  Protostellar jets are tightly connected to the accretion process and regulate
the angular momentum balance of accreting star-disk systems. The DG Tau jet is
one of the best-studied protostellar jets and contains plasma with temperatures
ranging over three orders of magnitude within the innermost 50 AU of the jet.
We present new Hubble Space Telescope (HST) far ultraviolet (FUV) long-slit
spectra spatially resolving the C IV emission (T~1e5 K) from the jet for the
first time, and quasi-simultaneous HST observations of optical forbidden
emission lines ([O I], [N II], [S II] and [O III]) and fluorescent H2 lines.
The C IV emission peaks at 42 AU from the stellar position and has a FWHM of 52
AU along the jet. Its deprojected velocity of around 200 km/s decreases
monotonically away from the driving source. In addition, we compare our HST
data with the X-ray emission from the DG Tau jet. We investigate the
requirements to explain the data by an initially hot jet compared to local
heating. Both scenarios indicate a mass loss by the T~1e5 K jet of ~1e-9
Msun/year, i.e., between the values for the lower temperature jet (T~1e4 K) and
the hotter X-ray emitting part (T>1e6 K). However, a simple initially hot wind
requires a large launching region (~1 AU), and we therefore favor local
heating.
