First results on the observation of 440-MHz high-latitude coherent echoes from the E region with the Millstone Hill Radar


  • J.-P. St.-Maurice,

  • J. C. Foster,

  • J. M. Holt,

  • C. del Pozo


During geomagnetically active periods, the 440-MHz steerable Millstone Hill incoherent scatter radar system often receives powerful coherent radar echoes when observing the lower ionosphere in regions roughly to the north of the antenna, and for distances ranging between 700 and 1000 km. With the exception of a few anomalous events, the echoes clearly originate from the E region, and the aspect angle of the irregularities responsible for the coherent echoes is within one half degree of perpendicularity to the magnetic field. The strength of these coherent E region echoes increases with the inferred electric field strength. In contrast with similar experiments reported earlier with the Homer system, the Doppler shift of the coherent spectrum that we observe seems to correspond to the line of sight component of the plasma drift unless this component exceeds the ion acoustic speed. In the latter case, the Doppler shift then appears to be limited to the ion acoustic speed as it does with other coherent radar systems. The mean height of the scattering layer varies between 105 and 115 km for any single event, while there is some evidence to suggest that the vertical width of the scattering volume may be as small as 4 to 5 km at times. We saw evidence for an occasional layering of the E region scattering into two distinct regions. As a result of the present study, we plan to exploit the unique sensitivity of the Millstone Hill radar to make direct simultaneous comparisons between mean F region plasma drifts and the Doppler shift of E region irregularities.