The relationships of broad band geomagnetic micropulsations (periods from 20-300 sec.) at Strawberry Hill, Concord, Mass, to some magnetospheric, interplanetary, and solar phenomena are examined to ascertain possible locations of energy sources for their excitation. The micropulsation activity is characterized by a one-hour, quasi-logarithmic index, M, which is chiefly a measure of hydromagnetic (HM) wave amplitudes. The strong diurnal variation in the HM wave energy continues to elude adequate explanation.However, it is apparent that the excitation of these waves is favored in the daylight hemisphere. High micrapulsation activity levels generally obtain during periods of sustained magnetic disturbance, but several exceptions preclude the existence of a direct cause-and-effect relationship. Indirect evidence suggests the importance of magnetospheric convection in controlling the micropulsation activity level. Daily micropulsation activity levels are more closely related to interplanetary magnetic field variability than to daily angle of the interplanetary magnetic field vector in solar ecliptic coordinates) and micropulsation activity is indicated by the present study. Superposed epoch analysis reveals an association of highly disturbed micropulsation days with the central meridian passage (CMP), 3 to 5 days earlier, of solar regions characterized by low levels of calcium plage activity. It is suggested that high micropulsation activity levels are more closely associated with the CMP of unipolar magnetic regions (UMR) than are high geomagnetic activity levels.

A 5‐month sample of geomagnetic micropulsation data from Strawberry Hill, Massachusetts, +54° 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 2° 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.