Letter to the Editor

The near-Sun streamer belt solar wind: turbulence and solar wind acceleration

¹ School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
e-mail: christopher.chen@qmul.ac.uk
² Department of Physics and Astronomy, University of New Hampshire, Durham, NH 03824, USA
³ Department of Physics, Imperial College London, London SW7 2AZ, UK
⁴ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ Department of Aerospace, Physics and Space Sciences, Florida Institute of Technology, Melbourne, FL 32901, USA
⁶ Johns Hopkins University Applied Physics Laboratory, Laurel, MD 20723, USA
⁷ Space Sciences Laboratory, University of California, Berkeley, CA 94720, USA
⁸ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85719, USA
⁹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 92195 Meudon, France
¹⁰ BWX Technologies, Inc., Washington, DC 20002, USA
¹¹ Climate and Space Sciences and Engineering, University of Michigan, Ann Arbor, MI 48109, USA
¹² Physics Department, University of California, Berkeley, CA 94720, USA

Received: 6 November 2020
Accepted: 4 January 2021

Abstract

The fourth orbit of Parker Solar Probe (PSP) reached heliocentric distances down to 27.9 R_⊙, allowing solar wind turbulence and acceleration mechanisms to be studied in situ closer to the Sun than previously possible. The turbulence properties were found to be significantly different in the inbound and outbound portions of PSP’s fourth solar encounter, which was likely due to the proximity to the heliospheric current sheet (HCS) in the outbound period. Near the HCS, in the streamer belt wind, the turbulence was found to have lower amplitudes, higher magnetic compressibility, a steeper magnetic field spectrum (with a spectral index close to –5/3 rather than –3/2), a lower Alfvénicity, and a ‘1∕f’ break at much lower frequencies. These are also features of slow wind at 1 au, suggesting the near-Sun streamer belt wind to be the prototypical slow solar wind. The transition in properties occurs at a predicted angular distance of ≈4° from the HCS, suggesting ≈8° as the full-width of the streamer belt wind at these distances. While the majority of the Alfvénic turbulence energy fluxes measured by PSP are consistent with those required for reflection-driven turbulence models of solar wind acceleration, the fluxes in the streamer belt are significantly lower than the model predictions, suggesting that additional mechanisms are necessary to explain the acceleration of the streamer belt solar wind.