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

Abstract

[1] 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 Alfvén 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.

Association of radiation belt electron enhancements with earthward penetration of Pc5 ULF waves: a case study of intense 2001 magnetic storms

“…It was also during the most intense storm of 31 March 2001 that the peak Pc5 wave power was observed the deepest into the magnetosphere, the same as the peak of the relativistic electron flux and the inner boundary of the outer electron radiation belt. In the past, Pc5 waves had been observed at unusual depths in the inner magnetosphere during extreme magnetospheric events, such as the great magnetic storm of 24 March 1991 (Lee et al, 2007) and the magnetic super-storm of Halloween (Loto'aniu et al, 2006;Marin et al, 2014). This earthward penetration of ULF wave activity is not, however, limited to extreme magnetic storms.…”

The interaction of ultra-low-frequency pc3-5 waves with charged particles in Earth’s magnetosphere

“…There is already evidence of enhanced oxygen ion densities in the inner magnetosphere (e.g., Yau et al 1985;Hamilton et al 1988;) and significantly decreased eigenfrequencies of magnetic field lines (Fraser et al 2005). The source of this enhanced mass density may be drift-bounce resonant oxygen ions that mass load magnetic field lines, allowing Pc5 waves to penetrate to significantly lower L than normal (e.g., Lee et al 2007). These standing waves at relatively low frequency in the inner magnetosphere can accelerate radiation belt electrons efficiently in this region via drift-resonance.…”

Discovery of the action of a geophysical synchrotron in the Earth’s Van Allen radiation belts

“…2 (top panel) is remarkably monochromatic, having large amplitudes B400nT p-p at L ¼ 4.5 and the characteristics of a driven field line resonance (FLR) standing along the background magnetic field, as measured by European ground-based magnetometers in the local noon sector 17 . Typically FLRs have maximum power in the auroral zone around LB7-9 (ref.…”