Ground-based measurements of the phase velocities, group velocities, and rise times of the maagnetic fields of high-latitude (60¿--68¿N corrected geomagnetic latitude), substorm associated Pi 2's indicate that these pulsations may be caused by the transient reflection of an Alfv¿n wave from the auroral ionosphere. Individual pulsation wave packets can be very short, typically one cycle, with a duration of 100--150 s. The phase velocities are eastward to the east of the region of the onset of the field-aligned currents, and westward to the west of the region of the onset. The group velocities follow a pattern similar to that of the phase velocities but tend to have values somewhat lower than the phase velocities. The longitudinal phase velocities are very high, typically 20 km/s or greater, suggesting that the Pi 2's are not connected with auroral forms such as the westward traveling surge but are more likely connected with the initial brightening of the arc associated with the breakup. Consequently, the longitudinal expansion of the field-aligned currents and Alfv¿n waves causing the Pi 2's may be a direct result of the mechanism leading to the onset of the substorm expansive phase in the magnetotail.