A sudden commencement occurred at. 2348 UT on February 15, 1967 when the ATS-1 satellite was 2 hours past local noon at a geocentric distance of 6.6 Re. Plasma was observed by the Suprathermal ion Detector first to flow in the antisolar direction, as expected, but then to flow westward, for about 2 minutes, at about 50 km/sec. Analysis of ground magnetograms suggests that the surprising westward flow at 6.6 R resulted the ionosphere's reaction to the sudden commencement. Beginning about 2 minutes before the start of westward flow at ATS-1,ground magnetometers near the foot of the ATS-1 field line recorded average magnetic-field deflections of about 100Y ,to the northeast. For an assumed height-integrated Hall conductivity of 1 mho, and a standoff distance of 7.2 R derived from Explorer 33 plasma data, the ground magnetic variations imply an electric field that agrees in magnitude and direction with that required to produce the observed flow at ATS-1.