Preparing for the Solar Orbiter Mission: Properties of Solar Wind Charged Particle Populations

UCL/MSSL is the Principal Investigator Institute on an international consortium providing the Solar Wind Analyser suite (SWA) of instruments for the ESA Solar Orbiter mission, due for launch in 2018. SWA will sample electron, proton, alpha particle and heavy ion populations at various distances down to 0.28 AU from the Sun (i.e. around a quarter the distance from the Sun to the Earth). In particular, UCL/MSSL is designing and building the electron analyser system (EAS) for the SWA suite. In order to prepare for the mission, and to be able to use the 3 sensors to make optimum measurements in the solar wind, we will undertake studies of the nature of the solar wind particle populations, their variability and their links to the Sun using, where relevant, data from existing in-orbit missions.

For example, it is known that the solar wind electron population in general consists of 3 components: A 'core' population of the coldest electrons which is nearly isotropic - approximately the same flux of electrons of a given energy may be detected in any direction; A 'halo' population occurs at somewhat higher energies, and shows a slight shift in average velocity with respect to the core, and thus provides a 'heat flux' in the solar wind; Finally, a 'strahl' population is often seen as a more energetic beam of particles streaming along the magnetic field. Together these different electron populations contain information about the processes occurring at the source region on the Sun, the magnetic connections of the sampled plasma back to the Sun and on the plasma processes (e.g. turbulence, wave-particle interactions and magnetic reconnection) which may be occurring within the solar wind itself. Separating the effects of these processes is a complicated task requiring high-cadence, high resolution data of the type available during burst modes from the UCL/MSSL-built PEACE electron spectrometers on the 4 Cluster spacecraft when one or more of the spacecraft are located in the solar wind. A highly mission-relevant PhD project would, for example, use these data to explore the nature and variability of the electron populations, using the multi-point measurements to determine the level of variation between spacecraft.

A second mission-relevant project will address the links between the heavy ion measurements in the solar wind and the measurements of the Sun itself available from ultra-violet imaging and spectroscopic telescopes. Establishing such links between the SWA measurements of ion composition and the remote sensing measurements of the Sun has been identified as critical for the success of the Solar Orbiter mission. Thus establishing a reliable method to do this, with existing datasets, will be invaluable during the post-launch analysis phase. Such a project would benefit from the ready collaboration possible with researchers in the UCL/MSSL Solar Physics Group which possesses expertise in analysis of the remotely sensed observations.

The results of such projects are critical as preparation and inputs into the ESA Solar Orbiter program, and the student will thus also be an integral part of the MSSL science-planning team, with the responsibility of making scientific inputs to those processes. There will also be opportunity to collaborate with our partners in France, Italy and the USA, who will provide the Heavy Ion Sensor and Proton-Alpha Sensor for the SWA suite.