Stephen A. Fuselier

The 2016 Hannes Alfvén Medal is awarded to Stephen A. Fuselier for his fundamental contributions to understanding the physics of the interaction of the solar wind with Earth’s magnetosphere, comets, and the interstellar medium.

Fuselier has contributed seminal works on magnetic reconnection at Earth’s magnetopause that led to significant advances in the understanding of kinetic processes in reconnection, and which were crucial for the design of NASA’s recently launched Magnetospheric Multi-Scale (MMS) mission. Fuselier was among the first to investigate ion kinetic processes in magnetic reconnection, using observations from the ISEE (International Earth-Sun Explorer) and AMPTE (Active Magnetospheric Particle Tracer Explorers)/CCE (Charge Composition Explorer) spacecraft. This led to one of the most important results in reconnection physics, namely that in the de Hoffmann-Teller frame all plasma flows are field-aligned and Alfvénic. This result is crucial for all studies of reconnection rates because it established that the ion outflow speed is invariant so that the reconnection rate is determined only by the inflow velocity. Fuselier’s studies firmly established the quantitative behaviour of ion transmission and reflection across the reconnection layer at the magnetopause. Based on the knowledge of the ion behaviour in reconnection, Fuselier pioneered a remote sensing technique to estimate the locations of magnetic reconnection X-lines at the dayside magnetopause. His findings of such X-line locations based on Polar and Cluster spacecraft data are among the highlights of these missions. Using his novel technique to determine the reconnection X-line location at the magnetopause for various solar wind magnetic field orientations, Fuselier and co-workers tackled the long-standing controversy concerning ‘component’ versus ‘anti-parallel’ reconnection at the magnetopause. Their analysis, taking into account the realistic draping of the shocked solar wind magnetic field over the magnetopause surface, indicates that neither the ‘component’ nor ‘anti-parallel’ reconnection model correctly describe the distribution of reconnection sites at the magnetopause. Instead, the X-lines seem to occur along a tilted reconnection X-line across the dayside magnetopause where the magnetic shear maximises. The maximum shear model was recently confirmed by observations from the THEMIS (Time History of Events and Macroscale Interactions during Substorms) spacecraft. Fuselier’s determination of the likely X-line locations was crucial for the design of the MMS orbit configurations to maximise the chance of spacecraft encounters with the miniscule reconnection X-line region. The success of MMS, which aims to investigate electron physics that causes reconnection at the X-line, will depend critically on having a sufficient number of X-line encounters. Fuselier is also the lead investigator for a new type of hot-plasma composition analyzer (HPCA) on MMS that will allow measurement of heavy ions at the dayside magnetosphere for the first time. The operation of HPCA has been confirmed during the MMS commissioning phase.