Auroral oval micropulsations at a subauroral zone station

A 5-month sample of geomagnetic micropulsation data from Strawberry Hill, Massachusetts, +540 geomagnetic latitude, is analyzed in octave period bandwidths in the 0.5 to 1024-s period range. Hourly average amplitudes of the horizontal and vertical components are used as measures of the activity. The auroral oval (AO) regime of micropulsations is identified by large H and Z amplitudes combined with small H/Z ratios. It is observed during large nighttime disturbances within a ''micropulsation oval'' whose southern boundary extends 1 or 20 of latitude south of the oval delineated by particle precipitation and is attributed to hydromagnetic waves which propagate in the guided mode from the plasma sheet in the magnetospheric tail. Spatial variations of the source strength in the plasma sheet, which vary with micropulsation period, are reflected in the diurnal variations of the AO amplitudes