  The correlation scale and the Taylor scale are evaluated for interplanetary
magnetic field fluctuations from two-point, single time correlation function
using the Advanced Composition Explorer (ACE), Wind, and Cluster spacecraft
data during the time period from 2001 to 2017, which covers over an entire
solar cycle. The correlation scale and the Taylor scale are respectively
compared with the sunspot number to investigate the effects of solar activity
on the structure of the plasma turbulence. Our studies show that the Taylor
scale increases with the increasing sunspot number, which indicates that the
Taylor scale is positively correlated with the energy cascade rate, and the
correlation coefficient between the sunspot number and the Taylor scale is
0.92. However, these results are not consistent with the traditional knowledge
in hydrodynamic dissipation theories. One possible explanation is that in the
solar wind, the fluid approximation fails at the spatial scales near the
dissipation ranges. Therefore, the traditional hydrodynamic turbulence theory
is incomplete for describing the physical nature of the solar wind turbulence,
especially at the spatial scales near the kinetic dissipation scales.
