  The transport of energetic particles in response to solar wind turbulence is
important for space weather. To understand charged particle transport, it is
usually assumed that the phase of the turbulence is randomly distributed (the
random phase approximation) in quasi-linear theory and simulations. In this
paper, we calculate the coherence index, $C_\phi$, of solar wind turbulence
observed by the Helios 2 and Parker Solar Probe spacecraft using the surrogate
data technique to check if the assumption is valid. Here, values of $C_\phi=0$
and $1$ indicate that the phase coherence is random and correlated,
respectively. We estimate that the coherence index at the resonant scale of
energetic ions ($10$ MeV protons) is $0.1$ at $0.87$ and $0.65$ au, $0.18$ at
$0.29$ au, and $0.3$ ($0.35$) at $0.09$ au for super (sub)-Alfv\'enic
intervals, respectively. Since the random phase approximation corresponds to
$C_\phi=0$, this may indicate that the random phase approximation is not valid
for the transport of energetic particles in the inner heliosphere, especially
very close to the Sun ($\sim0.09$ au).
