Generation of Pc 1 pulsations in the regime of backward wave oscillator

Satellite data [Loto’aniu, T.M., Fraser, B.J., Waters, C.L., 2005. Propagation of electromagnetic ion cyclotron wave energy in the magnetosphere. Journal of Geophysics Research 110 (A07), A07214, doi:10.1029/2004JA010816.] show that EMIC Pc 1 waves demonstrate, in a wide range of L   values, bidirectional energy flux propagation at low magnetic latitudes |MLat|<11∘|MLat|<11∘ and unidirectional (downward) propagation for |MLat|>11∘|MLat|>11∘. Loto’aniu et al. suggested that this fact can be due to generation of Pc 1 waves in the regime of backward wave oscillator (BWO) suggested earlier for ELF/VLF chorus generation. We give a quantitative consideration of this idea. According to the BWO model, waves are generated in the vicinity of the equatorial cross-section of a magnetic flux tube, and the MLat width of the generation region is estimated as |MLat|=(R0kAL)-1/3|MLat|=(R0kAL)-1/3, where R0R0 is the Earth radius and kAkA is the wave number. In the case of Pc 1 waves, for reasonable values of kA∼4×10-3km-1, |MLat|≃11∘|MLat|≃11∘ depends only weakly on L  . This is in close agreement with observations. We estimate the threshold energetic-ion flux for BWO generation as Sthr∼107cm-2s-1 and characteristic wave amplitude as Bwave≈0.01nT at L=6L=6.