  We present a model for magnetic energy dissipation in a pulsar wind nebula.
Better understanding of this process is required to assess the likelihood that
certain astrophysical transients may be powered by the spin-down of a
"millisecond magnetar." Examples include superluminous supernovae, gamma-ray
bursts, and anticipated electromagnetic counterparts to gravitational wave
detections of binary neutron star coalescence. Our model leverages recent
progress in the theory of turbulent magnetic relaxation to specify a
dissipative closure of the stationary magnetohydrodynamic (MHD) wind equations,
yielding predictions of the magnetic energy dissipation rate throughout the
nebula. Synchrotron losses are treated self-consistently. To demonstrate the
model's efficacy, we show that it can reproduce many features of the Crab
Nebula, including its expansion speed, radiative efficiency, peak photon
energy, and mean magnetic field strength. Unlike ideal MHD models of the Crab
(which lead to the so-called sigma problem) our model accounts for the
transition from ultra to weakly magnetized plasma flow, and for the associated
heating of relativistic electrons. We discuss how the predicted heating rates
may be utilized to improve upon models of particle transport and acceleration
in pulsar wind nebulae. We also discuss implications for the Crab Nebula's
gamma-ray flares, and point out potential modifications to models of
astrophysical transients invoking the spin-down of a millisecond magnetar.
