Measurements of D- and E-Region Electron Densities by the Incoherent Scatter Technique at Millstone Hill


  • G. W. Armistead,

  • J. V. Evans,

  • W. A. Reid


At Millstone Hill it is difficult to make incoherent scatter measurements of the electron density in the ionosphere at altitudes below 160 km because of the presence of interfering ground-clutter echoes. Modifications have been made to the zenith-pointing 220-foot antenna to reduce the energy radiated at low angles, and as a result the ground-clutter echoes now do not normally exceed the dynamic range of the receiver. Accordingly, it is possible to filter the clutter echoes from the ionospheric echoes on the basis of their narrow spectrum. Employing this approach, we have been able to measure electron densities in the E and D regions down to an altitude of 75 to 80 km. In the measurements made so far, a 3-km-height resolution has been employed and the results clearly indicate the presence of sporadic E layers and other features regularly seen on rocket flights. Echoes appear to be obtained at altitudes below 80 km that are too strong to be caused by free electrons, and it may be that these echoes result from imperfect cancellation of clutter returns or from turbulent irregularities in the neutral atmosphere.