Using STIS on board the HST we have obtained a spectroscopic map of the bipolar jet from RW~Aur with the slit parallel to the jet axis and moved across the jet in steps of 0 \aas@@fstack { \prime \prime } 07 .
After applying a velocity correction due to uneven slit illumination we find signatures of rotation within the first 300 AU of the jet ( 1 \aas@@fstack { \prime \prime } 5 at the distance of RW Aur ) .
Both lobes rotate in the same direction ( i.e .
with different helicities ) , with toroidal velocities in the range 5 - 30 km s ^ { -1 } at 20 and 30 AU from the symmetry axis in the blueshifted and redshifted lobes , respectively .
The sense of rotation is anti-clockwise looking from the tip of the blue lobe ( P.A .
130 ^ { \circ } north to east ) down to the star .
Rotation is more evident in the [ OI ] and [ NII ] lines and at the largest sampled distance from the axis .
These results are consistent with other STIS observations carried out with the slit perpendicular to the jet axis , and with theoretical simulations .

Using current magneto-hydrodynamic models for the launch of the jets , we find that the mass ejected in the observed part of the outflow is accelerated from a region in the disk within about 0.5 AU from the star for the blue lobe , and within 1.6 AU from the star for the red lobe .
Using also previous results we estimate upper and lower limits for the angular momentum transport rate of the jet .
We find that this can be a large fraction ( two thirds or more ) of the estimated rate transported through the relevant portion of the disk .
The magnetic lever arm ( defined as the ratio r _ { A } / r _ { 0 } between the Alfvèn and footpoint radii ) is in the range 3.5 - 4.6 ( with an accuracy of 20 - 25 % ) , or , alternatively , the ejection index \xi = d \ln ( \dot { M } _ { \mathrm { acc } } ) / dr is in the range 0.025 - 0.046 ( with similar uncertainties ) .
The derived values are in the range predicted by the models , but they also suggest that some heating must be provided at the base of the flow .

Finally , using the general disk wind theory we derive the ratio B _ { \phi } / B _ { p } of the toroidal and poloidal components of the magnetic field at the observed location ( i.e .
about 80 - 100 AU above the disk ) .
We find this quantity to be 3.8 \pm 1.1 at 30 AU from the axis in the red lobe and -8.9 \pm 2.7 at 20 AU from the axis in the blue lobe ( assuming cylindrical coordinates centred on the star and with positive z along the blue lobe ) .
The toroidal component appears to be dominant , which would be consistent with magnetic collimation of the jet .
The field appears to be more tightly wrapped on the blue side .