The heliospheric current sheet at solar maximum

Abstract

Whilst near solar minimum it was possible to gain an understanding of the structure of the heliosphere in three dimensions using only low latitude in situ measurements, at solar maximum this was not the case. The Ulysses mission, with its unique almost polar orbit has provided an opportunity to investigate the structure of the heliosphere in three dimensions during a period of solar maximum. This thesis has focussed on investigating the topology and evolution of the HCS at solar maximum as well as the local structure of the boundary itself using magnetic field and plasma observations taken during the second Ulysses orbit. As a passive structure the Heliospheric Current Sheet (HCS) acts as a marker for the dynamic evolution of solar wind surrounding it while the large scale structure of the HCS also reflects the structure of the solar magnetic field. By combining simultaneous spacecraft observations at differing latitudes with potential field models the HCS was found to be near vertical at low latitudes and non planar at high latitudes. A second technique using minimum variance analysis to infer the orientation of the HCS concurred that the HCS extended to high latitudes, with the added suggestion that the topology was modified by stream interactions. Suprathermal electron heat flux measurements suggested the presence of substantial local HCS structure, especially during periods where the Ulysses trajectory skimmed along the HCS.