  Magnetospheres of pulsars are thought to be filled with plasma, and
variations in plasma supply can affect both pulsar emission properties and
spin-down rates. A number of recently discovered "intermittent" pulsars switch
between two distinct states: an "on", radio-loud state, and an "off",
radio-quiet state. Spin-down rates in the two states differ by a large factor,
$\sim 1.5-2.5$, which is not easily understood in the context of current
models. In this Letter we present self-consistent numerical solutions of "on"
and "off" states of intermittent pulsar magnetospheres. We model the "on" state
as a nearly ideal force-free magnetosphere with abundant magnetospheric plasma
supply. The lack of radio emission in the "off" state is associated with plasma
supply disruption that results in lower plasma density on the open field lines.
We model the "off" state using nearly vacuum conditions on the open field lines
and nearly ideal force-free conditions on the closed field lines, where plasma
can remain trapped even in the absence of pair production. The toroidal
advection of plasma in the closed zone in the "off" state causes spin-downs
that are a factor of $\sim 2$ higher than vacuum values, and we naturally
obtain a range of spin-down ratios between the "on" and "off" states, $\sim
1.2-2.9$, which corresponds to a likely range of pulsar inclination angles of
$30{-}90^\circ$. We consider the implications of our model to a number of
poorly understood but possibly related pulsar phenomena, including nulling,
timing noise, and rotating radio transients.
