Alfvénic versus non-Alfvénic turbulence in the inner heliosphere as observed by Parker Solar Probe

¹ Earth, Planetary, and Space Sciences, University of California, Los Angeles, Los Angles, California, USA
e-mail: cshi1993@ucla.edu
² Advanced Heliophysics, Pasadena, CA, USA
³ Department of Physics, University of Texas at Austin, Austin, Texas, USA
e-mail: anna.tenerani@austin.utexas.edu
⁴ IRAP, Université Toulouse III-Paul Sabatier, CNRS, CNES, Toulouse, France
⁵ Physics Department, University of California, Berkeley, CA 94720-7300, USA
⁶ Space Sciences Laboratory, University of California, Berkeley, CA 94720-7450, USA
⁷ The Blackett Laboratory, Imperial College London, London, SW7 2AZ, UK
⁸ School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
⁹ University of Michigan, Ann Arbor, MI, USA
¹⁰ Smithsonian Astrophysical Observatory, Cambridge, MA, USA
¹¹ LPC2E, CNRS and University of Orléans, Orléans, France
¹² Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, Colorado, USA
¹³ School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA
¹⁴ Solar System Exploration Division, NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
¹⁵ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 place Jules Janssen, 92195 Meudon, France

Received: 31 October 2020
Accepted: 29 December 2020

Abstract

Context. Parker Solar Probe (PSP) measures the magnetic field and plasma parameters of the solar wind at unprecedentedly close distances to the Sun. These data provide great opportunities to study the early-stage evolution of magnetohydrodynamic (MHD) turbulence in the solar wind.

Aims. In this study, we make use of the PSP data to explore the nature of solar wind turbulence focusing on the Alfvénic character and power spectra of the fluctuations and their dependence on the distance and context (i.e., large-scale solar wind properties), aiming to understand the role that different effects such as source properties, solar wind expansion, and stream interaction might play in determining the turbulent state.

Methods. We carried out a statistical survey of the data from the first five orbits of PSP with a focus on how the fluctuation properties at the large MHD scales vary with different solar wind streams and the distance from the Sun. A more in-depth analysis from several selected periods is also presented.

Results. Our results show that as fluctuations are transported outward by the solar wind, the magnetic field spectrum steepens while the shape of the velocity spectrum remains unchanged. The steepening process is controlled by the “age” of the turbulence, which is determined by the wind speed together with the radial distance. Statistically, faster solar wind has higher “Alfvénicity,” with a more dominant outward propagating wave component and more balanced magnetic and kinetic energies. The outward wave dominance gradually weakens with radial distance, while the excess of magnetic energy is found to be stronger as we move closer toward the Sun. We show that the turbulence properties can significantly vary from stream to stream even if these streams are of a similar speed, indicating very different origins of these streams. Especially, the slow wind that originates near the polar coronal holes has much lower Alfvénicity compared with the slow wind that originates from the active regions and pseudostreamers. We show that structures such as heliospheric current sheets and velocity shears can play an important role in modifying the properties of the turbulence.