This study presents measurements of group delays, rise times, fall times, frequencies, and effective phase velocities of mid-latitude (39¿--55¿ geomagnetic latitude) Pi 2's recorded by the Air Force Geophysics Laboratory array of flux gate magnetometers. Measured group delays over the array are sometimes less than 5--10 s, indicating that in some instances Pi 2's can be used to give excellent timing of the onset of substorm expansive phases. Rise and fall times for the envelopes of Pi 2 oscillations are typically 60--100 s, bringing into doubt resonance theories for these types of waves. Most phase velocities indicate westward propagation, particularly near the center of the substorm current wedge, although there is some indication of eastward velocities far to the east of the center. We argue that these observations indicate that the magnetic fields associated with mid-latitude Pi 2's are due to transient changes in the field-aligned currents feeding the auroral electrojets at the beginning of the expansive phase of the substorm.