On Influence of Oblique Density Fluctuations on the Linear Growth of the Parametric Decay Instability of Circularly Polarized Alfvén Waves

The decay instability of low-frequency, circularly polarized Alfvén waves in the presence of oblique density fluctuations is discussed by using the three-dimensional magnetohydrodynamic (MHD) simulation. The wavenumber mode with the maximum linear growth rate is weakly suppressed due to the presence of oblique density fluctuations with larger initial amplitude, which can preferentially resonate with the parent wave. However, numerical results also indicate that the growth of the wavenumber mode with the maximum linear growth rate can also be enhanced as a consequence of nonlinear wave-wave interactions among obliquely propagating waves. These processes suggest that the influence of non-uniform wavenumber spectra of density fluctuations on the linear growth of the decay instability is not monotonous.