Analysis of selected observations of magnetic turbulence in the solar wind
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Models for the modulation of galactic cosmic rays, such as those used to study space weather (see discussion by Moloto et al., 2018), require as input a di usion tensor, which in turn depends on the magnetic turbulence spectrum (see, e.g., Bieber et al., 1994; Engelbrecht and Burger, 2013a,b). It is convention to assume that the inertial range of the magnetic turbulence spectrum is a power law with the Kolmogorov spectral index. The current project introduces a numerical technique, which will analyse the power spectra derived from Voyager 1 magnetic eld data, in order to characterise the form of the turbulence power spectrum. The most common form taken on by the power spectrum is that with a Kolmogorov spectral index associated with the inertial range of the turbulence spectrum. Driving by pickup ions (see, e.g., Zank, 1999; Isenberg, 2005; Cannon et al., 2014; Aggarwal et al., 2016; Cannon et al., 2017), at scales around the proton gyro frequency are suggested to influence the form of the turbulence power spectrum. If this driving is signi cant, it should show up in Voyager 1 spacecraft measurements beyond about 20 AU. The current project shows however no signi cant changes to the underlying power spectrum in the presence of pickup ion peaks. The presence of pickup ions is indicated by small enhancements above the background spectrum at the gyrofrequency of the pickup ion species. The current project finds evidence for helium and proton pickup ion species. The results of the current project show that peaks associated with helium pickup ions are more frequent at smaller radial distances and those associated with proton pickup ions are more frequent at larger radial distances. The current project also includes a theoretical discussion of the total turbulence power spectrum and compares the result to that presented in Bieber (1996). The current results are more general and agrees with that presented in Bieber (1996) for inertial range spectral index q = 2.