@article{85c3bb111d7a465786cad8293ec0a506,
title = "Sub-Alfv{\'e}nic Solar Wind Observed by the Parker Solar Probe: Characterization of Turbulence, Anisotropy, Intermittency, and Switchback",
abstract = "In the lower solar coronal regions where the magnetic field is dominant, the Alfv{\'e}n speed is much higher than the wind speed. In contrast, the near-Earth solar wind is strongly super-Alfv{\'e}nic, i.e., the wind speed greatly exceeds the Alfv{\'e}n speed. The transition between these regimes is classically described as the {"}Alfv{\'e}n point{"}but may in fact occur in a distributed Alfv{\'e}n critical region. NASA's Parker Solar Probe (PSP) mission has entered this region, 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 ≈16 R ⊙ from the Sun, PSP sampled four extended periods in which the solar wind speed was measured to be smaller than the local Alfv{\'e}n speed. These are the first in situ detections of sub-Alfv{\'e}nic solar wind in the inner heliosphere by PSP. Here we explore properties of these samples of sub-Alfv{\'e}nic solar wind, which may provide important previews of the physical processes operating at lower altitude. Specifically, we characterize the turbulence, anisotropy, intermittency, and directional switchback properties of these sub-Alfv{\'e}nic winds and contrast these with the neighboring super-Alfv{\'e}nic periods.",
author = "R. Bandyopadhyay and Matthaeus, {W. H.} and McComas, {D. J.} and R. Chhiber and Usmanov, {A. V.} and J. Huang and R. Livi and Larson, {D. E.} and Kasper, {J. C.} and Case, {A. W.} and M. Stevens and P. Whittlesey and Romeo, {O. M.} and Bale, {S. D.} and Bonnell, {J. W.} and {Dudok De Wit}, T. and K. Goetz and Harvey, {P. R.} and Macdowall, {R. J.} and Malaspina, {D. M.} and M. Pulupa",
note = "Funding Information: We are deeply indebted to everyone that helped make the Parker Solar Probe (PSP) mission possible. This work was supported as a part of the PSP mission under contract NNN06AA01C. This research was partially supported by the Parker Solar Probe Plus project through Princeton/IS⊙IS subcontract SUB0000165, in part by PSP GI grant 80NSSC21K1765 at the University of Delaware, and in part by PSP GI grant 80NSSC21K1767 at Princeton University. The Parker Solar Probe was designed, built, and is now operated by the Johns Hopkins Applied Physics Laboratory as part of NASA{\textquoteright}s Living with a Star (LWS) program (contract NNN06AA01C). Support from the LWS management and technical team has played a critical role in the success of the Parker Solar Probe mission. Publisher Copyright: {\textcopyright} 2022. The Author(s). Published by the American Astronomical Society.",
year = "2022",
month = feb,
day = "1",
doi = "10.3847/2041-8213/ac4a5c",
language = "English (US)",
volume = "926",
journal = "Astrophysical Journal Letters",
issn = "2041-8205",
publisher = "IOP Publishing Ltd.",
number = "1",
}