  Winds in Titan's lower and middle atmosphere have been determined by a
variety of techniques, including direct measurements from the Huygens Probe
over 0-150 km, Doppler shifts of molecular spectral lines in the optical,
thermal infrared and mm ranges, probing altogether the ~100-450 km altitude
range, and inferences from thermal field over 10 mbar - 10 -3 mbar (i.e.
~100-500 km) and from central flashes in stellar occultation curves. These
measurements predominantly indicated strong prograde winds, reaching maximum
speeds of ~150-200 m/s in the upper stratosphere, with important latitudinal
and seasonal variations. However, these observations provided incomplete
atmospheric sounding; in particular, the wind regime in Titan's upper
mesosphere and thermosphere (500- 1200 km) has remained unconstrained so far.
Here we report direct wind measurements based on Doppler shifts of six
molecular species observed with ALMA. We show that unlike expectations, strong
prograde winds extend up to the thermosphere, with the circulation
progressively turning into an equatorial jet regime as altitude increases,
reaching ~340 m/s at 1000 km. We suggest that these winds may represent the
dynamical response of forcing by waves launched at upper
stratospheric/mesospheric levels and/or magnetospheric-ionospheric interaction.
We also demonstrate that the HNC distribution is restricted to Titan's
thermosphere above ~870 km altitude.
