  We present an analysis of STIS/HST optical spectra of a sample of ten Seyfert
galaxies aimed at studying the structure and physical properties of the
coronal-line region (CLR). The high-spatial resolution provided by STIS allowed
us to resolve the CLR and obtain key information about the kinematics of the
coronal-line gas, measure directly its spatial scale, and study the mechanisms
that drive the high-ionisation lines. We find CLRs extending from just a few
parsecs (~10 pc) up to 230 pc in radius, consistent with the bulk of the
coronal lines (CLs) originating between the BLR and NLR, and extending into the
NLR in the case of [FeVII] and [NeV] lines. The CL profiles strongly vary with
the distance to the nucleus. We observed line splitting in the core of some of
the galaxies. Line peak shifts, both red- and blue-shifts, typically reached
500 km/s, and even higher velocities (1000 km/s) in some of the galaxies. In
general, CLs follow the same pattern of rotation curves as low-ionisation lines
like [OIII]. From a direct comparison between the radio and the CL emission we
find that neither the strength nor the kinematics of the CLs scale in any
obvious and strong way with the radio jets. Moreover, the similarity of the
flux distributions and kinematics of the CLs and low-ionisation lines, the low
temperatures derived for the gas, and the success of photoionisation models to
reproduce, within a factor of few, the observed line ratios, point towards
photoionisation as the main driving mechanism of CLs.
