Characterization of the trans-Alfvenic Solar Wind region from the Parker Solar Probe Mission

The solar wind consists of the ionized and magnetized plasma that flows from the Sun’s corona into interplanetary space. In the lower solar coronal regions, where the magnetic field is dominant, the Alfven speed is much higher than the wind speed. In contrast, the near-Earth solar wind is strongly super-Alfvenic, i.e., the wind speed greatly exceeds the Alfven speed. The transition between these regimes occurs in a region classically described as the “Alfven surface”. In the late 2021, NASA’s Parker Solar Probe (PSP) spacecraft entered this surface, as it follows a series of orbits that gradually approach more closely to the Sun. During its 8th and 9th solar encounters, at a distance of about 16 solar radii from the Sun, PSP sampled four extended periods in which the solar wind speed was measured to be smaller than the local Alfven speed. These are the first in situ detections of sub-Alfvenic solar wind in the inner heliosphere by PSP. In this talk, I will discuss the properties of these recently observed sub-Alfvenic solar wind, which may provide important previews of the physical processes operating at lower altitude.