The magnetic power spectrum in the solar wind is known to be characterized by a double power law at scales much larger than the proton gyroradius (from intermediate to large scales), with flatter spectral exponents close to -1 found at the lower frequencies below an inertial range with indices closer to [-1.5, -1.67]. This double power law is usually found in fast solar wind, in very long intervals of slow wind, and also in extremely long intervals without regard to wind speed. The low frequency range of the spectrum has been considered the energy reservoir that facilitates the turbulence cascade in the solar wind.

The origin and formation mechanism of the 1/f range is still not well understood and is under active debate [1, 2, 3]. Some studies suggested that the 1/f scaling is produced at the sun as a result of uncorrelated samples of fluctuations originating from different regions of the solar surface [1]. While other authors have attributed the 1/f scaling in the fast solar wind to turbulent dynamics within the solar wind [2, 4]. In fact, understanding the origin of the interplanetary 1/f observations was counted among the scientific motivations for the design of the Parker Solar Probe (PSP) mission [5].

We studied the presence of the 1/f spectrum using observational data from PSP and direct numerical simulation of MHD turbulence. We compare two regimes between having a strong and weak magnetic field. For the simulations, this implies fixing the value of the mean magnetic field guide, and for in-situ data, this translates into having measurements with greater (aphelion case) or lesser (perihelion) heliocentric distance and therefore, weak and strong mean magnetic field, respectively. In all observational cases, we used fast solar wind events. We compared these results by using a moving window so as to fit and determine (and quantify) the slope and range of 1/f of the magnetic energy spectra. We observed a range of one decade of 1/f spectrum for the strong magnetic field regime, which is larger than the weak magnetic field case in numerical simulations. As for the observational data, no significant 1/f range has been found. Moreover, the break between the two different scaling laws is only observed in the perihelion case.

[1] Matthaeus, W. H., & Goldstein, M. L. 1986, PhRvL, 57, 495.

[2] Velli, M., Grappin, R., & Mangeney, A. 1989, PhRvL, 63, 1807.

[3] Dmitruk, P., & Matthaeus, W. H. 2007, PhRvE, 76, 036305.

[4] Verdini, A., Grappin, R., Pinto, R., & Velli, M. 2012, ApJL, 750, L33.

[5] Fox, N. J., Velli, M. C., Bale, S. D., et al. 2016, SSRv, 204, 7.