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 ∼ 5/3 and ∼ 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 ∼ 3 × 10 −1 Hz, and a flattening towards 1/ f at frequencies lower than ∼ 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.

3D Feature of Self-correlation Level Contours at 1010 cm Scale in Solar Wind Turbulence

“…They show a case with the break located around 10 −4 Hz and an average velocity of 316 km/s. Our scale in the slow solar wind is smaller than the approximate break scale 3 × 10 11 cm and is in the typical quasi-Kolmogorov range, as in Bruno et al (2019). Our 3-D self-correlation level contour analysis of both the magnetic and velocity field shows a 3-D isotropic feature.…”

Dependence of 3D Self-correlation Level Contours on the Scales in the Inertial Range of Solar Wind Turbulence

“…At the large scale side of the inertial range, the fast solar wind often presents a robust 1/f scaling injection range (Horbury et al 1996;Matthaeus et al 2007;Bruno & Carbone 2013) for both the magnetic field and the velocity. The low-frequency break between the inertial range and the injection range is around 10 −3 Hz, a typical value for the fast wind at 1AU (Bruno & Carbone 2013;Bruno et al 2019). In the slow solar wind, the 1/f scaling is also present for the magnetic field with a smaller low-frequency break around 10 −4 Hz.…”

“…In the slow solar wind, the 1/f scaling is also present for the magnetic field with a smaller low-frequency break around 10 −4 Hz. However, the velocity spectrum keeps the Kolmogorov-like scaling throughout the analyzed frequency range by Bruno et al (2019). Matthaeus et al (1990) developed the 2-D self-correlation function method to study the solar wind turbulence and obtained the famous Maltese cross.…”

The multi-scale nature of the solar wind

“…The fluctuations exhibit Alfvénic polarization properties (see Sect. 4.3.1) and B ≈ constant (Matteini et al, 2018;Bruno et al, 2019).…”