Nightside, high-latitude (68¿--80¿ magnetic) Pi1 waves, measured with a ground array of induction magnetometers, are studied and compared with magnetic field measurements made at synchronous orbit near the meridian of the ground measurements. The objectives of the study are to relate the ground signatures of Pi1B and PiC to the auroral substorm and its manifestation at synchronous orbit in an attempt to understand the origin of the Pi1 waves. Pi1 waves are measured in the equatorial plane by the GOES spacecraft and appear to be initiated by the dipolarization process of the nightside tail magnetic field at the onset of substorms. Across two meridional arrays of ground stations the earliest onset of Pi1B generally occurred at the lowest-latitude station and, in many instances, this burst was superimposed on a ground signature of a sudden onset of the westward electrojet. In one instance, where good coverage of ground optical data was available, this sudden onset Pi1B was time related to the overhead passage of a westward traveling auroral surge. The timing of maximum Pi1B across the array in both longitude and latitude agrees with the westward motion of the local auroral surge and the poleward motion of the aurora after the surge has arrived at a given site, suggesting a local ionospheric source for some of the Pi1 waves. However, across the entire array, extending about 12¿ in latitude and 20¿ in longitude, there often was nearly simultaneous Pi1B wave power at all sites which occurred before the maximum signal at a given site (and presumably local aurora), suggesting horizontal ducting of wave power from the onset of Pi1B seen earlier at the auroral zone latitude. Prompt turn-on of PiC waves across the array also indicates ducting of these waves. The narrow bandwidth of the PiC waves themselves suggests a resonant cavity source for them which would indicate that some wave power enters the ionosphere from space (consistent with the GOES in situ data) as the trigger wave for this resonant source mechanism. In conclusion, this study finds evidence for local ionospheric currents, magnetospheric waves, and resonant cavity modes as sources for Pi1 ground waves. ¿ 1998 American Geophysical Union