  M82 is an archetypal starburst galaxy in the local Universe. The central
burst of star formation, thought to be triggered by M82's interaction with
other members in the M81 group, is driving a multiphase galaxy-scale wind away
from the plane of the disk that has been studied across the electromagnetic
spectrum. Here, we present new velocity-resolved observations of the [CII]
158$\mu$m line in the central disk and the southern outflow of M82 using the
upGREAT instrument onboard SOFIA. We also report the first detections of
velocity-resolved ($\Delta V = 10$ km s$^{-1}$) [CII] emission in the outflow
of M82 at projected distances of $\approx1-2$ kpc south of the galaxy center.
We compare the [CII] line profiles to observations of CO and HI and find that
likely the majority ($>55$%) of the [CII] emission in the outflow is associated
with the neutral atomic medium. We find that the fraction of [CII] actually
outflowing from M82 is small compared to the bulk gas outside the midplane
(which may be in a halo or tidal streamers), which has important implications
for observations of [CII] outflows at higher redshift. Finally, by comparing
the observed ratio of the [CII] and CO intensities to models of
photodissociation regions, we estimate that the far-ultraviolet (FUV) radiation
field in the disk is $\sim10^{3.5}~G_0$, in agreement with previous estimates.
In the outflow, however, the FUV radiation field is 2-3 orders of magnitudes
lower, which may explain the high fraction of [CII] arising from the neutral
medium in the wind.
