The low-frequency break observed in the slow solar wind magnetic spectra

Abstract

Fluctuations of solar wind magnetic field and plasma parameters exhibit a typical turbulence power spectrum with a spectral index ranging between $\sim -5/3$ and $\sim -3/2$. In particular, at $1$ AU, the magnetic field spectrum, observed within fast corotating streams, also shows a clear steepening for frequencies higher than the typical proton scales, of the order of $\sim 3\times 10^{-1}$ Hz, and a flattening towards $1/f$ at frequencies lower than $\sim 10^{-3}$ Hz. However, the current literature reports observations of the low-frequency break only for fast streams. Slow streams, as observed to date, have not shown a clear break, and this has commonly been attributed to slow wind intervals not being long enough. Actually, because of the longer transit time from the Sun, slow wind turbulence would be older and the frequency break would be shifted to lower frequencies with respect to fast wind. Based on this hypothesis, we performed a careful search for long-lasting slow wind intervals throughout $12$ years of Wind satellite measurements. Our search, based on stringent requirements not only on wind speed but also on the level of magnetic compressibility and Alfvénicity of the turbulent fluctuations, yielded $48$ slow wind streams lasting longer than $7$ days. This result allowed us to extend our study to frequencies sufficiently low and, for the first time in the literature, we are able to show that the $1/f$ magnetic spectral scaling is also present in the slow solar wind, provided the interval is long enough. However, this is not the case for the slow wind velocity spectrum, which keeps the typical Kolmogorov scaling throughout the analysed frequency range. After ruling out the possible role of compressibility and Alfvénicity for the 1/f scaling, a possible explanation in terms of magnetic amplitude saturation, as recently proposed in the literature, is suggested.

References in corpus (3)

• Radial Dependence of the Frequency Break Between Fluid and Kinetic Scales in the Solar Wind Fluctuations
• Soliton Turbulence in Shallow Water Ocean Surface Waves
• On the 1/f spectrum in the solar wind and its connection with magnetic compressibility

Cited by in corpus (11)

• The Solar Orbiter mission -- Science overview
• Spacecraft and interplanetary contributions to the magnetic environment on-board LISA Pathfinder
• New Observations of Solar Wind 1/f Turbulence Spectrum from Parker Solar Probe
• Magnetic field intermittency in the solar wind: PSP and SolO observations ranging from the Alfven region out to 1 AU
• The Evolution of the 1/f Range Within a Single Fast-Solar-Wind Stream Between 17.4 and 45.7 Solar Radii
• Energy Supply for Heating the Slow Solar Wind Observed by Parker Solar Probe between 0.17 and 0.7 au
• Phase mixing and the 1/f spectrum in the solar wind
• Dominance of 2-Minute Oscillations near the Alfvén Surface
• Two Types of $1/f$ Range in Solar Wind Turbulence
• Interplanetary magnetic correlation and low-frequency spectrum over many solar rotations
• Statistical study of electron density turbulence and ion-cyclotron waves in the inner heliosphere: Solar Orbiter observations