Field-determined oscillations in the magnetosheath as possible source of medium-period, daytime micropulsations

Studies of the magnetosheath show it to be characterized by the variable occurrence of large amplitude, sometimes quasiperiodic, field oscillations at diverse distances inward from the bow shock, including the region in proximity with the magnetopause. One established cause of such oscillations near the shock is local oblique orientation of the interplanetary field to the nominal shock surface, and there is evidence that the oscillations, or pulsations, composing the oblique structure appear also downstream in the sheath. It is suggested that medium-period micropulsations, exemplified by pc 3, might be excited in the magnetosphere when proper alignment of the interplanetary field allows shock pulsations to be convected to the magnetopause by plasma flow through the subsolar section of the magnetosheath.