Alpha-proton Differential Flow of the Young Solar Wind: Parker Solar Probe Observations

TL;DR

This study uses Parker Solar Probe data to analyze alpha-proton differential flow in the young solar wind near the Sun, revealing its radial evolution and relation to the Alfvén speed, with implications for solar wind acceleration.

Contribution

First observations of alpha-proton differential flow very close to the Sun, showing its radial dependence and relation to the Alfvén surface using PSP data.

Findings

Differential speed is greater in fast solar wind.

Differential flow decreases with distance from the Sun.

Alpha particles often move faster than protons near the Sun.

Abstract

The velocity of alpha particles relative to protons can vary depending on the solar wind type and distance from the Sun (Marsch 2012). Measurements from the previous spacecraft provided the alpha-proton's differential velocities down to 0.3 au. Parker Solar Probe (PSP) now enables insights into differential flows of newly accelerated solar wind closer to the Sun for the first time. Here, we study the difference between proton and alpha bulk velocities near PSP perihelia of Encounters 3-7 when the core solar wind is in the field of view of the Solar Probe Analyzer for Ions (SPAN-I) instrument. As previously reported at larger heliospheric distances, the alpha-proton differential speed observed by PSP is greater for fast wind than the slow solar wind. We compare PSP observations with various spacecraft measurements and present the radial and temporal evolution of the alpha-proton differential speed. The differential flow decreases as the solar wind propagates from the Sun, consistent with previous observations. While Helios showed a small radial dependence of differential flow for the slow solar wind, PSP clearly showed this dependency for the young slow solar wind down to 0.09 au. Our analysis shows that the alpha-proton differential speed's magnitude is mainly below the local Alfv\'en speed. Moreover, alpha particles usually move faster than protons close to the Sun. PSP crossed the Alfv\'en surface during its eighth Encounter and may cross it in future Encounters, enabling us to investigate the differential flow very close to the solar wind acceleration source region for the first time.