Spectral characteristics of Pc 3 and Pc 4/5 magnetic pulsation bands observed near L = 6

From dynamic spectral analysis of magnetic pulsations observed at auroral zone stations near L = 6, it has been found that there are two predominant spectral bands throughout the daytime, one in the Pc 3 range with frequencies of ∼20–80 mHz and the other in the Pc 4/5 range with frequencies of ∼3‐10 mHz. At synchronous orbit, corresponding spectral bands were seen in the azimuthal component with a harmonic structure when the satellite was located at the off‐magnetic equator. The result suggests that the 3‐ to 10‐mHz band is the fundamental mode of standing shear Alfvén waves and the 20‐ to 80‐mHz band is the higher harmonics. However, both bands were not always enhanced simultaneously. Switching of activity between both bands was frequently observed on magnetically quiet days. This result indicates that the Pc 3 and Pc 4/5 bands are excited by different external driving sources. It is likely that when the frequency range of the external driving source is in the Pc 3 range, this driving source excites the higher harmonics of standing Alfvén waves near L = 6, while when the frequency range is in the Pc 4/5 range, the external driving source excites the fundamental mode.