Ion pickup and acceleration: Measurements from planetary missions

Ion pickup is a key mechanism in the plasma interaction with objects in the solar system. It is important wherever flowing plasma interacts with neutral particles. In many cases, it provides an initial acceleration process prior to further acceleration by other processes. Here, we first introduce the ion pickup process. Pickup alone provides up to 4 times the plasma flow energy per amu. We then focus on observations of pickup at planetary bodies in the last 10 years. These include several locations within the Saturn magnetosphere, such as Enceladus, Titan, Saturn's inner water-dominated magnetosphere and Rhea, where pickup of both positive and negative ions is evident from Cassini measurements. In addition in the last decade there is some data from in-situ measurements close to and far from comets. From the inner solar system, we review observations near Mars, Venus, Mercury and the Moon. In the latter case, this includes ‘self-pickup’ of reflected particles, where the pickup energy can be even higher at 9 times the plasma flow energy per amu. We then examine the exciting prospects for the next 10 years for measurements at Saturn, Mars, Venus, comets, Pluto, Mercury and the Moon – and hopefully a return to Jupiter and its satellites.