The interplanetary magnetic field underwent a strong southward turning at the location of the ISEE 3 spacecraft, with Bz (in satellite coordinates) decreasing from +15&ggr; to -15&ggr; in the interval from 1540 UT to 1600 UT on August, 1979. Solar wind speed measurements at the spacecraft indicated that the southward turning of the IMF should have reached the front of the magnetosphere around 1625 UT. At that time the solar proton event of August 18, 1979, was still in progress. Observations from balloon-borne and ground-based instrumentation in Greenland in that period, around 1415 magnetic local time, showed an expansion of the polar cap, the cusp moving southward by at least 5¿ in latitude in 15 min, starting around 1635 UT. From changes in the energetic photon spectrum at ballon altitude, it is shown that the proton cutoff energy at Sondre Str¿mfjord, Greenland, prior to the cusp motion was in the range 6--10 MeV.