  We investigate the radial dependence of the spectral break separating the
inertial from the dissipation range in power density spectra of interplanetary
magnetic field fluctuations, between $0.42$ and $5.3$ AU, during radial
alignments between MESSENGER and WIND for the inner heliosphere and between
WIND and ULYSSES for the outer heliosphere. We found that the spectral break
moves to higher and higher frequencies as the heliocentric distance decreases.
The radial dependence of the corresponding wavenumber is of the kind
$\kappa_b\sim R^{-1.08}$ in good agreement with that of the wavenumber derived
from the linear resonance condition for proton cyclotron damping. These results
support conclusions from previous studies which suggest that a
cyclotron-resonant dissipation mechanism must participate into the spectral
cascade together with other possible kinetic noncyclotron-resonant mechanisms.
