[1] Narrow-band ultra-low-frequency (ULF) waves at frequencies greater than the He + cyclotron frequency (f cHe+ ) were detected during MESSENGER's first two Mercury flybys. The waves were observed primarily between closest approach (CA) and the outbound magnetopause. The magnetosphere was very quiet during the first flyby (M1) and highly disturbed during the second flyby (M2); that ULF waves were observed during both flybys despite these different magnetospheric conditions is remarkable. The wave frequency structure in the boundary layer (BL) was similar between M1 and M2. Between CA and the BL, for M1 the wave frequency rose systematically from f cHe+ to the proton cyclotron frequency (f cH+ ), while during M2 two frequency bands were observed, one near the He ++ cyclotron frequency and one near f cH+ . The main difference in the waves between the two flybys, apart from their frequency structure, was their power, which was 4 to 5 times larger during M2 than during M1. Citation: Boardsen
