Narrow-band ultra-low-frequency wave observations by MESSENGER during its January 2008 flyby through Mercury's magnetosphere

Abstract

During MESSENGER'S first flyby of Mercury, numerous narrow-band ultra-low-frequency (ULF) waves and their harmonics were detected between closest approach (CA) and the outbound magnetopause (MP) crossing. The fundamental mode was at frequencies between the He+ (f cHe+) and the H+ (fcH+) cyclotron frequencies. A boundary layer (BL) was detected before the MP crossing. The ULF frequency and amplitude increased from CA to the edge of the BL. In the BL the frequency dropped by a factor of 2, and the amplitude increased by an order of magnitude. There was a large variation in the wave-normal angle (Ψ), with a slight tendency for Ψ to be perpendicular to the ambient magnetic field near CA and parallel away from CA. Near CA the parallel power tended to be larger than the perpendicular power, while away from CA the perpendicular power dominated. Large variations in wave polarization properties were observed. © 2009 by the American Geophysical Union.