A group of equations governing the incompresible small time scale fluctuations in interplanetary magnetic field are derived from the MHD equations.The discusion of the governing equations show that the radial variation of the amplitude of the fluctuation can be discribed by the Alfven wave WKB model only at the small amplitude limite.When the amplitude of the fluctuation is finite with respect to the average magnetic field intensity,the nonlinear terms in the garvening equations can not be droped and the WKB aproxima-tion fails.That is the reason why the radial variation of the amplitude of the Alfvenic fluctuations from 0.3 AUto 1 AU observed by Heios does not agree with the WKBwaves model.According to the properties of the governing equations,a qualytative model for the propegation of Alfvenic fluctuations is presented.Alfvenic fluctuations can be considered to be in zeroth-order conpoused of many waves with different wave numbers propogating from the Sun towards away,but some wave modes in first-order will be generated,the nonlinear interactions between the zeroth order waves and the first order waves cause an energy cascading to high frequency range.The amplitude of the zeroth-order wave will be modified not only by the large scale variation of the solar wind but also by the cascading process.This model can explain some properties of Alfvenic fluctuations qualitatively.
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