Mechanisms of pc 2, 3 (5-40sec), pc 4 (40-150sec) and pc 5 (150-900sec) are studied. More than 6, 000 series of continuous pulsations are analyzed from the magnetograms at Fredericksburg, Byrd Station, Yakutsk, Onagawa and other auxiliary 20 stations. Results are summarized as follows:
1) Geomagnetic micropulsations of pc 4 and pc5 are caused by hydromagnetic standing oscillations along the geomagnetic field-lines, while pc 2, 3 are induced by hydromagnetic standing oscillation below the region of maximum Alfvén wave velocity. The disturbed region on the frontal surface of the earth's cavity generated by solar wind would be the source of the hydromagnetic waves responsible for the pulsations.
2) The world-wide shape of the base of the protonosphere surrounding the earth is inferred from the distribution of pc 2, 3 period, namely, an inverted U-type diurnal variation and a latitudinal dependence of the period. The various types of pc 2, 3 activity diurnal variation (M-, N- and A-types) and of its seasonal variation (S-, E- and C-types) may be due to the F2 control and the axial effect of the sun.
3) As for pc 4 and pc 5, a clear U-type diurnal variation in period seems to be attributed to the day-side compression of the geomagnetic field-line by solar wind.
4) Non-seasonal annual variations in the pc 4 and pc 5 periods are detected. It is found that the non-seasonal variations are related closely with those in whistler dispersion, satellite drag, f₀F2 and f₀E. Among several possibilities of the mechanism, the effect of the elipticity of the earth's orbit on the ionospheric density is deemed the most dominant cause of the common non-seasonal variation. It is stressed that pc 4 and pc 5, like whistlers, have powerful competence for exploring the physical state of the exosphere by considering both diurnal and seasonal variations between whistlers and pc's 4 and 5 phenomena.