Following "Voyager's" encounters with Enceladus in the early 1980s, scientists postulated it to be geologically active based on its young, reflective surface and location near the core of the E ring. Based on the connection between Enceladus and the E ring, scientists suspected that Enceladus was the source of material in the E ring, perhaps through venting of water vapor. Readings from "Cassini's" 2005 passage suggested that cryovolcanism, where water and other volatiles are the materials erupted instead of silicate rock, had been discovered on Enceladus. The first "Cassini" sighting of a plume of icy particles above Enceladus's south pole came from the Imaging Science Subsystem (ISS) images taken in January and February 2005, though the possibility of a camera artifact delayed an official announcement. Data from the magnetometer instrument during the February 17, 2005, encounter provided evidence for a planetary atmosphere. The magnetometer observed a deflection or "draping" of the magnetic field, consistent with local ionization of neutral gas. In addition, an increase in the power of ion cyclotron waves near the orbit of Enceladus was observed, which was further evidence of the ionization of neutral gas. These waves are produced by the interaction of ionized particles and magnetic fields, and the waves' frequency is close to the gyrofrequency of the freshly produced ions, in this case water vapor. During the two following encounters, the magnetometer team determined that gases in Enceladus's atmosphere are concentrated over the south polar region, with atmospheric density away from the pole being much lower. The Ultraviolet Imaging Spectrograph (UVIS) confirmed this result by observing two stellar occultations during the February 17 and July 14 encounters. Unlike the magnetometer, UVIS failed to detect an atmosphere above Enceladus during the February encounter when it looked over the equatorial region, but did detect water vapor during an occultation over the south polar region during the July encounter.