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

Abstract

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 ∘ and unidirectional (downward) propagation for | 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 | = ( R 0 k A L ) - 1 / 3 , where R 0 is the Earth radius and k A is the wave number. In the case of Pc 1 waves, for reasonable values of k A ∼ 4 × 10 - 3 km - 1 , | 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 S thr ∼ 10 7 cm - 2 s - 1 and characteristic wave amplitude as B wave ≈ 0.01 nT at L = 6 .