Global Pc5 pulsations observed at unusually low L during the great magnetic storm of 24 March 1991

Using CANOPUS, GREENLAND, IMAGE, SAMNET, and 210 MM ground-based magnetometer array data, we examine the global characteristics of ULF waves during the onset day of the great magnetic storm of 24 March 1991. We focus on the main phase in the period of decreasing Dst and investigate the characteristics of discrete frequency Pc5 pulsations. Pc5 pulsations with multiple discrete spectral peaks including a 2.8 mHz pulsation were seen at 0815-0915 UT, and a monochromatic 1.9 mHz wave appeared at 1010-1110 UT. Also, a very monochromatic and large-amplitude 1.7 mHz wave was observed at 1200-1340 UT across a wide range of dayside local times and at unusually low latitudes. We have analyzed the amplitude and phase relationships of waves at these frequencies. The 2.8 mHz, 1.9 mHz, and 1.7 mHz spectral peaks have maximum amplitudes at L = 3.6, L = 4, and L = 4.3, respectively, and exhibit the characteristic features of field line resonances ( FLRs). The discrete mHz frequencies are consistent with some of the discrete peaks seen by Samson et al. ( 1992) and support the hypothesis of the existence of MHD cavity or waveguide modes in the magnetosphere. However, our results show evidence for the penetration of ULF wave power in the Pc5 band to much lower L-values than normal, suggesting a significant reduction of the local Alfven eigenfrequency continuum as compared to nonstorm times. This may have considerable significance for the interaction between ULF waves and MeV electrons in the outer radiation belt during storms.