Analytical investigation of turbulence quantities and cosmic ray mean free paths from 1995-2017

Abstract

We use OMNI 1-minute resolution data sets from 1995 through 2017, covering about two consecutive solar cycles, to investigate the solar cycle dependence of various turbulence quantities and cosmic ray (CR) mean free paths. We employ quasi-linear theory (QLT) and nonlinear guiding center theory (NLGC) to evaluate the CR parallel and perpendicular diffusion. We find that in the ecliptic plane at 1 au (1) the fluctuating magnetic energy density ⟨z±2⟩, residual energy ED, and corresponding correlation functions all have an obvious solar cycle dependence. The residual energy ED is always negative, which indicates that the energy in magnetic fluctuations is larger than the energy in kinetic fluctuations, especially at solar maximum; (2) the correlation length λ for magnetic fluctuations does not show significant solar cycle variation; (3) the temporally varying shear source of turbulence, which is most important in the inner heliosphere, depends on the solar cycle; and (4) high level turbulence will increase CR perpendicular diffusion and decrease CR parallel diffusion, but this trend can be masked if the background interplanetary magnetic field (IMF) changes in concert with turbulence in response to solar activity. These results provide quantitative inputs for both turbulence transport models and CR diffusion coefficient models.