Characteristics of quasi-monochromatic ULF waves in the Venusian foreshock

Abstract : The statistical properties of ULF waves observed upstream of Venus foreshock are investigated. The study is restricted to waves which are observed well below the local proton cyclotron frequency. Using the magnetic field observations from Venus Express between May 2006 and February 2012, 115 quasi-monochromatic ULF wave trains have been identified. Statistical results show that the wave periods are mainly from 20 to 30 s in the spacecraft frame, which is about 2-3 times of the local proton cyclotron period. The transverse power dominates the power spectrum, and most of the waves display nearly circular or slightly elliptical polarization in the spacecraft frame. Moreover, these ULF waves mainly have small relative amplitudes with respect to the ambient field magnitude B₀ for parallel component (δB_||/B₀ less than 0.3), while the range of relative amplitudes for perpendicular component δB_⊥/B₀ is from ~0.1 to ~1.0. Wave propagation angles are mainly less than 30° with respect to the mean magnetic field direction. The obtained results are very similar to the wave properties seen for ULF waves present in the terrestrial foreshock, which suggests that backstreaming ions in the Venusian foreshock form an important energy source for the generation of the waves.