Measurements of the afternoon radio aurora at 1295 MHz


  • W. G. Abel,

  • R. E. Newell


Afternoon radio aurora was observed on 26 days out of 54 during the period May 1965 to January 1966, with the Millstone Hill, Massachusetts, radar. The completely steerable 84-foot-diameter antenna provided a 0.6° half-power beamwidth at 1295 MHz. Echoes were obtained at azimuths within ±30° of the magnetic meridian (345°T) and at elevation angles up to 13°. They were returned from a layer-like region having a thickness of 5–20 km and located at an average height near 110 km. The echoes are strongly aspect sensitive, and none were obtained from regions where the aspect angle was greater than about 3°. Both the occurrence of auroral echoes and the southernmost limit of the echo region were found to correlate with the magnetic index Kp. The Doppler spectra of the echoes show predominantly approaching velocities to the east of the magnetic meridian and receding velocities to the west. The magnitude of the Doppler offset and the variation of offset with azimuth favor the two-stream plasma instability theory of Farley as an explanation of auroral echoes. The direction of plasma wave motion together with the variation of electrojet current with altitude may explain the presence of echoes at the magnetic meridian (not expected from the Farley theory for an east-west current).