2022/02/01 by Parisa Mostafavi, P. Mostafavi, Robert C. Allen +8
Physics and Astronomy · #Astro and Planetary Science #Astrophysics #Coronal mass ejection #Interplanetary medium #Interplanetary spaceflight #Ionosphere and magnetosphere dynamics #Magnetopause #Nuclear physics #Physics #Plasma #Proton #Solar and Space Plasma Dynamics #Solar physics #Solar wind #astro-ph.SR #physics.space-ph
paper · pdf · doi:10.3847/2041-8213/ac51e1
Accepted paper
openalex publication_date 2022/02/01 · arxiv created 2022/02/07 · arxiv updated 2022/03/02 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/06
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 Alfvén speed. Moreover, alpha particles usually move faster than protons close to the Sun. PSP crossed the Alfvén 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.