A study of ionopause perturbation and associated boundary wave formation at Venus

Magnetometer and electron spectrometer data from Venus Express (VEX) are used to investigate the occurrence of boundary wave phenomenon occurring in the vicinity of the Venusian terminator. On 26 June 2006, VEX data show the ionosphere to be unmagnetized (≈0 nT), a common observation usually expected during solar maximum. This results in the respective Venusian boundaries to be higher than the nominal altitudes. This paper reports the occurrence of rippling of the ionopause boundary in the terminator plane. The ripples appear to propagate mainly in the Y direction in the Venus solar orbital coordinates. Further examination of the first oscillation in magnetic field suggests that it is a flux rope. The flux rope is found to be just inside the ionosphere close to the ionopause. The diameters of the flux rope and boundary wave oscillations are estimated to be ∼113 km and ∼133 km, respectively. We suggest that the boundary wave is generated by the Kelvin-Helmholtz instability. This research provides evidence of the ionopause boundary existing in a wave-like state and its relation to Venus atmospheric loss into space.