Observation of F Region Vertical Velocities at Millstone Hill, 1, Evidence for Drifts Due to Expansion, Contraction, and Winds


  • J. V. Evans


The theory for the drift of ions in the F region is briefly reviewed, and it is shown that, for the restricted case of no external electric fields or neutral air motion, the velocity of the ions along the magnetic field lines in the upper part of the F2 region can be derived from the continuity equation. Experimental results for the electron density, electron and ion temperatures, and O+ vertical velocity obtained by the incoherent (Thomson) scatter technique at Millstone Hill are presented for two 24-hour periods taken a year apart in March 1969 and 1970. The largest vertical velocities encountered occur around sunrise and sunset and give rise to vertical fluxes of the order of 5 × 108 ions/cm2/sec through 600-km altitude. It is shown that these fluxes are a result primarily of the growth or decay of the layer, though in the case of sunset the thermal contraction resulting from the decrease in the electron temperature with time assists in establishing the observed fluxes. The vertical velocity observed near 300 km is compared with the expected ambipolar diffusion velocity that should result from the observed pressure gradients in the plasma and from acceleration due to gravity. The difference between the observed and computed velocities is attributed to thermospheric neutral winds which have diurnal behavior similar to those determined by Vasseur (1969a, b) from incoherent scatter observations in France.