Magnetic field fluctuations in the Venus magnetosheath

Using a model for the convection pattern of the shocked solar wind flow around the Venus obstacle, Pioneer Venus observations of ultra‐low‐frequency (∼10‐40s period) magnetic field fluctuations in the magnetosheath have been traced along streamlines to the region of the quasi‐parallel bow shock. The periods and polarizations of the sinusoidal fluctuations are similar to those observed upstream of the quasi‐parallel bow shock, where streaming superthermal particles are believed to produce MHD waves by a beam‐plasma instability. The results suggest that both disturbances at the ionopause at Venus and the earth's magnetopause may be caused by convection of turbulent magnetic fields from the subsolar bow shock when the interplanetary field direction produces a quasi‐parallel shock there.