The Radial Dependence of Proton-scale Magnetic Spectral Break in Slow Solar Wind during PSP Encounter 2

Magnetic field fluctuations in the solar wind are commonly observed to follow a power-law spectrum. Near proton-kinetic scales, a spectral break occurs that is commonly interpreted as a transition to kinetic turbulence. However, this transition is not yet entirely understood. By studying the scaling of the break with various plasma properties, it may be possible to constrain the processes leading to the onset of kinetic turbulence. Using data from the Parker Solar Probe, we measure the proton-scale break over a range of heliocentric distances, enabling a measurement of the transition from inertial to kinetic-scale turbulence under various plasma conditions. We find that the break frequency f_b increases as the heliocentric distance r decreases in the slow solar wind following a power law of f_b ∼ r^-1.11. We also compare this to the characteristic plasma ion scales to relate the break to the possible physical mechanisms occurring at this scale. The ratio f_b/f_c (f_c for Doppler-shifted ion cyclotron resonance scale) is close to unity and almost independent of plasma β_p. While f_b/f_ρ (f_ρ for Doppler-shifted proton thermal gyroradius) increases with β_p approaching to unity at larger β_p, f_b/f_d (f_d for Doppler-shifted proton inertial length) decreases with β_p from unity at small β_p. Due to the large comparable Alfvén and solar wind speeds, we analyze these results using both the standard and modified Taylor hypotheses, demonstrating the robust statistical results.