Near-Sun Switchback Boundaries: Dissipation with Solar Distance

The most surprising result from the first solar encounters by the Parker Solar Probe (PSP) is the large amount of brief magnetic field reversals often referred to as switchbacks. Switchbacks have previously been observed further downstream in the solar wind by spacecraft such as Helios 2 at 62 R _s from the Sun. However, these observations lack a distinct proton temperature increase detected inside switchbacks by PSP, implying that they are evolving over time to eventually reach a pressure balance at the switchback boundaries. We look at the evolution of switchback boundaries as a function of radial distance from the Sun, from closest approach at 35.7 R _s during PSP's first two encounters to beyond 80 R _s. Using magnetic field and proton data from PSP's FIELDS and SWEAP instruments, we perform a day-by-day superposed epoch analysis of the 25 switchbacks with the sharpest step-like boundaries. During both encounters we found the proton temperature spikes to gradually decline before vanishing completely around 55 R _s. Magnetic reversals and velocity spikes also steadily drop in magnitude, but eventually flatten out instead of disappearing. Most interestingly, proton temperature change ΔT _p across switchback boundaries is found to reach 2 × 10⁵ in magnitude below 40 R _s during PSP's outbound trajectory, but is an order of magnitude less on the inbound trajectory, suggesting a possible common change in switchback characteristics near closest approach during both encounters.