QUANTIFYING THE ANISOTROPY AND SOLAR CYCLE DEPENDENCE OF “1/f” SOLAR WIND FLUCTUATIONS OBSERVED BY ACE

The solar corona expands non-uniformly into space as a supersonic plasma outflow known as the solar wind (Parker 1958). The solar wind carries signatures of coronal dynamics as well as locally generated turbulent phenomena, which span a broad range of scales. In situ spacecraft observations of fluctuations in solar wind parameters such as velocity and magnetic field (for example, Ruzmaikin et al. (1993) in the ecliptic plane and Horbury et al. (1995a) in polar flows) typically reveal an inertial range (IR) of turbulence with a “5/3” inverse power-law scaling at high frequencies and a flatter “1/f”-like scaling range at lower frequencies (Matthaeus & Goldstein 1986). The breakpoint between these two ranges is seen to evolve radially (Bavassano et al. 1982; Horbury et al. 1996a) with the inertial range extending to lower frequencies with increasing radial distance, suggesting a turbulent energy cascade in the solar wind. The solar wind also has a background magnetic field and is therefore a highly anisotropic plasma environment (Shebalin et al. 1983; Oughton et al. 1994). The strength of this background field relative to the amplitude of the fluctuations determines whether the turbulence is “strong”, i.e the amplitude of fluctuations is comparable to that of the background magnetic field (Sridhar & Goldreich 1994; Goldreich & Sridhar 1995) or “weak”, i.e the background magnetic field is dominant (Ng & Bhattacharjee 1997; Galtier et al. 2000). This inertial range has been extensively studied using timeseries analysis techniques including power spectra (Marsch & Tu 1990a), probability density functions (PDFs) (Marsch & Tu 1997; Padhye et al. 2001; Bruno et al. 2004) and generalised structure funtions (GSFs) (e.g. Horbury & Balogh 1997; Hnat et al. 2005; Sorriso-Valvo et al. 2007; Chapman & Hnat 2007; Nicol et al. 2008). In this paper we focus on the low frequency “1/f” range, where the observed α ∼ 1 for