Magnetic data observed at conjugate pair stations near L=4 have been used to compute dynamic power spectra in the frequency range 1-9 MHz (T=1000-110 s) for 15 days with various levels of magnetic activity. It is found that the dynamic spectral patterns at the conjugate points are quite similar to each other, often even in their fine structure. The dynamic spectra show a significant diurnal variation of the power intensity with a large power during the daytime hours, regardless of the level of magnetic activity. During the nightime hours on extremely quiet days the power intensity remains at much lower levels compared with the daytime intensity level, while on disturbed days the power amplitude during the nighttime hours is enhanced in association with the occurrences of substorms. It is also found that the conjugate power ratio has a well-defined local time dependence. The results suggest that the magnetic fluctuations in this frequency range arise principally from energy within the magnetosphere. The results also suggest that a main energy source in the Pc 5 frequency range comes from the day side of the magnetosphere, and the energy sources in the night side are closely related to the substorm mechanism. It is alos suggested that the fluctuation of the Spq current is important as an energy source in the Pc 5 frequency range. |