Statistical study of low-frequency magnetic field fluctuations near Venus under the different interplanetary magnetic field orientations

The magnetic field fluctuations near Venus are investigated in the frequency range 0.03-0.3 Hz on the basis of the measurements observed by Venus Express from April 2006 to December 2008. The data are sorted by the angle between interplanetary magnetic field (IMF) and solar wind flow. The spatial distributions of fluctuation properties under the different IMF orientations are presented and a comparative study is performed. In the mantle and tail regions, the magnetic field is fairly quiet and the fluctuations are almost linearly polarized. There are two possible sources for the fluctuations in the magnetosheath: convection from the upstream foreshock and local generation. When the IMF is nearly perpendicular to the solar wind flow, the fluctuations in the magnetosheath are mainly generated locally by an ion cyclotron instability due to planetary ion pickup. The wave intensity is relatively low and the transverse component is dominant. The waves are left-handed, elliptically polarized, and propagating parallel to the mean magnetic field. When the IMF is nearly aligned to the solar wind flow, foreshock waves are convected into the magnetosheath. The fluctuations in the magnetosheath become more intensive. Their polarization properties are very mixed and there is no clear tendency. It indicates that the waves convected from the foreshock may be the mixture of multiple wave types and incoherent noise.