  The C II] feature at ~2325 Angstrom is very prominent in the spectra of T
Tauri stars (TTSs). This feature is a quintuplet of semiforbidden transitions
excited at electron temperatures around 10,000 K that, together with the nearby
Si II] and Fe II] features, provides a reliable optically thin tracer for
accurate measurement of the plasma properties in the magnetospheres of TTSs.
The spectra of 20 (out of 27) TTSs observed with the Space Telescope Imaging
Spectrograph (STIS) on board the Hubble Space Telescope (HST) have good enough
signal to noise ratio (S/N) at the C II] wavelength. For these stars we have
determined electron densities (ne) and temperatures (Te) in the line emission
region as well as the profile broadening (sigma). For most of the stars in the
sample (17) we obtain 10^{4.1} < Te < 10^{4.5} K and 10^{8} < ne < 10^{12}
cm^{-3}. These stars have suprathermal line broadening (35 < sigma < 165 km
s^{-1}), except TW Hya and CY Tau with thermal line broadening. Both C II] line
luminosity and broadening are found to correlate with the accretion rate. Line
emission seems to be produced in the magnetospheric accretion flow, close to
the disk. There are three exceptions: DG Tau, RY Tau and FU Ori. The line
centroids are blueshifted indicating that the line emission in these three
stars is dominated by the outflow.
