Photoelectrons at Venus in the Ionosphere and the Tail

Pioneer Venus Orbiter has shown that there is no intrinsic magnetic field at Venus. Instead, there is an induced magnetosphere caused by the solar wind interaction with the upper atmosphere of the planet. This interaction causes the solar wind's magnetic field lines to drape around the planet, dragging through the ionosphere where they continue on to form a magnetotail. Ionisation of the ionosphere by the solar HeII 30.4nm line leads to the production of ionospheric photoelectrons, recognisable by their characteristic spectral shape in the electron energy spectrum. These photoelectrons have been seen at Venus by the electron spectrometer, part of the ASPERA instrument, onboard the Venus Express spacecraft due to its energy resolution of ~7%. In addition, these ionospheric photoelectrons have now been seen in the induced magnetotail for the first time at Venus. Tailward photoelectrons have previously been seen at Earth, Mars and Titan. At Titan, they have been linked to ambipolar diffusion.