First upstream proton cyclotron wave observations at Venus

The escape of particles from planetary atmospheres, especially hydrogen, is an important key towards understanding the atmospheric composition and evolution over the lifetime of the solar system. For an unmagnetized planet such as Venus or Mars when the neutral exosphere extends into the flowing solar wind plasma, loss of pick-up ions upstream of the bow shock can play a significant role in the escape process. Cyclotron waves from pick-up of planetary hydrogen in the solar wind have been previously observed at Mars and other solar system bodies. At Venus, they were reported within the magnetosheath, but not upstream of the bow shock. Here we present observations of the magnetometer aboard the Venus Express spacecraft showing waves at the proton cyclotron frequency in the solar wind; this provides direct evidence that the solar wind is removing hydrogen from the planetary exosphere.