Quasi-monochromatic Foreshock ULF Waves: Comparison between Earth and Venus

Large amplitude ultralow frequency (ULF) waves are observed in the ion foreshock upstream from the terrestrial bow shock inside a specific boundary called the ULF foreshock boundary. Many observations have been provided by the WIND spacecraft and by the Cluster mission for which wave-particle interaction has been extensively studied. More recently, with the magnetic field observations from Venus Express, an abundance of quasi-monochromatic ULF waves (with frequency below and far enough from the local proton cyclotron frequency) have been identified by an automatic survey upstream from the Venusian bow shock. One objective of this study was first to derive the relative occurrence of such foreshock waves and proton cyclotron waves associated to local pickup ions linked to exospheric hydrogen previously reported upstream from the Venusian shock. The second objective was to compare with the observed properties of the similar ULF waves in the terrestrial foreshock and discuss their association with ion distributions. The transverse part of the power spectrum dominates the parallel part for the reported Venusian foreshock ULF waves. Most of the waves display left-hand polarization in the spacecraft frame. Taking into account the Doppler-shift by the high-speed solar wind, they may be right-hand polarized in the solar wind frame. These characteristics suggest that they are RH mode waves generated by a cyclotron resonant process in the ion foreshock region by the field aligned beam protons reflected at the shock as in the terrestrial case.