown that ULF waves are primarily compressional near the equator and transverse with linear polarization at higher latitude, the underlying reason for this distribution of wave polarization has not been understood. In order to address this key question, we have developed a two-dimensional, finite element code that solves the full wave equations in global magnetospheric geometry. Using this code, we show that (1) efficient mode conversion from the fast compressional waves to the ion-ion hybrid resonance occurs at Mercury consistent with previous calculations; (2) such mode-converted waves globally oscillate similar to field line resonance at Earth; and (3) compressional wave energy is primarily localized near the equator, while field-aligned transverse, linearly polarized waves generated by mode conversion at the ion-ion hybrid resonance radiate to higher latitude. Based on these wave solutions, we suggest that the strong transverse component of observed ULF waves at Mercury in high magnetic latitude can be explained as excitation of the field line resonant waves at the ion-ion hybrid resonance.