  Using data from orbits one to eleven of the Parker Solar Probe (PSP) mission,
the magnetic field spectral index was measured across a range of heliocentric
distances. The previously observed transition between a value of $-5/3$ far
from the Sun and a value of $-3/2$ close to the Sun was recovered, with the
transition occurring at around $50 \, R_{\odot}$ and the index saturating at
$-3/2$ as the Sun is approached. A statistical analysis was performed to
separate the variation of the index on distance from its dependence on other
parameters of the solar wind that are plausibly responsible for the transition;
including the cross helicity, residual energy, turbulence age and the magnitude
of magnetic fluctuations. Of all parameters considered the cross helicity was
found to be by far the strongest candidate for the underlying variable
responsible. The velocity spectral index was also measured and found to be
consistent with $-3/2$ over the range of values of cross helicity measured.
Possible explanations for the behaviour of the indices are discussed, including
the theorised different behaviour of imbalanced, compared to balanced,
turbulence.
