Solar wind magnetic field background spectrum from fluid to kinetic scales

Abstract

The solar wind is highly structured in fast and slow flows. These two dynamical regimes remarkably differ not only for the average values of magnetic field and plasma parameters but also for the type of fluctuations they transport. Fast wind is characterized by large amplitude, incompressible fluctuations, mainly Alfvénic, slow wind is generally populated by smaller amplitude and less Alfvénic fluctuations, mainly compressive. The typical corotating fast stream is characterized by a stream interface, a fast wind region and a slower rarefaction region formed by the trailing expansion edge of the stream. Moving {between these two regions}, from faster to slower wind, we observe the following behavior: a) the power level of magnetic fluctuations within the inertial range largely decreases, keeping the typical Kolmogorov scaling; b) at proton scales, for about one decade right beyond the high frequency break, the spectral index becomes flatter and flatter towards a value around -2.7; c) at higher frequencies, before the electron scales, the spectral index remains around -2.7 and, {based on suitable observations available for $4$ corotating streams}, the power level does not change, irrespective of the flow speed. All these spectral features, characteristic of high speed streams, suggest the existence of a sort of magnetic field background spectrum. This spectrum would be common to both faster and slower wind but, any time the observer would cross the inner part of a fluxtube channeling the faster wind into the interplanetary space, a turbulent and large amplitude Alfvénic spectrum would be superposed to it.

arXiv comment7 pages, 7 figures

References in corpus (8)

• Universality of solar wind turbulent spectrum from MHD to electron scales
• Anisotropic scaling of magnetohydrodynamic turbulence
• Solar Wind Turbulence and the Role of Ion Instabilities
• Radial Dependence of the Frequency Break Between Fluid and Kinetic Scales in the Solar Wind Fluctuations
• Spectral Slope Variation at Proton Scales from Fast to Slow Solar Wind
• Spectral Features of Magnetic Fluctuations at Proton Scales from Fast to Slow Solar Wind
• MHD-kinetic transition in imbalanced Alfvénic turbulence
• Linking Fluid and Kinetic Scales in Solar Wind Turbulence

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• Spacecraft and interplanetary contributions to the magnetic environment on-board LISA Pathfinder
• Solar Orbiter Observations of the Kelvin-Helmholtz Instability in the Solar Wind
• Inverse cascade and magnetic vortices in kinetic Alfvén-wave turbulence
• Modeling imbalanced collisionless Alfvén wave turbulence with nonlinear diffusion equations