In-flight scientific performance of magnetically clean spacecraft: Ulysses and Cluster

Accurate measurements of weak magnetic fields in space are essential for the in situ study of space environments in the solar system. DC magnetometers typically used on space missions (fluxgate and He vector magnetometers) have an intrinsic performance that can meet the requirements on accuracy and precision. The requirement on the spacecraft is, ideally, that the background magnetic field that it generates at the location of the magnetometer sensor is small enough not to affect the magnetic field to be measured. Usually, this is achieved by a combination of removing the magnetometer sensor far from the spacecraft body, to the end of a generally deployable boom. However, in order for the spacecraft-generated disturbance to be small while the length of the boom remains feasible, the signature of the spacecraft itself has to be made small by a programme of magnetic cleanliness. In addition, the spacecraft magnetic field must be measured so that the background field at the sensors can be modelled. In this paper, the scientific achievements of two magnetically clean and fully characterised spacecraft, the ESA/NASA Ulysses and ESA´s four Cluster spacecraft are illustrated with measurements that were enabled by the high level of cleanliness of these spacecraft.