Geomagnetic pulsations from 0.2 to 5.0 Hz (Pc 1) observed on the ground could be used to monitor the magnetospheric processes generating them if their source regions were known. We report simultaneous observations of Pc 1 bursts at South Pole Station and in space at 9 Re geocentric distance near the magnetic equator showing that the subsolar outer magnetosphere, earthward of the low latitude boundary layer, is the source region of Pc 1 bursts. The source extends 1 to 2 hours azimuthally and 1 to 2 Re radially. Correlation of magnetopause displacements and bursts at South Pole indicates that the bursts are stimulated by compressions of the dayside magnetosphere. In space the bursts occur above the He+ gyrofrequency implying that the H+-He+ bi-ion resonance does not prevent the signals from reaching the ionosphere. ¿ American Geophysical Union 1996