The Observational Anatomy of Externally Photoevaporating Planet-Forming
  Discs I: Atomic Carbon
  We demonstrate the utility of CI as a tracer of photoevaporative winds that
are being driven from discs by their ambient UV environment. Commonly observed
CO lines only trace these winds in relatively weak UV environments and are
otherwise dissociated in the wind at the intermediate to high UV fields that
most young stars experience. However, CI traces unsubtle kinematic signatures
of a wind in intermediate UV environments ($\sim1000$G$_0$) and can be used to
place constraints on the kinematics and temperature of the wind. In CI
position-velocity diagrams external photoevaporation results in velocities that
are faster than those from Keplerian rotation alone, as well as emission from
quadrants of position-velocity space in which there would be no Keplerian
emission. This is independent of viewing angle because the wind has components
that are perpendicular to the azimuthal rotation of the disc. At intermediate
viewing angles ($\sim 30-60^\circ$) moment 1 maps also exhibit a twisted
morphology over large scales (unlike other processes that result in twists,
which are typically towards the inner disc). CI is readily observable with
ALMA, which means that it is now possible to identify and characterise the
effect of external photoevaporation on planet-forming discs in intermediate UV
environments.
