Broad Band Micropulsation Activity at a Geomagnetic Midlatitude Station

The relationships of broad band geomagnetic micropulsations (periods from 20-300sec.) 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 micropulsation 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 averages of the solar wind velocity. No significant relationship between the sign of θ (the latitude 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.