The Quasi-monochromatic ULF Wave Boundary in the Venusian Foreshock: Venus Express Observations

The location of ultralow-frequency (ULF) quasi-monochromatic wave onset upstream of Venus bow shock is explored using Venus Express magnetic field data. We report the existence of a spatial foreshock boundary behind which ULF waves are present. We have found that the ULF wave boundary at Venus is sensitive to the interplanetary magnetic field (IMF) direction like the terrestrial one and appears well defined for a cone angle larger than 30 degrees. In the Venusian foreshock, the inclination angle of the wave boundary with respect to the Sun-Venus direction increases with the IMF cone angle. We also found that for the IMF nominal direction (theta(BX) = 36 degrees) at Venus' orbit, the value of this inclination angle is 70 degrees. Moreover, we have found that the inferred velocity of an ion traveling along the ULF boundary is in a qualitative agreement with a quasi-adiabatic reflection of a portion of the solar wind at the bow shock. For an IMF nominal direction at Venus, the inferred bulk speed of ions traveling along this boundary is 1.07 V-SW, sufficiently enough to overcome the solar wind convection. This strongly suggests that the backstreaming ions upstream of the Venusian bow shock provide the main energy source for the ULF waves.