Compressible Turbulence in the Near-Sun Solar Wind: Parker Solar Probe's First Eight Perihelia

Abstract

Many questions remain about the compressibility of solar wind turbulence with respect to its origins and properties. Low plasma beta (ratio of thermal to magnetic pressure) environments allow for the easier generation of compressible turbulence, enabling study of the relationship between density fluctuations and turbulent Mach number. Utilizing Parker Solar Probe plasma data, we examine the normalized proton density fluctuations $⟨\delta n_p^2{⟩}^{1/2}/⟨n_p⟩=\delta {n_p}_{rms}/⟨n_p⟩$ as a function of turbulent Mach number $M_t$ conditioned on plasma beta and cross helicity. With consideration of statistical error in the parameters computed from in-situ data, we find a general result that $\delta {n_p}_{rms}/⟨n_p⟩\sim M_t^{1.18\pm 0.04}$, consistent with both linear-wave theory, and nearly-incompressible turbulence in an inhomogeneous background field. We compare observational results conditioned on plasma beta and cross helicity with 3D magnetohydrodynamic simulations, and observe rather significant similarities with respect to how those parameters affect the proportionality between density fluctuations and turbulent Mach number. This study further investigates the complexity of compressible turbulence as viewed by the density scaling relationship, and may help better understand the compressible environment of the near-Sun solar wind.

8 pages, 3 figures, 1 table, submitted to ApJL

References in corpus (12)

• The Athena++ Adaptive Mesh Refinement Framework: Design and Magnetohydrodynamic Solvers
• The Evolution and Role of Solar Wind Turbulence in the Inner Heliosphere
• Density Fluctuations in MHD Turbulence: Spectra, Intermittency and Topology
• The Solar Probe Cup on Parker Solar Probe
• Energy Cascade Rate In Compressible Fast And Slow Solar Wind Turbulence
• Three-dimensional simulation of the fast solar wind driven by compressible magnetohydrodynamic turbulence
• Turbulence Transport Modeling and First Orbit Parker Solar Probe (PSP) Observations
• Contextual Predictions for Parker Solar Probe II: Turbulence Properties and Taylor Hypothesis
• On the Existence of Fast Modes in Compressible Magnetohydrodynamic Turbulence
• Fast magneto-acoustic wave turbulence and the Iroshnikov-Kraichnan spectrum
• Nature and Scalings of Density Fluctuations of Compressible MHD Turbulence with Applications to the Solar Wind
• On the Interpretation of the Scalings of Density Fluctuations from In-situ Solar Wind Observations: Insights from 3D Turbulence Simulations