Characterising Solar Wind Fluctuations at Ion-kinetic Scales

The availability of large in-situ datasets makes the solar wind an excellent astrophysical laboratory to probe kinetic processes in a collisionless plasma. As the solar wind turbulent cascade reaches ion-kinetic scales close to the proton gyro-radius, ρ_p, and inertial length, d_p, collective effects lead to interactions between electromagnetic fluctuations and particle velocity distributions. At these scales, wave-particle interactions can lead to the dissipation of turbulent fluctuations and instability growth, which in turn, moderates the macroscopic properties of the plasma.

In this thesis, I use over a decade of magnetic field and ion measurements from the Wind spacecraft to investigate the physical processes ongoing at these scales. I make the first in-flight determination of the magnetometer noise-floor, enabling the most accurate interpretation of magnetic field fluctuations at these scales with Wind to date. I then conduct three detailed statistical analyses of the spectral properties of these fluctuations.

I first show that the steepening of the power spectrum and a coherent signature in magnetic helicity at ion-kinetic scales are associated with the cyclotron resonance wave-number, k_c, providing evidence for ongoing wave-particle interactions at these scales. I then use magnetic helicity to characterise the polarisation properties of the fluctuations, identifying three populations at ion-kinetic scales: quasi-parallel propagating Alfvén-ion cyclotron and fast magnetosonic-whistler waves driven by proton temperature anisotropy instabilities, as well as highly-oblique kinetic Alfvén wave-like fluctuations from the turbulent cascade. Finally, I show that the KAW-like fluctuations are associated with steeper spectra and higher proton temperatures, suggesting damping of the turbulence.

The results presented in this thesis indicate that wave-particle interactions play an important role in the energy transfer between the turbulent fields and ions in the solar wind, in the absence of collisions. I also show that proton heating in the solar wind depends on the polarisation properties of the fluctuations at ion-kinetic scales and the radial direction in the solar wind, in contradiction to the ergodicity hypothesis. Further investigative work is proposed to confirm these findings and identify specific dissipation mechanisms responsible for turbulent heating.